What the result means
Velocity limits protect against noise, water hammer and erosion, especially in copper hot water lines. The calculator finds the inside diameter needed and picks the smallest listed size that meets it.
This is a velocity check only. Real sizing also considers fixture units, available pressure, friction loss and your local plumbing code.
How it's calculated
- Required ID = √(0.4085 × Q ÷ Vmax)
- Pick the smallest size with ID ≥ required
Q in GPM, Vmax in ft/s. Default 8 ft/s cold and 5 ft/s hot are common guidance, not code.
Worked example
10 GPM of hot water in Type L copper at 5 ft/s maximum.
- Required ID = √(0.4085 × 10 ÷ 5) = 0.904 in
- 3/4 in Type L = 0.785 in (too small)
- 1 in Type L = 1.025 in
Use 1 in Type L copper.
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 | Approx. max flow |
|---|---|---|
| 1/2 | 0.545 in | 3.64 GPM |
| 3/4 | 0.785 in | 7.54 GPM |
| 1 | 1.025 in | 12.86 GPM |
| 1-1/4 | 1.265 in | 19.59 GPM |
| 1-1/2 | 1.505 in | 27.72 GPM |
| 2 | 1.985 in | 48.23 GPM |
| 2-1/2 | 2.465 in | 74.37 GPM |
| 3 | 2.945 in | 106.16 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
Q in GPM, Vmax in ft/s. Default 8 ft/s cold and 5 ft/s hot are common guidance, not code. 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
Why lower velocity for hot water?
Hot water is more aggressive to copper, so lower velocity reduces erosion corrosion.
Does this replace fixture unit sizing?
No. Code sizing methods use fixture units and pressure. This is a velocity check.
Why does PEX need a bigger size?
PEX has a thicker wall, so its inside diameter is smaller than copper of the same nominal size.
Can I change the velocity limit?
Yes. The field is editable. Some manufacturers publish their own limits.
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.