📏 Why Duct Velocity Matters

Duct velocity — the speed at which air travels through ductwork — is a fundamental parameter in HVAC system design. It directly impacts noise levels, pressure losses, fan energy consumption, and occupant comfort. Air that moves too fast creates distracting noise and wastes energy; air that moves too slowly may not reach its destination or may cause temperature stratification.

This duct velocity calculator computes the average air speed through any duct — round or rectangular — given the airflow rate (CFM or m³/h) and duct dimensions. It also calculates velocity pressure, Reynolds number, and provides a design assessment against ASHRAE-recommended velocity ranges.

🔧 Duct Velocity & Air Speed Calculator

Enter airflow rate and duct dimensions. The calculator determines average air velocity, velocity pressure, Reynolds number, and provides a design assessment.

Airflow Rate (Q)

m³/h

Duct Shape

Duct Diameter (D)

📊 Results

Duct Cross-Sectional Area

—

m² / ft²

Internal duct free area

Average Air Velocity (V)

—

m/s / fpm

V = Q / A

Velocity Pressure (Pv)

—

Pa / in.wg

Pv = ½ρV²

Reynolds Number (Re)

—

Flow regime indicator

Velocity Assessment

—

Based on ASHRAE recommended ranges

Note: Standard air properties used — density ρ = 1.2 kg/m³, kinematic viscosity ν = 1.5×10⁻⁵ m²/s. For rectangular ducts, the hydraulic diameter D_h = 2WH/(W+H) is used for Reynolds number calculation.

📐 The Duct Velocity Formula – V = Q / A

The fundamental relationship between airflow rate, duct cross‑sectional area, and average air velocity is expressed by the continuity equation:

V = Q / A

Variable Definitions

Symbol	Name	SI Unit	Imperial Unit	Description
V	Average Air Velocity	m/s	fpm (ft/min)	Mean speed of air traveling through the duct cross‑section
Q	Volumetric Airflow Rate	m³/s (or m³/h)	CFM (ft³/min)	Volume of air passing through the duct per unit time
A	Duct Cross‑Sectional Area	m²	ft²	Internal free area perpendicular to airflow

Area Formulas by Duct Shape

A_round = π D² / 4 A_rect = W × H

For rectangular ducts, the hydraulic diameter D_h = 2WH/(W+H) is used in pressure loss and Reynolds number calculations to approximate the behavior of a round duct with equivalent frictional characteristics.

💡 Unit Consistency is Critical: When Q is in m³/s and A in m², V is in m/s. When Q is in CFM (ft³/min) and A in ft², V is in fpm. Our calculator handles all unit conversions automatically. Remember: 1 m/s = 196.85 fpm, and 1 CFM = 1.699 m³/h.

🌬️ Air Velocity Explained – Speed, Comfort & Performance

Duct air velocity is the speed at which conditioned air moves through the ductwork. It's not just a number — it's a design parameter that balances multiple competing factors:

Higher Velocity: Smaller, cheaper ducts but more noise, higher pressure loss, and greater fan energy.
Lower Velocity: Larger, more expensive ducts but quieter operation, lower pressure loss, and better energy efficiency.

Velocity Classification

Classification	Velocity Range (fpm)	Velocity Range (m/s)	Typical Application
Low Velocity	300 – 900	1.5 – 4.6	Residential, acoustic spaces, theaters
Medium Velocity	900 – 1,500	4.6 – 7.6	Commercial offices, retail, schools
High Velocity	1,500 – 2,500	7.6 – 12.7	Industrial, high‑pressure systems
Very High Velocity	2,500+	12.7+	Dust collection, pneumatic conveying

⚪ Round Duct Velocity – Diameter & Airflow

For a round duct, the velocity formula becomes:

V = 4Q / (π D²)

Round ducts are the most aerodynamically efficient shape — they have the smallest perimeter for a given cross‑sectional area, minimizing friction. A 200 mm round duct carrying 500 m³/h has a velocity of approximately 4.42 m/s. The same airflow through a 150 mm duct jumps to 7.86 m/s — nearly double the speed and roughly four times the velocity pressure.

