Pipe size calculator from flow rate
This pipe size calculator turns a flow rate into a pipe diameter. Enter the flow in litres per second, litres per minute, m³/h, US gallons per minute or cubic feet per minute, choose a velocity target, and it returns the minimum inside diameter and the next standard size up. A second mode takes a pipe you already have and reports the water velocity and Hazen-Williams friction loss.
It works as a pipe size calculator for water in plumbing, irrigation and pumping jobs, and as a pipe size calculator for compressed air: enter the free-air delivery of the compressor and the gauge pressure, and the calculator converts to actual flow before sizing. Use it with the water pump HP calculator once you know the pipe.
Everything is computed in your browser. Velocity limits and roughness values are design practice, not legal limits, so both are editable — your plumbing code, the pipe maker and the project engineer decide the final size.
How to use the Pipe Size Calculator
- Choose Find the pipe size or Check a pipe, and water or compressed air.
- Enter the flow rate and pick its unit. For compressed air enter the free-air flow and the gauge pressure in bar.
- In size mode pick a velocity target (or type your own); in check mode enter the pipe inside diameter in mm.
- Enter the pipe length and the material, which sets the Hazen-Williams C value you can still overwrite.
- Read the minimum diameter, the next standard size, velocity and head loss, then copy the summary.
Pipe Size Calculator features
Two directions
Flow to diameter, or diameter to velocity and friction loss.
Every flow unit
L/s, L/min, m³/h, US gpm and cfm, shown back in the other units.
Water and air
Free-air flow is corrected to actual flow at line pressure before sizing.
Friction loss
Hazen-Williams head loss per 100 m and for your length, for several nearby sizes.
Editable limits
No hidden hard limits: velocity and C are inputs.
How the pipe size is calculated
The continuity equation says flow equals area times velocity: Q = A × v. Solving for the diameter gives d = √(4Q ÷ (π v)). For 10 L/s (0.01 m³/s) at 2 m/s, d = √(0.04 ÷ 6.283) = 0.0798 m, so a minimum inside diameter of 79.8 mm and the next standard size is DN80, which then runs at 1.99 m/s.
Friction loss uses the Hazen-Williams equation in SI form, h = 10.67 L Q¹·⁸⁵² ÷ (C¹·⁸⁵² d⁴·⁸⁷) with Q in m³/s and d and L in metres. For the 80 mm pipe above with C = 140 it gives 4.91 m of head per 100 m. The equation matches the US form on the Engineering ToolBox Hazen-Williams page (checked 19 October 2026) and is valid for water at normal temperatures in turbulent flow.
Typical velocity targets and C values
C values used for the material presets: PVC/CPVC/PE 150, copper and cement-lined ductile iron 140, new steel 120, old cast iron or steel 100. These are textbook design values; check the pipe maker, and remember they fall as the pipe ages.
| Use | Common target | Note |
|---|---|---|
| Pump suction line | about 1 m/s (3 ft/s) | Low velocity helps avoid cavitation |
| Water supply branches | about 1.5 m/s (5 ft/s) | Keeps noise and erosion down |
| Water mains and pump discharge | about 2 m/s (6.5 ft/s) | Often allowed up to 3 m/s |
| Compressed-air mains | about 6 m/s (20 ft/s) | Keeps pressure drop low and moisture out of the stream |
Limits of this calculator
- Standard sizes assume the inside diameter equals the nominal size (DN). Real inside diameters differ with material and schedule, so use Check a pipe with the actual inside diameter before you buy.
- Domestic supply sized by fixture units (the IPC Hunter-curve method, or IS 1172 and IS 2065 loads) and drainage sized by drainage fixture units are not included; those tables were not checked here, so size them from the code you work under.
- Hazen-Williams is for water. Compressed air is sized here by velocity only; pressure drop in an air line depends on pressure and length and should be checked separately.
- For pipe mass and cost see the pipe weight calculator.
Pipe Size Calculator FAQ
How do I find the pipe size from flow rate?
Choose a velocity, then use d = √(4Q ÷ (π v)). The calculator does this for you and rounds up to the next standard size. Ten litres per second at 2 m/s needs about 80 mm.
What pipe size is needed for 50 gpm?
50 gpm is 3.15 L/s. At 2 m/s the minimum inside diameter is about 45 mm, so a 2 in (DN50) pipe is the next standard size; at 1.5 m/s it is about 52 mm, so a 2½ in pipe.
What is a good water velocity in a pipe?
Roughly 1 to 2 m/s (3 to 6.5 ft/s) for most building and pumping work. Higher speeds raise friction loss, noise and wear. Your code or the pipe maker may set a lower limit.
How do I size a pipe for compressed air?
Convert free-air delivery to actual flow at line pressure, then size for about 6 m/s. This tool does the conversion from the gauge pressure you enter. Add a generous margin for future tools and check the pressure drop.
What does Hazen-Williams C mean?
It is a roughness coefficient: higher means smoother. PVC is about 150, new steel about 120. The head loss rises sharply as C falls and as the diameter shrinks, because d is raised to the power 4.87.
Is the result a plumbing-code size?
No. It is an engineering estimate from flow and velocity. Fixture-unit tables, minimum pipe sizes and pressure requirements in your local plumbing code still apply.