What the result means
A bucket test is the fastest field check: time how long a fixture takes to fill a known container. The velocity method is useful when you know or have measured velocity in a pipe.
Use flow to check a well pump, confirm a tankless water heater will get its minimum activation flow, or show a homeowner why a fixture is weak.
How it's calculated
- From velocity: Q = V × d² ÷ 0.4085
- Bucket test: Q = gallons ÷ seconds × 60
V in ft/s, d = inside diameter in inches, Q in GPM.
Worked example
Water moving at 5 ft/s in 1 in Type L copper (ID 1.025 in).
- Q = 5 × 1.025² ÷ 0.4085
- Q = 12.86 GPM
The line is carrying about 12.9 GPM.
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 | Inside diameter | At 3 ft/s | At 5 ft/s | At 8 ft/s |
|---|---|---|---|---|
| 1/2 | 0.545 in | 2.18 GPM | 3.64 GPM | 5.82 GPM |
| 3/4 | 0.785 in | 4.53 GPM | 7.54 GPM | 12.07 GPM |
| 1 | 1.025 in | 7.72 GPM | 12.86 GPM | 20.58 GPM |
| 1-1/4 | 1.265 in | 11.75 GPM | 19.59 GPM | 31.34 GPM |
| 1-1/2 | 1.505 in | 16.63 GPM | 27.72 GPM | 44.36 GPM |
| 2 | 1.985 in | 28.94 GPM | 48.23 GPM | 77.16 GPM |
| 2-1/2 | 2.465 in | 44.62 GPM | 74.37 GPM | 119.00 GPM |
| 3 | 2.945 in | 63.69 GPM | 106.16 GPM | 169.85 GPM |
Reference values are examples based on the stated assumptions. Verify manufacturer data, conductor ampacity and local requirements before using a result.
Sources and assumptions
V in ft/s, d = inside diameter in inches, Q in GPM. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
What container should I use?
A 5-gallon bucket with a marked line gives better accuracy than a small container.
Should the fixture be fully open?
Yes, for maximum flow. Aerators and flow restrictors will limit the result.
How do I measure velocity?
With an ultrasonic or insertion flow meter. Otherwise use the bucket test.
Why does the pipe ID matter so much?
Flow scales with the square of diameter, so a small ID change makes a big difference.
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.