🔑 Key Insight: Velocity is inversely proportional to the square of duct diameter. Reducing the diameter by just 25% increases velocity by about 78%. Always verify that the resulting velocity falls within the recommended range for your application.

⬛ Rectangular Duct Velocity – Aspect Ratio Effects

For a rectangular duct, velocity is simply V = Q / (W × H). However, the aspect ratio (W/H) significantly affects airflow distribution within the duct. Higher aspect ratios create more surface area per unit cross‑section, increasing friction and potentially causing non‑uniform velocity profiles.

For pressure loss calculations, rectangular ducts are converted to an equivalent round diameter. While velocity depends only on the cross‑sectional area (not the shape), the velocity distribution and friction characteristics differ. Keep the aspect ratio ≤ 4 for energy‑efficient design.

📋 Recommended Duct Velocities – ASHRAE & SMACNA Guidelines

The table below summarizes industry‑standard recommended air velocities for various HVAC applications. These values balance noise control, pressure loss, and duct sizing economics.

Application	Recommended Velocity (fpm)	Recommended Velocity (m/s)	Noise Sensitivity	Pressure Class
Residential – Main Supply Trunk	600 – 900	3.0 – 4.6	Low Noise	Low
Residential – Branch Supply	400 – 600	2.0 – 3.0	Very Low Noise	Low
Residential – Return Air	500 – 700	2.5 – 3.6	Low Noise	Low
Commercial – Main Duct	1,000 – 1,500	5.1 – 7.6	Moderate	Medium
Commercial – Branch Duct	600 – 900	3.0 – 4.6	Low Noise	Low–Medium
Commercial – Return / Exhaust	800 – 1,200	4.1 – 6.1	Moderate	Medium
Industrial – Supply	1,500 – 2,500	7.6 – 12.7	High Noise	High
Industrial – Exhaust / Fume	2,000 – 3,500	10.2 – 17.8	Very High Noise	High
Kitchen Exhaust (Grease)	1,500 – 2,000	7.6 – 10.2	Moderate	Medium–High
Laboratory / Fume Hood	1,000 – 2,000	5.1 – 10.2	Moderate	Medium–High
Acoustic Ducts (Theaters)	300 – 500	1.5 – 2.5	Critical Quiet	Very Low
Data Center / Server Room	800 – 1,200	4.1 – 6.1	Low Noise	Low–Medium
Clean Room – ISO 7/8	400 – 800	2.0 – 4.1	Very Low Noise	Low

⚠️ Important: These are general guidelines. Actual velocity limits may be dictated by project specifications, local codes, acoustical requirements, and equipment manufacturer recommendations. Always consult the relevant standards (ASHRAE, SMACNA, CIBSE) for your specific application.

📊 Velocity Pressure – The Kinetic Energy of Airflow

Velocity pressure (Pv) is the kinetic energy per unit volume of moving air. It's a critical component of total pressure in duct systems and is calculated directly from air velocity:

Pv = ½ ρ V²

Where ρ is air density (standard: 1.2 kg/m³). Velocity pressure is always positive and represents the pressure required to accelerate air from rest to velocity V. It's measured using a pitot tube connected to a manometer.

Total Pressure Relationship

Pt = Ps + Pv

Total Pressure (Pt) = Static Pressure (Ps) + Velocity Pressure (Pv). The fan in an HVAC system must generate sufficient total pressure to overcome all duct friction losses (static pressure) and provide the required airflow velocity (velocity pressure).

Velocity (m/s)	Velocity (fpm)	Velocity Pressure (Pa)	Velocity Pressure (in.wg)
2.0	394	2.4	0.010
4.0	787	9.6	0.039
5.0	984	15.0	0.060
7.5	1,476	33.8	0.136
10.0	1,969	60.0	0.241
12.5	2,461	93.8	0.377
15.0	2,953	135.0	0.542
20.0	3,937	240.0	0.964

💡 Practical Note: Doubling the velocity quadruples the velocity pressure (Pv ∝ V²). This exponential relationship is why small increases in duct velocity can significantly impact fan energy consumption and system total pressure requirements.

