What the result means
Water loses pressure as it rubs against the pipe wall. Loss rises quickly with flow and drops sharply as diameter increases, so one size up can cut friction by more than half.
Compare total loss with your available pressure to check whether a fixture at the end of a long run will get enough. Velocity is shown too, since high velocity causes noise and erosion.
How it's calculated
- psi per ft = 4.52 × Q^1.852 ÷ (C^1.852 × d^4.8704)
- Head loss (ft) = psi × 2.31
- Velocity (ft/s) = 0.4085 × Q ÷ d²
Q in GPM, d = inside diameter in inches. Default C: copper 140, PVC/CPVC 150, PEX 150, new steel 120, old steel or cast iron 100.
Worked example
10 GPM through 3/4 in Type L copper (ID 0.785 in) with C = 150.
- psi/ft = 4.52 × 10^1.852 ÷ (150^1.852 × 0.785^4.8704)
- = 9.75 psi per 100 ft
- Velocity = 0.4085 × 10 ÷ 0.785² = 6.63 ft/s
Over a 50 ft run that's about 4.9 psi lost before fittings.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Nominal size | 2 GPM | 4 GPM | 6 GPM | 8 GPM | 10 GPM | 15 GPM | 20 GPM |
|---|---|---|---|---|---|---|---|
| 1/2 | 2.93 | 10.57 | 22.39 | 38.14 | 57.66 | 122.18 | 208.15 |
| 3/4 | 0.49 | 1.79 | 3.79 | 6.45 | 9.75 | 20.66 | 35.20 |
| 1 | 0.13 | 0.49 | 1.03 | 1.76 | 2.66 | 5.64 | 9.60 |
| 1-1/4 | 0.05 | 0.17 | 0.37 | 0.63 | 0.95 | 2.02 | 3.45 |
| 1-1/2 | 0.02 | 0.08 | 0.16 | 0.27 | 0.41 | 0.87 | 1.48 |
| 2 | 0.01 | 0.02 | 0.04 | 0.07 | 0.11 | 0.23 | 0.38 |
† Gray cells exceed 8 ft/s. The 3/4 in, 10 GPM check uses C = 150, giving 9.75 psi/100 ft. Default calculator C remains 140.
Sources and assumptions
Q in GPM, d = inside diameter in inches. Default C: copper 140, PVC/CPVC 150, PEX 150, new steel 120, old steel or cast iron 100. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Is Hazen-Williams accurate for hot water?
It's calibrated for water around 60 °F. It's commonly used for hot water too, with slightly conservative results.
How do I include fittings?
Add their equivalent length (from manufacturer or code tables) to the pipe length.
Why is my C factor editable?
Older pipe gets rougher. Lower C for scaled or corroded pipe.
What's a reasonable velocity?
Common guidance is up to 8 ft/s for cold and 5 ft/s for hot copper. Check your local code and manufacturer.
Does elevation count?
Not here. Subtract 0.433 psi per foot of rise separately.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a size is nominal or an inside diameter and, where a field asks for a length, whether it includes the equivalent length of fittings. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different pipe material, water temperature, fitting count or installation method can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
TierMenu provides these free calculators without an email gate. The results stay in this browser unless you choose to copy a shareable URL or print the page. A shared URL includes the calculator inputs in its query string; do not put personal or sensitive information in calculator fields.