Air Flow Rate Calculator
Professional HVAC airflow calculator for engineers, contractors, and ventilation designers. Calculate CFM, m³/h, duct airflow, air changes per hour (ACH), and ventilation rates instantly. Includes duct sizing, velocity recommendations, and comprehensive HVAC engineering reference.
Interactive Air Flow Rate Calculator
💡 Enter any two values — the third is calculated automatically using Q = A × V.
🔧 Enter duct dimensions and velocity to compute airflow. Toggle between round and rectangular ducts.
🔄 Enter a value in either field — the other updates instantly.
Conversion factor: 1 CFM = 1.699 m³/h | 1 m³/h = 0.5886 CFM
🏠 Enter room volume and target ACH, or enter a known CFM to compute actual ACH.
📘 The Fundamental Airflow Formula: Q = A × V
The cornerstone of all HVAC airflow calculations, duct sizing, and ventilation system design is the continuity equation for incompressible flow:
Where:
- Q = Volumetric flow rate (CFM, m³/h, or L/s) — the volume of air moving per unit time
- A = Cross-sectional area of the duct or opening (ft² or m²)
- V = Average air velocity (fpm or m/s) — the speed at which air travels through the duct
Engineering Significance
This equation assumes steady, incompressible flow — a valid assumption for most HVAC applications where air velocities remain well below Mach 0.3 (approximately 10,000 fpm). The relationship is linear: doubling the duct area doubles the airflow at constant velocity; doubling the velocity doubles the airflow through a fixed duct size.
Extended Formulas
- CFM from round duct: CFM = π × (diameterinches / 24)² × Velocityfpm
- CFM from rectangular duct: CFM = (Widthin × Heightin / 144) × Velocityfpm
- m³/h from duct: m³/h = Aream² × Velocitym/s × 3600
🔄 CFM to m³/h Conversion Guide
HVAC engineers frequently convert between imperial (CFM) and metric (m³/h) airflow units. The precise conversion factors are:
| CFM | m³/h | L/s | Typical Application |
|---|---|---|---|
| 50 | 84.95 | 23.6 | Small bathroom exhaust fan |
| 100 | 169.9 | 47.2 | Residential bathroom fan / small room |
| 200 | 339.8 | 94.4 | Residential kitchen range hood |
| 400 | 679.6 | 188.8 | Typical residential HVAC air handler (1 ton) |
| 800 | 1,359 | 377.5 | 2-ton residential AC / small commercial |
| 1,200 | 2,039 | 566.3 | 3-ton system / medium commercial zone |
| 2,000 | 3,398 | 943.9 | Large commercial AHU / 5-ton system |
| 5,000 | 8,495 | 2,360 | Industrial ventilation / data center cooling |
| 10,000 | 16,990 | 4,719 | Large industrial exhaust / clean room supply |
🔧 Duct Airflow Calculation – Round & Rectangular Ducts
Accurate duct airflow calculation is essential for HVAC design. The cross-sectional area of the duct directly determines the volumetric airflow capacity at a given velocity.
Round Duct Airflow
Rectangular Duct Airflow
Duct Velocity Recommendations
| Application | Recommended Velocity (fpm) | Velocity (m/s) | Notes |
|---|---|---|---|
| Residential supply ducts | 600 – 900 | 3.0 – 4.6 | Low noise priority |
| Residential return ducts | 500 – 800 | 2.5 – 4.1 | Lower velocity acceptable |
| Commercial supply (main) | 1,000 – 1,600 | 5.1 – 8.1 | Higher velocity tolerated |
| Industrial ventilation | 1,500 – 2,500 | 7.6 – 12.7 | Noise less critical |
| Clean room supply | 300 – 600 | 1.5 – 3.0 | Laminar flow priority |
| Data center cooling | 800 – 1,200 | 4.1 – 6.1 | Static pressure managed |
💨 Air Velocity in Ducts – Impact on System Performance
Air velocity is a critical parameter in HVAC duct design. It affects duct noise, pressure drop, energy efficiency, and overall air distribution quality.