📉 Duct Friction & Pressure Loss – The Velocity Connection

Friction loss in ductwork is strongly dependent on velocity. The Darcy‑Weisbach equation quantifies this relationship:

Δp/L = f × ( ρ V² ) / ( 2 D )

Pressure loss per unit length is proportional to the square of velocity. This means doubling the air speed quadruples the friction loss — a critical consideration when sizing ducts and selecting fans. The friction factor f itself depends on the Reynolds number (which incorporates velocity) and duct surface roughness.

🔑 Energy Impact: Since fan power is proportional to the cube of airflow (and thus velocity for a fixed duct), reducing design velocity from 6 m/s to 5 m/s (a 17% reduction) can lower friction pressure loss by about 31% and fan energy consumption by a similar margin. Always design for the lowest practical velocity consistent with space and cost constraints.

⚡ High Velocity vs Low Velocity HVAC Systems

Characteristic	Low Velocity Systems	High Velocity Systems
Typical Velocity	2–6 m/s (400–1200 fpm)	7.5–15 m/s (1500–3000 fpm)
Duct Size	Larger, heavier	Smaller, lighter, easier to route
Noise Level	Quiet — suitable for occupied spaces	Noisy — requires sound attenuation
Pressure Loss	Low — energy efficient	High — requires more fan power
First Cost	Higher duct material cost	Lower duct material cost
Operating Cost	Lower fan energy	Higher fan energy
Typical Use	Offices, homes, hospitals, schools	Industrial, retrofit, space‑constrained

⚖️ Airflow Balancing – Why Velocity Matters

Airflow balancing is the process of adjusting dampers and registers so that each branch duct delivers its design airflow. Velocity measurements are the primary method for verifying airflow: by measuring duct velocity and multiplying by the known cross‑sectional area, the balancing technician determines the actual airflow and adjusts accordingly. Consistent duct velocities throughout the system indicate proper sizing and balanced resistance paths.

🫁 Ventilation & Indoor Air Quality – Air Distribution

Proper duct velocity ensures that fresh, conditioned air reaches all occupied spaces effectively. Velocity that is too low can result in poor air mixing, temperature stratification, and inadequate contaminant removal. Velocity that is too high can cause drafts and discomfort. ASHRAE Standard 62.1 specifies minimum ventilation rates that depend on both occupancy and floor area — these translate into airflow requirements that must be delivered at appropriate velocities.

📝 Worked Engineering Examples

Example 1: Residential Main Supply Duct

Scenario: A 3‑ton residential system delivers 1,200 CFM through a 16‑inch round main duct. Find the velocity.

Area: A = π × (16/12)² / 4 = 1.396 ft². Velocity: V = 1,200 / 1.396 = 860 fpm (4.37 m/s). This falls within the recommended 600–900 fpm range for residential mains. The velocity pressure is approximately 0.046 in.wg.

Example 2: Commercial Office Branch Duct

Scenario: A VAV box supplies 250 L/s (900 m³/h) through a 400×200 mm rectangular duct. Find the velocity.

Area: A = 0.4 × 0.2 = 0.08 m². Velocity: V = 0.25 / 0.08 = 3.13 m/s (615 fpm). This is at the low end of the commercial branch recommendation (600–900 fpm), suitable for an open‑plan office. Velocity pressure = 5.9 Pa.

Example 3: Kitchen Exhaust Duct

Scenario: A commercial kitchen exhaust fan moves 2,000 CFM through a 14‑inch round duct. Find the velocity and assess.

Area: A = π × (14/12)² / 4 = 1.069 ft². Velocity: V = 2,000 / 1.069 = 1,871 fpm (9.5 m/s). This is within the recommended 1,500–2,000 fpm for grease exhaust. The velocity pressure is approximately 0.219 in.wg.

Example 4: Undersized Duct Warning

Scenario: 800 CFM forced through an 8‑inch round duct. Velocity: A = π × (8/12)² / 4 = 0.349 ft². V = 800 / 0.349 = 2,292 fpm (11.6 m/s). This far exceeds residential and commercial recommendations. The velocity pressure of 0.327 in.wg indicates severe energy waste. The calculator would flag this as overspeed — risk of noise and recommend a larger duct.

