Pipe volume calculator
Updated
Choose the pipe type and size and enter the length. The calculator uses the pipe's actual inside diameter, not the nominal size, and shows the gallons and the weight of the water.
Your pipe
How the calculator works
A pipe is a cylinder, so its volume is the inside cross-section area times the length:
- Area = π ÷ 4 x ID², in square inches, where ID is the inside diameter.
- Volume = area x length in feet x 12, in cubic inches.
- Gallons = cubic inches ÷ 231. Combined, that is about 0.0408 x ID² x feet.
Nominal size is not the inside diameter. A 3/4 inch Type L copper tube is 0.785 inch inside, while 3/4 inch PEX is only 0.681 inch, so the same length of PEX holds about 25% less water. The IDs come from the dimension tables in ASTM B88 for copper tube, ASTM F876 for PEX and ASTM D1785 for Schedule 40 PVC (average outside diameter minus twice the minimum wall).
| Nominal size | Copper L ID | PEX ID | Sch 40 ID |
|---|---|---|---|
| 1/2 in | 0.545 | 0.485 | 0.622 |
| 3/4 in | 0.785 | 0.681 | 0.824 |
| 1 in | 1.025 | 0.875 | 1.049 |
Worked example
100 feet of 3/4 inch Type L copper: π ÷ 4 x 0.785² = 0.484 sq in. Times 1,200 inches = 580.8 cu in. Divided by 231 = 2.51 gallons, which weighs about 21 lb.
Why it matters
Pipe volume tells you how much cold water has to run out of a hot line before hot water arrives, how much antifreeze or sanitizer a system needs, and how much water a line holds when you drain it. At 2 gallons per minute, that 2.51 gallon run takes about 75 seconds to clear.
Limits
Fittings, valves and the water heater are not included. Wall thickness tolerances mean real pipe can vary a little from the table.