Why use this calculator
Knowing the flow rate through a pipe is critical for plumbing design, ensuring adequate water supply to fixtures, and avoiding issues like water hammer or insufficient pressure. This calculator uses the industry-standard Hazen-Williams equation to estimate flow based on pipe size, length, and available pressure, helping plumbers and homeowners size pipes correctly.
How to use it
Select your preferred unit system, then enter the internal pipe diameter, the total pipe length, and the available water pressure. The calculator uses the Hazen-Williams formula with a C factor of 130 (typical for copper or PEX pipe) to estimate the flow rate and water velocity. In imperial mode, enter diameter in inches, length in feet, and pressure in PSI. In metric mode, enter diameter in mm, length in metres, and pressure in kPa.
The formula
Q = 0.2083 x C x d^2.63 x S^0.54, where Q is flow in GPM, C is the Hazen-Williams roughness coefficient (130 for copper/PEX), d is internal pipe diameter in inches, and S is the hydraulic slope (pressure head in feet / pipe length in feet). Pressure head = PSI x 2.31 ft/PSI.
Worked examples
1-inch copper pipe, 100 feet long, at 40 PSI
Approximately 9.5 GPM flow rate at 3.9 ft/s velocity
0.75-inch PEX pipe, 50 feet long, at 60 PSI
Approximately 7.8 GPM with higher velocity due to smaller diameter
When people use it
- Sizing water supply pipes for residential plumbing
- Verifying adequate flow to showers, faucets, and appliances
- Planning irrigation system pipe layouts
- Troubleshooting low water pressure issues
Tips
- Keep water velocity below 8 ft/s to prevent noise and pipe erosion; ideally between 3-5 ft/s for residential systems
- Fittings, valves, and elbows add equivalent pipe length and reduce flow. Add 10-30% to your pipe length to account for fittings
- Higher C values indicate smoother pipes. New copper is around 140, PVC is 150, while older galvanized pipe can drop to 100 or lower
Questions people ask
- What is the Hazen-Williams C factor?
- The C factor represents the internal roughness of the pipe. Smoother pipes have higher C values and allow more flow. Common values: PVC = 150, copper = 130-140, cast iron = 100, galvanized steel = 120 when new but degrades over time.
- Why is water velocity important?
- Excessive velocity (above 8 ft/s) can cause noise, water hammer, and accelerated pipe erosion. Very low velocity can lead to sediment buildup. The ideal range for residential plumbing is 3-5 ft/s.
- How does pipe length affect flow rate?
- Longer pipes create more friction, reducing the available pressure at the outlet and therefore reducing flow rate. Doubling the pipe length does not halve the flow, but it does reduce it significantly due to the exponential relationship in the formula.