Example 5: Server Room Cooling

Scenario: A data center CRAC unit supplies 5,000 CFM through a 30×24 inch rectangular duct. Velocity: A = (30/12) × (24/12) = 5.0 ft². V = 5,000 / 5.0 = 1,000 fpm (5.1 m/s). Acceptable for data center under‑floor supply plenums. The large duct area keeps velocity and noise low.

📊 Duct Velocity Quick Reference Charts

Airflow vs Velocity for Common Round Duct Sizes

Duct Diameter	Area (ft²)	CFM @ 600 fpm	CFM @ 900 fpm	CFM @ 1,500 fpm
6 inch	0.196	118	176	294
8 inch	0.349	209	314	524
10 inch	0.545	327	491	818
12 inch	0.785	471	707	1,178
14 inch	1.069	641	962	1,604
16 inch	1.396	838	1,257	2,094
18 inch	1.767	1,060	1,590	2,651
20 inch	2.182	1,309	1,964	3,273
24 inch	3.142	1,885	2,828	4,713

Velocity Pressure Quick Conversion

V (fpm)	V (m/s)	Pv (in.wg)	Pv (Pa)
500	2.54	0.016	3.9
800	4.06	0.040	10.0
1,000	5.08	0.062	15.5
1,200	6.10	0.090	22.4
1,500	7.62	0.140	34.9
2,000	10.16	0.249	62.0
2,500	12.70	0.389	96.9
3,000	15.24	0.561	139.7

Velocity Pressure vs Air Velocity

🏭 Common Applications

Residential HVAC: Verify that main trunks stay below 900 fpm and branches below 600 fpm to prevent audible airflow noise in living spaces.
Commercial Offices: Size VAV branch ducts for 600–900 fpm at design airflow. Use the velocity method for quick sizing of constant‑volume systems.
Kitchen & Bathroom Exhaust: Ensure grease exhaust ducts maintain 1,500+ fpm to keep particulates in suspension. Bathroom exhausts: 500–800 fpm.
Industrial Ventilation: Dust collection systems require 3,500–4,500 fpm to prevent particulate settling. Fume exhaust: 1,500–2,500 fpm depending on contaminant.
Data Centers: Under‑floor supply plenums designed for 800–1,200 fpm to balance cooling distribution with low noise.
Clean Rooms: Very high airflow rates (20–300 ACH) delivered at low velocities (200–600 fpm) through large ductwork and HEPA filter ceilings.
Air Conditioning Systems: DX systems require minimum velocities (typically 350–500 fpm) to ensure proper coil performance and prevent coil icing.

❓ Frequently Asked Questions – Duct Velocity & Airflow

▸1. How do you calculate duct velocity?

Duct velocity is calculated using V = Q / A, where Q is the volumetric airflow rate and A is the duct cross‑sectional area. For a round duct, V = 4Q/(πD²). For a rectangular duct, V = Q/(W×H). Use our calculator above for instant results with automatic unit conversions.

▸2. What is the duct velocity formula?

The duct velocity formula is V = Q / A. V is velocity (m/s or fpm), Q is volumetric airflow (m³/s or CFM), and A is duct cross‑sectional area (m² or ft²). For round ducts: V = 4Q/(πD²). For rectangular ducts: V = Q/(W×H).

▸3. What is a good duct velocity for residential HVAC?

For residential HVAC, main supply trunks should be 600–900 fpm (3.0–4.6 m/s). Branch supply ducts: 400–600 fpm (2.0–3.0 m/s). Return ducts: 500–700 fpm (2.5–3.6 m/s). These ranges minimize noise and pressure loss while keeping duct sizes reasonable.

▸4. How does airflow velocity affect HVAC systems?

Higher velocity increases noise, pressure loss, and fan energy consumption. Lower velocity reduces noise and energy use but requires larger, more expensive ducts. The optimal velocity balances these factors based on the application, space constraints, and acoustic requirements.

