Volume of a cylinder, using the inside bore
A pipe is a cylinder, so its capacity is the cross-section area (π times the inside radius squared) multiplied by the length. Everything must be in one unit: work in inches and divide the cubic inches by 231 for US gallons, or work in centimetres and divide by 1,000 for litres. The number that matters is the inside radius — the wall carries no fluid.
Capacity per foot for Schedule 40 steel
| Nominal size | Inside dia. | Gallons / ft | Litres / m |
|---|---|---|---|
| 1/2" | 0.622" | 0.016 | 0.180 |
| 3/4" | 0.824" | 0.028 | 0.314 |
| 1" | 1.049" | 0.041 | 0.488 |
| 1½" | 1.610" | 0.076 | 0.899 |
| 2" | 2.067" | 0.147 | 1.714 |
| 3" | 3.068" | 0.292 | 3.456 |
| 4" | 4.026" | 0.524 | 6.194 |
Multiply the per-foot figure by your run length. A 100 ft run of 2" Sch 40 holds about 14.7 gallons.
Two things that trip people up
- Nominal size is a label, not the bore. "2 inch" pipe (NPS 2) is 2.375" on the outside and 2.067" inside for Schedule 40. Don't plug the nominal number into the formula.
- Volume scales with the square of the diameter. Double the bore and capacity quadruples, so a small error in inside diameter grows fast over a long run.
Same nominal size, thicker wall: the higher schedule holds about 12% less because the bore is narrower.
Common questions
How do I calculate the volume of a pipe?
Use V = pi x r squared x L, where r is the inside radius and L is the length in the same units. In inches, divide the result by 231 to get US gallons.
Do I use the inside or outside diameter?
Always the inside diameter, because that is the space the fluid actually fills. If you only know the outside diameter and wall thickness, the inside diameter is outside minus twice the wall thickness.
Does a higher schedule pipe hold more?
No, less. A higher schedule means a thicker wall, which narrows the inside bore. Schedule 80 holds about 12 percent less than Schedule 40 at the same nominal size.
