Pipe Sizing Calculator

What Is Pipe Sizing — and Why Does It Matter?

Pipe sizing is the process of selecting the correct internal diameter for a pipe so it delivers the right volume of water (or other fluid) at the right pressure and speed. Choose a pipe that's too small and you get poor flow, noisy pipes and pressure loss. Choose one that's too big and you waste money on materials and get sluggish flow that encourages bacterial growth in hot water systems.

In UK domestic plumbing, pipe sizing is governed by BS EN 806 for cold and hot water supplies and by BS 6700 for hot and cold water in buildings. For central heating systems, BS EN 12831 and CIBSE Guide C set the design parameters. Getting these calculations right is essential for safety, comfort and compliance.

Three physical properties drive every pipe sizing calculation:

How to Use the Pipe Sizing Calculator

Three calculation modes cover the most common UK pipe sizing scenarios:

• Water Supply: Enter your design flow rate (from BS EN 806-3 loading unit tables), the pipe run length, material and available mains pressure. The calculator recommends the minimum pipe OD and checks velocity.
• Central Heating: Enter the heat load in kW and the system temperature differential (typically 10–20 °C for conventional systems, 5–10 °C for heat pumps). The calculator converts this to a flow rate and recommends a pipe size.
• Flow Rate → Diameter: The most fundamental pipe sizing calculation — enter any flow rate and a target velocity to get the minimum required internal diameter. Use this for any fluid system.

Pipe Sizing Formula — How the Calculation Works

Every pipe sizing calculation is based on the relationship between flow rate, pipe cross-section and velocity. Here are the core formulas used in professional pipe design, explained simply.

Formula 1: Pipe Diameter from Flow Rate and Velocity

In practical terms: divide flow rate by velocity, then multiply by 4/π, then take the square root. This gives you the minimum internal diameter in metres — multiply by 1000 for millimetres, then round up to the next standard pipe size.

Formula 2: Flow Rate from Known Pipe Size and Velocity

Formula 3: Pressure Drop (Darcy-Weisbach)

For practical domestic plumbing, engineers typically use a simplified pressure drop of 250–350 Pa/m (about 25–35 mm water column per metre of pipe) as a design target. For central heating, 100–300 Pa/m is typical.

Recommended Velocity Ranges

Keeping velocity within the right range is as important as calculating the correct diameter:

Pipe Sizing Charts — UK Reference Tables

These pipe sizing charts cover the most common UK scenarios. All values are based on standard pipe materials, typical UK mains pressure (2–4 bar) and recommended velocity ranges.

Water Pipe Sizing Chart — Flow Capacity by Diameter (Copper, UK)

Values based on Type R250 copper tube (EN 1057), water at 10 °C. Pressure drop is indicative at the stated velocity. Always verify against actual flow rate and pressure data.

Pipe Sizing Chart — kW to Pipe Diameter (Central Heating, ΔT = 20 °C)

Based on ΔT = 20 °C (flow/return 80/60 °C conventional system). For heat pump systems use ΔT = 5–10 °C — this significantly increases required flow rates and pipe sizes.

Water Pipe Sizing Chart — Flow Rate Through Common Pipe Sizes

MDPE Mains Water Pipe Sizes (UK)

Water Pipe Sizing — UK Domestic Guide

In the UK, domestic water pipe sizing follows BS EN 806-3 using a loading unit approach. Each outlet (tap, shower, WC, etc.) is assigned a loading unit (LU) value based on its demand, and a probability factor is applied to account for the fact that not all outlets run simultaneously. The result is the design flow rate for each pipe section.

Standard Water Pipe Sizes in UK Homes

BS EN 806-3 Loading Units — Quick Reference

What Size Water Pipe Do I Need for My House?

For a typical UK 3-bedroom house with one bathroom, one en-suite, a kitchen and a utility room, the design flow rate is typically 0.4–0.6 l/s. With average UK mains pressure of 2–4 bar, this is comfortably served by:

• A 32 mm MDPE service pipe from the street main
• A 22 mm copper main distribution pipe inside the property
• 15 mm copper spurs to individual outlets

For larger properties with multiple bathrooms or high-flow features like power showers, step up to a 28 mm internal distribution pipe. Properties with unvented hot water cylinders often require at least a 22 mm cold mains feed directly to the cylinder, with the full 28 mm recommended for cylinders over 250 litres.

Heating Pipe Sizing — Central Heating, Heat Pumps & UFH

Heating pipe sizing uses the same principles as water supply sizing, but the design conditions are different. The key variable is the temperature differential (ΔT) — the difference between flow and return temperatures. A larger ΔT means a smaller flow rate is needed for the same heat output, allowing smaller pipes.

