Enter the measured inside diameter in inches and the straight-run length in feet. Use product dimensions for the actual material and wall specification, and split a run into separate calculations when its bore changes. Fittings and valves require their own internal volumes if the fill quantity must include them.
The cylinder model assumes a full straight pipe. Fittings, valves and branches add internal volume that the length-and-bore calculation excludes. Trapped air and fluid expansion affect the actual fill requirement.
A volume estimate does not select freeze protection, pipe material or a hydronic-system design.
Pipe volume (gallons) from pipe diameter
| Pipe diameter | 2 |
|---|---|
| Pipe length | 100 |
| Pipe volume (gallons) | 16.3 |
| Pipe volume (cubic feet) | 2.2 |
Common error: Using the nominal size as the inside diameter. The error is squared in the volume, so a small difference in diameter becomes a larger difference in gallons.
Pipe Volume: formula
Pipe Volume: changing Pipe diameter
| Pipe diameter | Pipe volume (gallons) | Pipe volume (cubic feet) |
|---|---|---|
| 1.6 | 10.4 | 1.4 |
| 1.8 | 13.2 | 1.8 |
| 2 (example) | 16.3 | 2.2 |
| 2.2 | 19.7 | 2.6 |
| 2.4 | 23.5 | 3.1 |
Pipe Volume: compare input ranges
| Input | Tested from | To | Pipe volume (gallons): low input | Pipe volume (gallons): high input | Swing |
|---|---|---|---|---|---|
| Pipe diameter | 1.8 | 2.2 | 13.2 | 19.7 | 6.5 (40%) |
| Pipe length | 90 | 110 | 14.7 | 18 | 3.3 (20%) |
Pipe Volume: individual input comparisons
| Input | Value | Pipe volume (gallons) | Pipe volume (cubic feet) |
|---|---|---|---|
| Pipe diameter | 1.6 | 10.4 | 1.4 |
| 1.8 | 13.2 | 1.8 | |
| 2 | 16.3 | 2.2 | |
| 2.2 | 19.7 | 2.6 | |
| 2.4 | 23.5 | 3.1 | |
| Pipe length | 80 | 13.1 | 1.7 |
| 90 | 14.7 | 2 | |
| 100 | 16.3 | 2.2 | |
| 110 | 18 | 2.4 | |
| 120 | 19.6 | 2.6 |
Pipe volume (gallons): Pipe diameter against Pipe length
| Pipe diameter / Pipe length | 80 | 100 | 120 |
|---|---|---|---|
| 1.6 | 8.4 | 10.4 | 12.5 |
| 1.8 | 10.6 | 13.2 | 15.9 |
| 2 | 13.1 | 16.3 | 19.6 |
| 2.2 | 15.8 | 19.7 | 23.7 |
| 2.4 | 18.8 | 23.5 | 28.2 |
Pipe Volume: values and definitions
| Quantity and definition | Value used |
|---|---|
| Pipe diameter — Inside diameter of the pipe, in inches. | 2 |
| Pipe length — Length of the pipe run, in feet. | 100 |
| Pipe volume (gallons) | 16.3 |
| Pipe volume (cubic feet) | 2.2 |
Pipe diameter: half and double
| Pipe diameter | Pipe volume (gallons) |
|---|---|
| 1 | 4.1 |
| 2 | 16.3 |
| 4 | 65.3 |
The water a pipe holds grows with the square of its diameter, so a modest change in size changes the volume a lot. One hundred feet of 2 inch pipe holds about 16.3 gallons, and the same run in 4 inch pipe holds about 65.3, four times as much. That matters for how long hot water takes to arrive, how much to drain before a repair and how much fluid a closed loop needs.
Use the inside diameter, which is smaller than the nominal size.
Look up the actual inside diameter for your pipe material and schedule. Copper, PVC and pex with the same nominal size have different inside diameters.
See flow through the pipe on the flow rate calculator, pressure loss on the pipe friction calculator and slope on the drain slope page. The cylinder volume and volume converter calculators handle the geometry and units, and the water heater sizing page covers hot water supply.
Why does a larger pipe take longer to run hot?
Because it holds much more cold water that must be flushed first, in proportion to the square of the diameter.
How many gallons are in 100 feet of 2-inch pipe?
For an assumed actual inside diameter of exactly 2 inches and 100-foot length, π × 1² × 1200/231 = about 16.32 US gallons.
Should I use the nominal or actual pipe diameter?
Use the actual inside diameter, since the nominal size is only a name and the real bore differs by material and wall thickness.