OneTaskBy Tim King · Updated
Pipe Volume Calculator
Litres and Gallons of Water in a Pipe
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How the Pipe Volume Calculator works
This pipe volume calculator works out how much water a pipe holds. Enter the internal diameter and the length, in millimetres and metres or inches and feet, and it gives the volume in litres, US gallons and UK gallons, the weight of that water, and the litres per metre. It is handy for dosing inhibitor into heating systems or flushing a new line.
A pipe is a long cylinder, so its volume is the area of the circle inside it times its length. Use the internal (bore) diameter, not the outside size the pipe is sold by: a 15 mm copper pipe has a bore of about 13.6 mm, and a 22 mm pipe about 20.2 mm.
How the pipe volume calculator works
The formula is volume = π × (d ÷ 2)² × length. For example, 10 m of pipe with a 15 mm bore holds 1.77 litres (0.47 US gal), or 0.18 litres per metre. A second example: 50 ft of 3/4 inch pipe holds 4.34 litres, which is 1.15 US gallons and weighs about 4.3 kg.
A litre of water weighs a kilogram, which lets you check how many kilograms a full pipe run adds to a support. To total a whole heating system, add the pipe runs to the radiator and boiler volumes, or measure what drains out.
Volume of water per metre and per 10 m of pipe by internal diameter
| Internal diameter | Litres per metre | Litres per 10 m | US gallons per 10 m |
|---|---|---|---|
| 6 mm (0.24 in) | 0.028 | 0.28 | 0.07 |
| 8 mm (0.31 in) | 0.050 | 0.50 | 0.13 |
| 10 mm (0.39 in) | 0.079 | 0.79 | 0.21 |
| 12 mm (0.47 in) | 0.113 | 1.13 | 0.30 |
| 13 mm (0.51 in) | 0.133 | 1.33 | 0.35 |
| 15 mm (0.59 in) | 0.177 | 1.77 | 0.47 |
| 19 mm (0.75 in) | 0.284 | 2.84 | 0.75 |
| 20 mm (0.79 in) | 0.314 | 3.14 | 0.83 |
| 22 mm (0.87 in) | 0.380 | 3.80 | 1.00 |
| 25 mm (0.98 in) | 0.491 | 4.91 | 1.30 |
| 28 mm (1.10 in) | 0.616 | 6.16 | 1.63 |
| 32 mm (1.26 in) | 0.804 | 8.04 | 2.12 |
| 35 mm (1.38 in) | 0.962 | 9.62 | 2.54 |
| 40 mm (1.57 in) | 1.257 | 12.57 | 3.32 |
| 42 mm (1.65 in) | 1.385 | 13.85 | 3.66 |
| 50 mm (1.97 in) | 1.963 | 19.63 | 5.19 |
| 54 mm (2.13 in) | 2.290 | 22.90 | 6.05 |
| 63 mm (2.48 in) | 3.117 | 31.17 | 8.23 |
| 75 mm (2.95 in) | 4.418 | 44.18 | 11.67 |
| 90 mm (3.54 in) | 6.362 | 63.62 | 16.81 |
| 100 mm (3.94 in) | 7.854 | 78.54 | 20.75 |
| 110 mm (4.33 in) | 9.503 | 95.03 | 25.11 |
| 150 mm (5.91 in) | 17.671 | 176.71 | 46.68 |
| 200 mm (7.87 in) | 31.416 | 314.16 | 82.99 |
FAQ
How much water is in a 15 mm pipe?
Using the nominal 15 mm, there are about 0.18 litres per metre. Real 15 mm copper has a 13.6 mm bore, which holds about 0.145 litres per metre, so 20 m of it holds roughly 2.9 litres. Use the real internal diameter for accurate results.
How do I calculate the volume of a pipe in gallons?
Work out cubic inches with π × r² × length in inches, then divide by 231 for US gallons. Or use litres and divide by 3.785 for US gallons or 4.546 for UK gallons. The calculator does all three conversions at once.
Why use the internal diameter?
The water fills only the bore of the pipe, so the wall thickness does not count. Pipes are sold by nominal or outside size, and the wall can take a large share of that on small plastic pipe, so using the outside size overstates the volume.
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