Pipe Sizing Calculator

Q: What is a good water velocity for a pipe?

For most building water systems, aim for these speeds: Small pipes (15 mm and under): up to 1.0 m/s (3.3 ft/s) Medium pipes (up to 50 mm): 0.75–1.5 m/s (2.5–5 ft/s) Large mains (over 50 mm): 1.25–3.0 m/s (4–10 ft/s) Going faster causes noise and pipe wear. Going slower means you paid for pipe you did not need, and air can get stuck.

Q: How do you calculate flow velocity in a pipe?

Divide the flow rate by the inside area of the pipe: v = Q ÷ A Where v is velocity in m/s, Q is flow in m³/s, and A is area in m². Area is π × (D ÷ 2)² using the real internal bore in metres. Example: 0.25 L/s (0.00025 m³/s) in a 22 mm copper pipe (20.2 mm bore). Area = 0.00032 m². Velocity = 0.00025 ÷ 0.00032 = 0.78 m/s .

Q: What is the Darcy-Weisbach equation?

It is the standard formula for pressure loss from friction in a pipe: Δp/L = f × (ρ × v²) ÷ (2 × D) Δp/L = pressure loss per metre (Pa/m) f = Darcy friction factor ρ = fluid density (kg/m³) v = velocity (m/s) D = internal bore (m) Notice velocity is squared. Double the speed and friction loss goes up four times.

Q: What is the internal bore of a 22 mm copper pipe?

About 20.2 mm . The 22 mm is the outside diameter, not the inside. Here are common copper bores: 15 mm pipe → 13.6 mm bore 22 mm pipe → 20.2 mm bore 28 mm pipe → 26.2 mm bore 35 mm pipe → 32.6 mm bore 42 mm pipe → 39.6 mm bore 54 mm pipe → 51.6 mm bore Always use the bore for flow maths. Using 22 mm instead of 20.2 mm would give you a velocity about 16% too low.

Q: What is a good pressure drop per metre for pipework?

The normal target is 100 to 300 Pa/m (about 1 to 3 ft H₂O per 100 ft). Under 100 Pa/m: pipe is likely oversized and costing extra money 100–300 Pa/m: good balance of pipe cost and pump energy 300–500 Pa/m: fine for short runs only Over 500 Pa/m: pump energy gets expensive fast

Q: What is the Reynolds number and why does it matter for pipe sizing?

Reynolds number tells you how the fluid moves inside the pipe. The formula is Re = (ρ × v × D) ÷ μ , where μ is the fluid viscosity. Re under 2300: laminar, smooth layers, rare in heating and cooling Re 2300–4000: transitional, unstable and hard to predict Re over 4000: turbulent, normal for water pipework It matters because the friction factor is worked out differently for each zone. Turbulent flow also moves heat much better than laminar flow.

Q: Does glycol need bigger pipes than water?

Often yes. Glycol is thicker and heavier than plain water. A 50% glycol mix at −20 °C has about four times the viscosity of water at 10 °C. At the same flow and pipe size, glycol gives more pressure loss. You either upsize the pipe or fit a stronger pump. Glycol also carries less heat per litre, so you usually need more flow to move the same heat, which pushes the pipe size up again.

Q: What happens if a pipe is too small?

Water moves too fast, and that causes real problems: Noise – hissing and rushing sounds in walls and rooms Erosion – fast water wears away the pipe wall, especially in copper elbows High pump cost – friction loss rises with velocity squared Poor flow – far ends of the system may starve Going up one standard size can cut pressure loss by half or more.

Q: What happens if a pipe is too big?

Oversized pipe wastes money and can cause faults: Higher cost – more copper or steel, bigger fittings, more insulation Trapped air – slow water below about 0.5 m/s cannot push air bubbles to the vents More heat loss – bigger surface area on hot pipes Dirt build-up – slow flow lets sludge settle

Q: Why does pipe material change pressure loss?

Each material has a different inside wall roughness, measured as ε in mm: Copper and PVC: 0.0015 mm (very smooth) MDPE plastic: 0.007 mm Stainless steel: 0.015 mm Mild steel: 0.046 mm Galvanized steel: 0.15 mm Cast iron: 0.26 mm (rough) Rougher walls grab the water more, so friction goes up. At the same size and flow, cast iron can lose noticeably more pressure than copper.

