Pipe Flow Calculator
Pipe flow rate, velocity and Hazen–Williams head loss for water.
About this tool
Enter the pipe's inside diameter in millimetres and either the water velocity or the flow rate. The tool gives flow in L/s, m³/h and US gallons per minute, the velocity, and the friction loss over a length of pipe for the chosen material.
Flow, velocity and friction loss
For a full pipe, flow = area × velocity, where area = π × (diameter ÷ 2)². A 50 mm inside-diameter pipe with water moving at 1.5 m/s carries about 2.95 L/s, which is 10.6 m³/h or 46.7 US gpm. Pipes are sized by keeping velocity sensible: supply lines are usually designed for about 1 to 2 m/s, and above roughly 3 m/s you get noise, water hammer and erosion. The friction loss uses the Hazen–Williams equation for water, h = 10.67 × L × Q1.852 ÷ (C1.852 × d4.87), where C reflects pipe roughness: about 140–150 for plastic, 130 for copper and 100 for old steel. The example loses about 5 m of head (about 49 kPa) per 100 m of PVC. Use the actual inside diameter, as wall thickness varies with pipe class. Honest limits: Hazen–Williams is for water at ordinary temperatures in full pipes; fittings and valves add more loss. Not engineering advice. The CFM calculator and the pool volume calculator are related.
Frequently asked questions
How do I calculate flow rate in a pipe?
Multiply the pipe's internal cross-sectional area by the fluid velocity. In SI, m² × m/s gives m³/s.
What velocity should water flow at in a pipe?
Usually 1 to 2 m/s for supply pipes, staying below about 3 m/s to avoid noise and water hammer.
What is the Hazen-Williams C factor?
A roughness coefficient: about 140 to 150 for PVC, 130 to 140 for copper and around 100 for old steel or cast iron.
How do I convert L/s to gpm?
Multiply litres per second by 15.85 to get US gallons per minute.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.