▸5. What causes noisy air ducts?

Noisy air ducts are most commonly caused by excessively high air velocity. Other causes include sharp bends, abrupt transitions, loose duct connections, vibrating duct walls, and poorly sized grilles. Keeping velocity within recommended ranges is the primary noise control strategy.

▸6. What is velocity pressure?

Velocity pressure (Pv) is the kinetic energy per unit volume of moving air: Pv = ½ρV². It's the pressure required to accelerate air from rest to velocity V. It's always positive and is measured as the difference between total pressure and static pressure using a pitot tube.

▸7. How does duct size affect airflow velocity?

For a given airflow rate, velocity is inversely proportional to duct cross‑sectional area. A larger duct reduces velocity; a smaller duct increases it. Doubling the duct area halves the velocity. This is why undersized ducts cause high‑velocity noise and pressure loss.

▸8. What is the difference between airflow and velocity?

Airflow (Q) is the volume of air moving per unit time (CFM or m³/h). Velocity (V) is the speed of that air through the duct (fpm or m/s). They are related by V = Q/A. Airflow describes how much air is delivered; velocity describes how fast it travels.

▸9. How do you improve duct airflow?

Improve duct airflow by: cleaning or replacing filters, sealing duct leaks, opening closed dampers, ensuring adequate return air paths, upsizing undersized ducts, reducing sharp bends, and verifying fan speed settings. Proper duct sizing and sealing are the most impactful measures.

▸10. What is the recommended velocity for supply ducts?

Supply duct velocity recommendations: Residential main trunk 600–900 fpm; residential branch 400–600 fpm; commercial main 1,000–1,500 fpm; commercial branch 600–900 fpm. See the full recommended velocity table on this page for detailed values by application.

▸11. What causes poor airflow in HVAC systems?

Poor airflow can result from: undersized ductwork, dirty filters, blocked coils, closed or stuck dampers, duct leakage, excessive duct length, too many bends, undersized return paths, and incorrectly set fan speeds. Systematic diagnosis starting with filter inspection is recommended.

▸12. How does duct velocity affect pressure loss?

Pressure loss in ducts is proportional to the square of velocity (Δp ∝ V²). Doubling the velocity quadruples the friction loss per unit length. This exponential relationship means that even modest velocity increases significantly impact fan energy requirements.

▸13. What is high velocity HVAC?

High velocity HVAC systems operate at duct velocities of 1,500–3,000+ fpm (7.6–15+ m/s). They use smaller, more flexible ducts that are easier to route in retrofit applications. They typically require special high‑pressure fans, sound attenuators, and high‑velocity diffusers to manage noise.

▸14. What is low velocity HVAC?

Low velocity HVAC systems operate at duct velocities below 900 fpm (4.6 m/s). They use larger ducts that minimize noise and pressure loss, making them ideal for noise‑sensitive spaces like theaters, recording studios, hospitals, and high‑end residential applications.

▸15. How do you balance airflow in ducts?

Airflow balancing involves measuring velocity at each supply diffuser using an anemometer or flow hood, calculating actual airflow, and adjusting branch dampers to achieve design airflow at each outlet. The process is iterative — adjusting one branch affects others due to system pressure changes.

▸16. What is airflow velocity in ventilation systems?

Airflow velocity in ventilation systems is the speed at which fresh outdoor air or conditioned air travels through ductwork. It directly affects ventilation effectiveness, energy consumption, and occupant comfort. ASHRAE 62.1 specifies minimum ventilation airflow rates but velocity must also be appropriate for the system design.

▸17. How do you calculate duct velocity from CFM and duct size?

Convert duct dimensions to area in ft²: for round duct, A = π×(D/12)²/4; for rectangular, A = (W/12)×(H/12). Then V (fpm) = CFM / A (ft²). For example, 500 CFM through a 12‑inch round duct (A=0.785 ft²) gives V = 500/0.785 = 637 fpm.

▸18. What is the maximum duct velocity for commercial HVAC?