Pipe Sizing Calculator

Updated Sep 1, 2026 By Jehan Wadia

Introduction

This pipe sizing calculator helps you pick the right pipe for a water system. You enter the flow rate, the pipe material, and the fluid type. The tool then shows the flow speed, the pressure loss, and whether the size is a good fit.

It works two ways. In Standard mode, you choose a pipe size and see how it performs. In Find Optimal Pipe Size mode, the tool checks every standard size and tells you the best one for your flow.

You can work in metric or imperial units. Pick from copper, steel, stainless, plastic, cast iron, galvanized, or PVC pipe. Fluid choices include hot water (LTHW, MTHW, DHW), chilled water, cold water, condenser water, and glycol mixes. Each one has its own density and thickness, so the math stays true to real jobs.

The results give you flow velocity, pressure drop per metre or foot, Reynolds number, flow regime, and total pressure loss over the whole pipe run. Color badges show if each value is in range, close to the limit, or too high. A step-by-step section shows the full working, using the Darcy–Weisbach equation and the Swamee–Jain friction factor. A chart and table let you compare all sizes side by side.

Use it to size heating, cooling, and water pipework fast, cut pump energy, and avoid noisy or oversized pipes.

How to use our Pipe Sizing Calculator

Pick your pipe material, fluid, and flow rate, and the calculator gives you the flow velocity, pressure loss per metre, total pressure drop, Reynolds number, and the best pipe size for the job.

Unit System: Choose Metric (SI) or Imperial (US). All inputs and results switch over, and your entered numbers are converted for you.

Calculator Mode: Choose Standard if you already know the pipe size you want to check. Choose Find Optimal Pipe Size if you want the tool to rank every standard size for your flow.

Pipe Material: Pick copper, steel, plastic, cast iron, and more. This sets the pipe roughness used in the friction calculation.

System / Fluid Type: Pick the fluid in the pipe, such as LTHW, chilled water, or glycol. This sets the density and viscosity at that temperature.

Standard Nominal Pipe Size: In Standard mode, choose the pipe size to test. The true internal bore is used for all maths, not the nominal name.

Flow Rate: Type the flow going through the pipe and pick a unit: L/s, L/min, m³/h, or US GPM. It must be more than zero.

Total Pipe Run Length: Type the length of the pipe run in m, ft, mm, or in. This gives you the total pressure loss. Leave it blank or zero to skip it.

Target Velocity Range: In Find Optimal Pipe Size mode, set the lowest and highest speed you will accept. Sizes inside this band get a green score.

Output Display Units: Choose how you want the results shown for velocity, pressure loss per length, total pressure loss, and internal bore.

Calculate and Reset: Press Calculate to see your results, chart, size table, and step-by-step working. Press Reset to go back to the default values.

Pipe Sizing: What It Means and Why It Matters

Pipe sizing is the job of picking the right pipe diameter to carry a set flow of water. If the pipe is too small, the water moves too fast. That makes noise, wears the pipe walls, and forces the pump to work harder. If the pipe is too big, it costs more to buy and install, and slow water can trap air pockets. Good pipe sizing finds the middle ground.

The Two Numbers That Decide Pipe Size

Engineers size pipes using two key results:

Flow velocity – how fast the water travels, in metres per second (m/s) or feet per second (ft/s). Most building services work aims for about 0.75–1.5 m/s in small and medium pipes, and up to about 3 m/s in large mains.
Pressure loss per metre – how much pressure the pipe friction eats up along the run. A common target is 100–300 Pa/m. Higher numbers mean bigger pumps and higher energy bills.

How the Maths Works

Velocity comes from the flow rate divided by the inside area of the pipe: v = Q ÷ A. Pressure loss uses the Darcy–Weisbach equation: Δp/L = f × (ρv²) ÷ (2D). The friction factor f depends on the Reynolds number, which tells you if the flow is smooth (laminar) or mixed up (turbulent). Almost all heating and cooling pipes run turbulent, and the friction factor is found with the Swamee–Jain formula.

Why Material and Fluid Change the Answer

Pipe material sets the roughness of the inside wall. Copper and PVC are very smooth (about 0.0015 mm), while cast iron is rough (about 0.26 mm). Rougher walls mean more pressure loss for the same size and flow.