Central Heating Pipe Sizing

Most UK gas boiler systems run at 80 °C flow / 60 °C return (ΔT = 20 °C). To find the required flow rate:

For a 12 kW zone at ΔT = 20 °C: Q = 12 ÷ (4.19 × 20) = 0.143 l/s. At 1.0 m/s target velocity, the required internal diameter = 13.5 mm → use 22 mm OD copper pipe (19.6 mm ID).

Heat Pump Pipe Sizing

Air source and ground source heat pumps operate at much lower temperature differentials — typically ΔT = 5–10 °C. This means the required flow rate for the same heat output is 2–4× higher than a gas boiler system. Always use heat pump pipe sizing tables and increase pipe sizes accordingly.

Underfloor Heating Pipe Sizing

UFH systems use small bore pipe (typically 16 mm or 20 mm OD PEX or PEX-Al-PEX) with very low flow velocities. The key sizing considerations are:

• Loop length: Maximum 100–120 m per loop (16 mm pipe) to avoid excessive pressure drop
• Spacing: Typically 150–200 mm centres for heating; 100 mm for bathrooms
• Flow rate per loop: Typically 1.5–2.5 l/min per circuit at 45/35 °C (ΔT = 10 °C)
• Manifold supply: Size the manifold supply pipes based on the total combined flow of all active loops

Use the heating pipe sizing calculator above to determine manifold supply pipe size. Individual UFH loops are standardised at 16 mm or 20 mm — use the manufacturer's pressure drop tables for loop length design.

Pipe Size Based on Flow Rate — What You Need to Know

The most fundamental pipe sizing question is: "for a given flow rate, what pipe diameter do I need?" The answer depends on one key choice — the target velocity. Different systems have different optimal velocity ranges.

Why Velocity Matters as Much as Flow Rate

Imagine a 22 mm copper pipe carrying 0.4 l/s. That gives a velocity of about 1.3 m/s — well within the acceptable range. Now try to push 0.8 l/s through the same pipe: velocity climbs to 2.6 m/s. That's above the maximum recommended for domestic plumbing. You'll hear the pipe vibrate, erosion corrosion accelerates at fittings, and pressure loss quadruples.

Flow Rate Through 15 mm Pipe

A 15 mm copper pipe (13.6 mm ID) can comfortably carry up to 0.22 l/s at 1.5 m/s. This is adequate for a single outlet — a basin, kitchen tap or WC. It is not adequate as a shared supply for multiple outlets or as a main distribution pipe in anything other than the smallest property.

Flow Rate Through 22 mm Pipe

A 22 mm copper pipe (19.6 mm ID) carries up to 0.45 l/s at 1.5 m/s. This is sufficient for most individual circuits in a typical 3-bedroom house — the cold water circuit to a bathroom, the hot water circuit from a cylinder, or a central heating zone up to about 22 kW (ΔT = 20 °C).

Pipe Size for Flow Rate — Quick Lookup

Copper vs Plastic Pipe Sizes — UK Comparison

Both copper and plastic pipes are widely used in UK plumbing and heating. They share the same nominal OD sizes (15 mm, 22 mm, 28 mm) allowing them to be connected with the same push-fit and compression fittings, but they have different internal diameters and wall thicknesses.

The key practical rule: if a copper sizing calculation shows 22 mm is marginal, use 28 mm plastic. The thicker wall of plastic pipe reduces internal diameter enough to push a borderline calculation over the velocity limit.

Commercial Pipe Sizing — BS EN 806 & CIBSE Guidance

Commercial water supply pipe sizing follows the same physical principles as domestic, but the scale and regulatory requirements are more complex. The two primary references for UK commercial water pipe sizing are BS EN 806-3 (design of water supply) and CIBSE Guide G (public health engineering).

Loading Units in Commercial Buildings

BS EN 806-3 assigns each outlet type a loading unit (LU) value. The total loading units for all outlets connected to a pipe section are converted to a design flow rate using a probability curve that accounts for the fact that not all outlets operate simultaneously. For large buildings, this results in substantially lower design flow rates than summing all outlet minimum flows directly.

Key Standards for Commercial Pipe Sizing

• BS EN 806-3: Sizing of pipe installations — the primary UK and European standard
• BS EN 806-2: Design of water supply — Legionella risk, temperatures, pipe materials
• CIBSE Guide G: Public health engineering — comprehensive design guidance for large buildings
• CIBSE Guide C: Reference data — pressure drop tables, pipe friction charts
• Water Regulations Advisory Scheme (WRAS): Material approval for pipes and fittings

Step-by-Step Pipe Sizing Examples

Example 1: Domestic Cold Water Supply — 3-Bed House

Example 2: Central Heating Zone — 14 kW at ΔT = 20 °C

Example 3: Heat Pump Primary Circuit — 12 kW at ΔT = 7 °C

Common Pipe Sizing Mistakes to Avoid

Frequently Asked Questions — Pipe Sizing UK

Related Calculators & Tools

Use these specialist calculators alongside the pipe sizing calculator for complete system design.