For commercial HVAC, the maximum recommended velocity for main ducts is typically 1,500 fpm (7.6 m/s). Branch ducts should not exceed 900 fpm (4.6 m/s). Exceeding these limits increases noise complaints and energy costs. Some industrial applications allow higher velocities with appropriate acoustic treatment.

▸19. How do you measure duct velocity in the field?

Duct velocity is measured using a pitot tube and manometer (for velocity pressure), a hot‑wire anemometer, or a vane anemometer. For accurate results, traverse the duct in a grid pattern at a straight section at least 7.5 duct diameters downstream from any disturbance. Average the readings to obtain mean velocity.

▸20. What is the relationship between velocity and static pressure?

Total pressure = static pressure + velocity pressure. Static pressure is the potential energy (pressure exerted on duct walls); velocity pressure is the kinetic energy. As air speeds up in a smaller duct section, velocity pressure increases and static pressure decreases (Bernoulli's principle).

▸21. What happens if duct velocity is too low?

If duct velocity is too low (below ~300 fpm/1.5 m/s), air may not mix adequately in the occupied zone, leading to temperature stratification and poor ventilation effectiveness. In cooling ducts, low velocity can also cause condensation on duct surfaces if the duct is uninsulated.

▸22. How do you calculate rectangular duct velocity?

V = Q / (W × H). Ensure consistent units: for CFM and inches, convert W and H to feet first (divide by 12). For m³/h and mm, convert to m (divide by 1000) and Q to m³/s (divide by 3600). Our calculator handles all conversions automatically.

▸23. What is the velocity pressure formula?

Velocity pressure Pv = ½ρV², where ρ is air density (standard 1.2 kg/m³) and V is velocity in m/s. In imperial units: Pv (in.wg) = (V/4005)² where V is in fpm. For example, at 800 fpm, Pv = (800/4005)² ≈ 0.040 in.wg.

▸24. How does flexible duct affect velocity?

Flexible duct has higher friction loss than rigid duct at the same velocity due to its corrugated inner surface. For the same airflow and diameter, flex duct may require a slightly larger size or the velocity should be kept at the lower end of the recommended range to compensate for the increased resistance.

▸25. What is the recommended velocity for return air ducts?

Return air duct velocity recommendations: Residential 500–700 fpm (2.5–3.6 m/s); Commercial 800–1,200 fpm (4.1–6.1 m/s). Return ducts are often sized slightly larger (lower velocity) than supply ducts to accommodate filter loading and maintain proper building pressure balance.

▸26. What is the velocity for kitchen exhaust ducts?

Grease exhaust ducts should maintain 1,500–2,000 fpm (7.6–10.2 m/s) to keep grease particles in suspension and prevent deposition inside the duct. Velocity should not exceed 2,500 fpm to avoid excessive noise and pressure loss. Always use smooth rigid duct (no flex) for grease exhaust.

▸27. How does altitude affect duct velocity?

At higher altitudes, air density decreases. This means that for the same mass flow, volumetric flow (CFM) increases. Ducts may need to be sized larger to maintain the same velocity. Additionally, velocity pressure decreases with lower density, which affects pressure loss calculations and fan performance.

▸28. What is the difference between duct velocity and face velocity?

Duct velocity is the air speed inside the duct. Face velocity is the air speed at the face of a grille, diffuser, coil, or filter. Face velocity is usually much lower than duct velocity because the face area is larger. For example, a duct velocity of 1,200 fpm might correspond to a diffuser face velocity of 400 fpm.

▸29. How do you convert m/s to fpm?

Multiply m/s by 196.85 to get fpm. For example, 5 m/s = 5 × 196.85 ≈ 984 fpm. To convert fpm to m/s, divide by 196.85. These conversions are built into our calculator for automatic unit switching.

▸30. What is the Reynolds number in ducts?

The Reynolds number (Re = VD/ν) indicates whether duct airflow is laminar or turbulent. In HVAC ducts, Re is almost always > 4,000 (turbulent flow). This is important because the Darcy friction factor — and thus pressure loss — depends on the flow regime. Our calculator displays Re for reference.