The fluid matters too. Hot water is thinner than cold water, so it flows with less friction. Glycol mixes used in chilled and frost-protected systems are thicker and heavier, so they need more pump pressure than plain water at the same flow.

Nominal Size vs. Real Bore

The name on a pipe is not its true inside width. A 22 mm copper pipe has about a 20.2 mm bore, and a DN50 steel pipe has about a 52.5 mm bore. All velocity and pressure loss maths must use the real internal bore, not the nominal label.

Where Pipe Sizing Is Used

LTHW and MTHW heating circuits
Chilled water and condenser water systems
Domestic hot and cold water services
Glycol-filled cooling and frost protection loops

Remember that friction in straight pipe is only part of the story. Bends, valves, tees, and fittings add more loss, so allow extra when you set the pump duty for the full circuit.

Formulas used

Cross-sectional area of pipe

A = \pi \left(\frac{D}{2}\right)^2

Flow velocity

v = \frac{Q}{A}

Reynolds number

Re = \frac{\rho v D}{\mu}

Darcy friction factor - laminar flow (Re <= 2300)

f = \frac{64}{Re}

Darcy friction factor - turbulent flow (Swamee-Jain)

f = \frac{0.25}{\left[\log_{10}\!\left(\frac{\varepsilon}{3.7 D} + \frac{5.74}{Re^{0.9}}\right)\right]^2}

Darcy friction factor - transitional flow (2300 < Re <= 4000, interpolated)

f = f_{lam} + (f_{turb} - f_{lam})\,w, \quad w = \frac{Re - 2300}{1700}

Pressure loss per unit length (Darcy-Weisbach)

\frac{\Delta p}{L} = f\,\frac{\rho v^2}{2D}

Total pressure loss over pipe run

\Delta p_{total} = \frac{\Delta p}{L} \times L

Frequently asked questions

What is a good water velocity for a pipe?

For most building water systems, aim for these speeds:

Small pipes (15 mm and under): up to 1.0 m/s (3.3 ft/s)
Medium pipes (up to 50 mm): 0.75–1.5 m/s (2.5–5 ft/s)
Large mains (over 50 mm): 1.25–3.0 m/s (4–10 ft/s)

Going faster causes noise and pipe wear. Going slower means you paid for pipe you did not need, and air can get stuck.

How do you calculate flow velocity in a pipe?

Divide the flow rate by the inside area of the pipe:

v = Q ÷ A

Where v is velocity in m/s, Q is flow in m³/s, and A is area in m². Area is π × (D ÷ 2)² using the real internal bore in metres.

Example: 0.25 L/s (0.00025 m³/s) in a 22 mm copper pipe (20.2 mm bore). Area = 0.00032 m². Velocity = 0.00025 ÷ 0.00032 = 0.78 m/s.

What is the Darcy-Weisbach equation?

It is the standard formula for pressure loss from friction in a pipe:

Δp/L = f × (ρ × v²) ÷ (2 × D)

Δp/L = pressure loss per metre (Pa/m)
f = Darcy friction factor
ρ = fluid density (kg/m³)
v = velocity (m/s)
D = internal bore (m)

Notice velocity is squared. Double the speed and friction loss goes up four times.

What is the internal bore of a 22 mm copper pipe?

About 20.2 mm. The 22 mm is the outside diameter, not the inside. Here are common copper bores:

15 mm pipe → 13.6 mm bore
22 mm pipe → 20.2 mm bore
28 mm pipe → 26.2 mm bore
35 mm pipe → 32.6 mm bore
42 mm pipe → 39.6 mm bore
54 mm pipe → 51.6 mm bore

Always use the bore for flow maths. Using 22 mm instead of 20.2 mm would give you a velocity about 16% too low.

What is a good pressure drop per metre for pipework?

The normal target is 100 to 300 Pa/m (about 1 to 3 ft H₂O per 100 ft).

Under 100 Pa/m: pipe is likely oversized and costing extra money
100–300 Pa/m: good balance of pipe cost and pump energy
300–500 Pa/m: fine for short runs only
Over 500 Pa/m: pump energy gets expensive fast

What is the Reynolds number and why does it matter for pipe sizing?

Reynolds number tells you how the fluid moves inside the pipe. The formula is Re = (ρ × v × D) ÷ μ, where μ is the fluid viscosity.

