Pipe Size Calculator
Enter the flow rate and the velocity you want in the pipe. The calculator works out the minimum inside diameter, picks the Schedule 40 size and shows the real velocity and friction loss.
- Flow to diameter
- Schedule 40 sizes
- Head loss per 100 m
For clean water in steel or plastic pipe. Use a lower velocity, about 1 m/s, on pump suction lines.
Pipe size is a balance between cost and friction. A small pipe is cheaper but water rushes through it, which is noisy, wastes pump energy and can cause water hammer. A bigger pipe slows the water down. Most water systems aim for 1 to 2.5 metres per second.
Pipe size and flow chart
Schedule 40 at 1.5 m/s. Search for a size.
| Nominal size | Inside dia (mm) | Flow at 1.5 m/s (L/min) | Flow (US gpm) |
|---|---|---|---|
| 1/2 in | 15.8 | 18 | 4.7 |
| 3/4 in | 20.9 | 31 | 8.2 |
| 1 in | 26.6 | 50 | 13.2 |
| 1-1/4 in | 35.1 | 87 | 23 |
| 1-1/2 in | 40.9 | 118 | 31 |
| 2 in | 52.5 | 195 | 51 |
| 3 in | 77.9 | 429 | 113 |
| 4 in | 102.3 | 740 | 195 |
| 6 in | 154.1 | 1678 | 443 |
Copper and plastic pipes have different inside diameters from steel of the same nominal size. Check the maker's table for exact bore.
Flow and pipe diameter
At three velocities. Slide the flow rate.
Faster water means a smaller, cheaper pipe, but more friction and noise. 1.5 m/s is a common design value for building water services.
How to size a pipe
From flow and velocity. Step through it.
Convert flow
To cubic metres per second.
Long pipes or tall buildings may need a size up to keep pressure loss within what the pump or mains can supply.
Slow, normal or fast
Velocity changes how the system behaves. Pick a velocity.
Quiet and low loss, but a larger, more expensive pipe.
The usual range for supply and distribution pipes.
Small pipe, but noisy, erosive and prone to water hammer.
Water hammer, the bang when a valve shuts quickly, grows with velocity. Keep it lower on systems with fast-closing valves.
Before choosing a pipe
Check more than size. Tick each item.
Tick each item as you go.
Plastic pipe ratings fall as temperature rises. A PVC pipe rated for 10 bar at 20°C may only take half that at 50°C.
What you get
Size and performance. Each tile explains one result.
Schedule 40 nominal size.
Minimum and chosen bore.
Real speed in the chosen pipe.
Metres per 100 m of pipe.
In litres per minute.
Head loss per 100 m lets you estimate the loss over your actual length. Multiply by the run in metres and divide by 100.
Frequently Asked Questions
No question matches that search.
How do I calculate pipe size from flow rate?
Divide flow by velocity to get the area, then find the diameter from d = √(4A ÷ π) and round up to a standard size.
What size pipe for 100 litres per minute?
About 1-1/2 in at 1.5 m/s, or 1-1/4 in at 2 m/s.
What velocity should water flow in a pipe?
Usually 1 to 2.5 m/s. Under 1 m/s on pump suction lines.
What is Schedule 40?
A wall thickness standard for steel and PVC pipe. Higher schedules have thicker walls and smaller bores.
How much flow does a 1 inch pipe carry?
About 50 L/min (13 gpm) at 1.5 m/s.
Does pipe length affect size?
It affects pressure loss. Long runs may need a size up to keep pressure at the far end.
What is head loss?
The pressure lost to friction, expressed as metres of water.
The right pipe for the flow
Pick a sensible velocity, work out the diameter and round up to a standard size. Check the head loss over your run and you will have a quiet system that delivers the pressure you need.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.