▸31. How do you size a duct based on velocity?

Select a target velocity from recommended ranges, then calculate required area A = Q/V. For round ducts, D = √(4A/π). For rectangular ducts, choose an aspect ratio and solve H = √(A/AR), W = AR×H. This is the "velocity method" — quick and effective for most HVAC designs.

▸32. What causes velocity pressure loss in ducts?

Velocity pressure is not "lost" — it's converted to static pressure as the duct expands (diffuser effect) and vice versa. However, friction and turbulence irreversibly convert some mechanical energy into heat. The total pressure decreases along the duct due to these irreversible losses, even though velocity pressure may increase or decrease with area changes.

▸33. What is the minimum duct velocity for cooling?

For cooling ducts, maintain at least 300–400 fpm (1.5–2.0 m/s) to ensure proper air throw from diffusers and adequate mixing in the occupied zone. Velocities below this may result in cold air "dumping" and poor temperature distribution. Some high‑induction diffusers can operate effectively at lower velocities.

▸34. How do you calculate airflow from velocity?

Airflow Q = V × A. Measure the average velocity V (using a pitot tube traverse or anemometer) and multiply by the duct cross‑sectional area A. For round ducts: Q = V × πD²/4. For rectangular: Q = V × W × H. Ensure consistent units.

▸35. What is the velocity limit for flexible duct?

Flexible duct manufacturers typically recommend maximum velocities of 1,200–1,500 fpm (6.1–7.6 m/s). Higher velocities can cause the inner liner to flutter, increase noise, and accelerate wear. For residential applications, keep flex duct below 900 fpm for best results.

▸36. What is the relationship between duct velocity and fan energy?

Fan power is proportional to the cube of airflow (and thus velocity for a fixed duct). If velocity increases by 25%, fan power increases by approximately 95% (1.25³ ≈ 1.95). This cubic relationship makes velocity optimization one of the most powerful energy‑saving strategies in HVAC design.

▸37. How do you calculate velocity pressure from duct velocity?

In imperial units: Pv (in.wg) = (V/4005)² where V is in fpm. In SI: Pv (Pa) = ½ × 1.2 × V² where V is in m/s. Our calculator computes velocity pressure automatically and displays it in both unit systems.

▸38. What is a good velocity for bathroom exhaust ducts?

Bathroom exhaust ducts should be sized for 500–800 fpm (2.5–4.0 m/s). This provides adequate moisture removal without excessive noise. A typical 50 CFM bathroom fan with a 4‑inch duct produces approximately 570 fpm. Use rigid duct or smooth‑bore flex to minimize pressure loss.

▸39. How does duct velocity affect indoor air quality?

Adequate duct velocity ensures proper air distribution and mixing, which is essential for diluting indoor contaminants and maintaining uniform temperature. Velocity that is too low results in poor mixing and potential contaminant buildup. Velocity that is too high can cause drafts that lead occupants to block diffusers, reducing effective ventilation.

▸40. What is the equivalent diameter of a rectangular duct?

The equivalent round diameter for a rectangular duct is the diameter of a round duct that would have the same friction loss at the same airflow. It's calculated using the Huebscher formula: Dₑ = 1.3×(W×H)⁰·⁶²⁵/(W+H)⁰·²⁵. This is used in pressure loss calculations for rectangular ducts.

▸41. How do you calculate round duct velocity from diameter and CFM?

V (fpm) = CFM / (π × (D/12)² / 4). Alternatively: V = CFM × 183.35 / D² where D is in inches. For example, 800 CFM through a 14‑inch round duct: V = 800 × 183.35 / 14² = 800 × 183.35 / 196 = 748 fpm. In SI: V (m/s) = Q(m³/s) / (πD²/4).

▸42. What are the consequences of oversizing ductwork?

Oversized ductwork results in very low velocity, which can cause: poor air mixing and temperature stratification, difficulty balancing the system, increased material and space costs, potential condensation issues in cooling ducts, and reduced air throw from diffusers. The duct velocity should be high enough to ensure proper air distribution — typically at least 300–400 fpm.