Re under 2300: laminar, smooth layers, rare in heating and cooling
Re 2300–4000: transitional, unstable and hard to predict
Re over 4000: turbulent, normal for water pipework

It matters because the friction factor is worked out differently for each zone. Turbulent flow also moves heat much better than laminar flow.

Does glycol need bigger pipes than water?

Often yes. Glycol is thicker and heavier than plain water. A 50% glycol mix at −20 °C has about four times the viscosity of water at 10 °C.

At the same flow and pipe size, glycol gives more pressure loss. You either upsize the pipe or fit a stronger pump. Glycol also carries less heat per litre, so you usually need more flow to move the same heat, which pushes the pipe size up again.

What happens if a pipe is too small?

Water moves too fast, and that causes real problems:

Noise – hissing and rushing sounds in walls and rooms
Erosion – fast water wears away the pipe wall, especially in copper elbows
High pump cost – friction loss rises with velocity squared
Poor flow – far ends of the system may starve

Going up one standard size can cut pressure loss by half or more.

What happens if a pipe is too big?

Oversized pipe wastes money and can cause faults:

Higher cost – more copper or steel, bigger fittings, more insulation
Trapped air – slow water below about 0.5 m/s cannot push air bubbles to the vents
More heat loss – bigger surface area on hot pipes
Dirt build-up – slow flow lets sludge settle

Why does pipe material change pressure loss?

Each material has a different inside wall roughness, measured as ε in mm:

Copper and PVC: 0.0015 mm (very smooth)
MDPE plastic: 0.007 mm
Stainless steel: 0.015 mm
Mild steel: 0.046 mm
Galvanized steel: 0.15 mm
Cast iron: 0.26 mm (rough)

Rougher walls grab the water more, so friction goes up. At the same size and flow, cast iron can lose noticeably more pressure than copper.

How do you convert litres per second to GPM?

Multiply L/s by 15.85 to get US gallons per minute.

1 L/s = 15.85 GPM
0.25 L/s = 3.96 GPM
1 L/min = 0.264 GPM
1 m³/h = 4.40 GPM

To go the other way, divide GPM by 15.85 to get L/s.

How do you work out flow rate from a heating load?

Use this formula for water systems:

Flow (L/s) = kW ÷ (4.18 × ΔT)

Where ΔT is the temperature drop across the circuit in °C.

Example: a 20 kW load with an 11 °C drop (82/71 °C flow and return) needs 20 ÷ (4.18 × 11) = 0.43 L/s. A bigger ΔT means less flow and smaller pipes.

Do fittings and valves add to pressure loss?

Yes, and a lot. Straight pipe friction is only part of the total. Every bend, tee, valve, and strainer adds loss.

A quick rule is to add 10% to 30% onto the straight pipe loss for a normal run. For short runs packed with fittings, the fitting loss can beat the pipe loss. Plant room headers and boiler connections are usually the worst.

Add plant items like coils, chillers, and control valves on top of that when you set the pump head.

What is the difference between DN50 and 2 inch pipe?

They are the same size, just named in different systems. DN means Diameter Nominal in millimetres.

DN15 = 1/2 inch
DN25 = 1 inch
DN50 = 2 inch
DN80 = 3 inch
DN100 = 4 inch

Neither name is the real bore. A DN50 steel pipe has about a 52.5 mm inside bore.

Does hot water flow easier than cold water?

Yes. Hot water is thinner, so it has less friction. At 80 °C, water viscosity is about 0.000355 Pa·s. At 10 °C, it is about 0.001307 Pa·s, nearly four times thicker.

That means a chilled water pipe carrying the same flow as a hot water pipe will show a bit more pressure loss. Density drops too, from about 1000 kg/m³ cold to 972 kg/m³ at 80 °C, which slightly offsets it.

What is the maximum velocity for copper pipe?

Keep copper under about 1.5 m/s for cold water and 1.0 to 1.2 m/s for hot water. Hot water speeds up erosion-corrosion, where fast water strips the protective film off the copper.

In small bores of 15 mm and under, stay at or below 1.0 m/s to keep noise down. Recirculating hot water loops should be even slower, around 0.5 to 1.0 m/s, because the water passes the same spot over and over.

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