Low Velocity vs. High Velocity Systems
- Low velocity (≤900 fpm): Quieter operation, lower friction losses, larger duct sizes required — ideal for residential and comfort-critical spaces
- Medium velocity (900–1,500 fpm): Balanced approach for commercial buildings, moderate noise, manageable pressure drops
- High velocity (≥1,500 fpm): Compact ducts, lower material costs, higher fan energy consumption, increased noise — suitable for industrial applications
🏠 Air Changes Per Hour (ACH) – Ventilation Rate Calculations
Air Changes Per Hour (ACH) quantifies how many times the entire air volume of a space is replaced with fresh air each hour.
Recommended ACH by Application
| Space Type | Recommended ACH | Standard / Guideline |
|---|---|---|
| Residential living areas | 0.35 – 4 | ASHRAE 62.2 |
| Residential bathrooms | 8 – 10 | IRC / IMC |
| Office spaces | 4 – 10 | ASHRAE 62.1 |
| Classrooms | 6 – 8 | ASHRAE 62.1 |
| Hospital patient rooms | 6 – 12 | ASHRAE 170 |
| Laboratories | 8 – 12 | ANSI/AIHA Z9.5 |
| Commercial kitchens | 20 – 40 | IMC / NFPA 96 |
| ISO Class 8 Clean Room | 20 – 40 | ISO 14644 |
| ISO Class 5 Clean Room | 200 – 600+ | ISO 14644 |
| Data centers | 15 – 30 | ASHRAE TC 9.9 |
🏗️ HVAC Ventilation Design Principles
Effective HVAC ventilation design balances supply air, return air, exhaust air, and fresh air intake to maintain indoor air quality (IAQ).
Supply Air vs. Exhaust Air
- Supply Air: Conditioned air delivered to occupied spaces through diffusers or grilles.
- Return Air: Air drawn back from occupied spaces to the AHU for reconditioning (typically 80-90% of supply).
- Exhaust Air: Air removed from the building and discharged outdoors — critical for removing contaminants.
- Fresh Air (Outdoor Air) Intake: Outside air introduced to meet ventilation standards (ASHRAE 62.1).
Pressure Relationships
- Balanced ventilation: Supply ≈ Exhaust + Return (neutral building pressure)
- Positive pressure: Supply > Exhaust (used in clean rooms, hospitals)
- Negative pressure: Exhaust > Supply (used in laboratories, isolation rooms)
📋 Worked Engineering Examples
Example 1: Residential HVAC System Airflow
Scenario: A 3-ton residential air conditioner requires 1,200 CFM total supply air through an 18-inch round main duct. What is the duct velocity?
- Duct area = π × (18/24)² = 1.767 ft²
- Velocity = CFM / Area = 1,200 / 1.767 = 679 fpm ✅ (within 600–900 fpm residential range)
Example 2: Bathroom Exhaust Fan Sizing
Scenario: 8 ft × 10 ft bathroom with 9 ft ceilings. Code requires 8 ACH.
- Room volume = 8 × 10 × 9 = 720 ft³
- Required CFM = (8 × 720) / 60 = 96 CFM → Select a 100 CFM exhaust fan.
Example 3: Commercial Office Ventilation
Scenario: 3,000 ft² office with 10 ft ceilings requires 6 ACH.
- Room volume = 30,000 ft³
- Required CFM = (6 × 30,000) / 60 = 3,000 CFM (5,097 m³/h)
Example 4: Clean Room Airflow Design
Scenario: ISO Class 7 clean room (10,000 ft², 12 ft ceiling) requires 60 ACH.
- Room volume = 120,000 ft³
- Required CFM = (60 × 120,000) / 60 = 120,000 CFM (203,880 m³/h)
🏭 Common Applications
❓ Frequently Asked Questions – Airflow Rate & HVAC Ventilation
Comprehensive answers to the most common airflow rate, CFM calculation, and HVAC ventilation questions.