What the result means
Pipe volume matters when you're filling a hydronic loop with glycol, sizing an expansion tank, flushing a line or dosing disinfectant. It's also how long you wait for hot water at a far fixture.
The built-in tables use actual inside diameters, not nominal sizes, which is why 1 in Type L copper holds more than 1 in PEX.
How it's calculated
- Gallons per foot = 0.0408 × ID²
- Total gallons = gallons per foot × length
- Liters = gallons × 3.785
ID in inches. 0.0408 ≈ π ÷ 4 × 12 ÷ 231.
Worked example
100 ft of pipe with a 1.000 in inside diameter.
- Gallons per foot = 0.0408 × 1.000² = 0.0408
- Total = 0.0408 × 100 = 4.08 gallons
Plan for about 4.1 gallons plus equipment and 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 | Inside diameter | Gallons per ft | Gallons per 100 ft |
|---|---|---|---|
| 1/2 | 0.545 in | 0.0121 | 1.21 |
| 3/4 | 0.785 in | 0.0251 | 2.51 |
| 1 | 1.025 in | 0.0429 | 4.29 |
| 1-1/4 | 1.265 in | 0.0653 | 6.53 |
| 1-1/2 | 1.505 in | 0.0924 | 9.24 |
| 2 | 1.985 in | 0.1608 | 16.08 |
| 2-1/2 | 2.465 in | 0.2479 | 24.79 |
| 3 | 2.945 in | 0.3539 | 35.39 |
Reference values are examples based on the stated assumptions. Verify manufacturer data, conductor ampacity and local requirements before using a result.
Sources and assumptions
ID in inches. 0.0408 ≈ π ÷ 4 × 12 ÷ 231. 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 are PEX values approximate?
PEX wall thickness varies by manufacturer. The table uses typical CTS SDR-9 values.
Does this include the boiler or water heater?
No. Add equipment volume from the manufacturer data.
How much does the water weigh?
About 8.34 lb per gallon, which the calculator shows for support planning.
How long until hot water arrives?
Divide the pipe volume by fixture flow in GPM for a rough minimum wait time in minutes.
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.
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