How this calculator works
The water in a pipe is the volume of a cylinder: π times the radius squared times the length. Work in feet, with the diameter converted from inches, and convert cubic feet to gallons at 7.48 per cubic foot. The inside diameter is what counts.
Worked example: 100 ft of 4 inch pipe holds 65.3 gallons (65.3 per 100 ft, or 0.653 gallons per foot) and the water weighs 545 lb. The same length of 1/2 inch pipe holds 1.02 gallons, and 6 inch pipe 147 gallons.
Gallons per 100 feet of pipe
By inside diameter.
| Inside diameter | Gallons per 100 ft | Gallons per foot | Liters per 100 ft |
|---|---|---|---|
| 0.5 in | 1.02 | 0.0102 | 3.9 |
| 0.75 in | 2.29 | 0.0229 | 8.7 |
| 1 in | 4.08 | 0.0408 | 15.4 |
| 1.5 in | 9.18 | 0.0918 | 34.7 |
| 2 in | 16.32 | 0.1632 | 61.8 |
| 3 in | 36.72 | 0.3672 | 139.0 |
| 4 in | 65.28 | 0.6528 | 247.1 |
| 6 in | 146.88 | 1.4688 | 555.9 |
| 8 in | 261.12 | 2.6112 | 988.3 |
Tips before you buy
- The volume goes up with the square of the diameter: doubling the pipe size holds four times the water.
- For a long run of hot water pipe, the volume tells you how much cold water you throw away before it gets hot.
- Fill a new line slowly with the end open, to let the air out.
- Check the real inside diameter of the pipe: schedule 40, schedule 80, copper and PEX all differ.
- The weight of water in a big pipe is significant: allow for it in the supports.
Tools & materials for this project
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Frequently asked questions
How many gallons are in 100 feet of pipe?
It depends on the inside diameter: 65.3 gallons for 4 inch pipe, 147 gallons for 6 inch and 1.02 gallons for 1/2 inch.
How do I calculate the volume of a pipe?
π × radius² × length. For 4 inch pipe, 100 ft is 8.73 cubic feet, or 65.3 gallons.
How much water is in a foot of 4 inch pipe?
0.653 gallons, or about 2.47 liters, weighing 5.4 lb.
Why does the inside diameter matter?
The wall takes up space, so the real diameter of the water is smaller than the nominal pipe size. Use the inside diameter of the pipe you have.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.