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Work out how much water is in a run of pipe. Choose copper, plastic, MDPE or your own bore, enter the length of each size, and press Calculate to see the litres in each size and in total, the litres per metre and, for central heating, the inhibitor to add.
Water In The Pipes
0litres
Check the box above. The result updates once it looks right.
| Pipe Size | Bore | Litres Per Metre | Your Litres |
|---|
Water in one metre of each size of this pipe, worked out from the bore.
You get the litres in each pipe size, the total in the pipes, the weight of the water and, for central heating, the whole system volume and the inhibitor dose. A drawing shows the pipes to scale, a table lists the litres per metre for every size, and you can copy the result or save it as a PDF.
The water in a pipe is a cylinder, so its volume is the area of the bore times the length. With the bore in millimetres and the length in metres, the answer in litres is:
Litres = π × bore² ÷ 4,000 × length
The bore is the internal diameter, the outside size less two wall thicknesses. UK pipes are named by their outside diameter, so a 15 mm copper pipe has a bore of about 13.6 mm, and a 15 mm plastic pipe, with its thicker wall, about 11.3 mm. Using the outside size would over-estimate the water by around 20% for copper and over 75% for plastic.
Copper tube for plumbing and heating is made to EN 1057, and the usual wall thickness in the UK is still called Table X, after the old British Standard. The Engineering ToolBox EN 1057 table lists every size and wall:
| Size | Wall | Bore | Litres Per Metre |
|---|---|---|---|
| 10 mm | 0.6 mm | 8.8 mm | 0.061 |
| 15 mm | 0.7 mm | 13.6 mm | 0.145 |
| 22 mm | 0.9 mm | 20.2 mm | 0.320 |
| 28 mm | 0.9 mm | 26.2 mm | 0.539 |
| 35 mm | 1.2 mm | 32.6 mm | 0.835 |
| 42 mm | 1.2 mm | 39.6 mm | 1.232 |
A handy rule: 15 mm copper holds about 0.15 litres a metre, so 7 metres hold about a litre, and 22 mm holds about a third of a litre a metre. Thicker Table Y tube holds slightly less.
Plastic push-fit pipe, such as polybutylene, has a thicker wall than copper, so the same nominal size holds less water. The figures below use typical polybutylene walls of 1.65 to 2.75 mm; PEX and other brands vary a little. Blue MDPE pipe for underground water supplies has an even thicker wall:
| Pipe | Bore | Litres Per Metre |
|---|---|---|
| 10 mm push-fit | 6.7 mm | 0.035 |
| 15 mm push-fit | 11.3 mm | 0.100 |
| 22 mm push-fit | 17.7 mm | 0.246 |
| 28 mm push-fit | 22.5 mm | 0.398 |
| 20 mm MDPE | 15.2 mm | 0.182 |
| 25 mm MDPE | 20.1 mm | 0.317 |
| 32 mm MDPE | 25.8 mm | 0.523 |
For steel, PVC, drainage or any other pipe, choose Other and enter the bore from the maker's data sheet, or measure the inside of a cut end.
The pipes are only part of a heating system. Most of the water sits in the radiators, and the boiler heat exchanger and any hot water cylinder coil add more. The radiator maker's data sheet gives the water content of each radiator, usually a few litres for a single panel and more for doubles. Add those figures in the radiators box to get the whole system volume.
Knowing the volume matters when you size an expansion vessel, add inhibitor or cleaner, or plan a drain-down. For a heat pump, the installer also checks the system holds enough water for the unit to run without short cycling; our U-value calculator helps with the heat loss side of a heat pump design.
Central heating inhibitor protects the boiler, pipes and radiators from corrosion and scale. Standard inhibitors, such as Sentinel X100, are dosed at 1 litre per 100 litres of system water, a 1% dose, which suits a typical home with 8 to 10 radiators. Concentrated versions do the same job with a smaller bottle, so always follow the dose on the label.
Clean and flush the system before dosing an old or sludgy system, and check the strength with a test kit after any work that drains and refills it.
Every time you open a hot tap, the cooled water sitting in the pipe from the cylinder or boiler runs to waste before hot water arrives. Ten metres of 22 mm copper hold over 3 litres, so long hot water runs waste water and energy. Keep them short, use 15 mm where the flow allows, and lag them.
When flushing or disinfecting a new pipe run, plumbers usually draw off several times the pipe volume so all the water inside is replaced. The calculator shows three times the volume as a guide.
A central heating system with 40 m of 15 mm copper and 12 m of 22 mm copper, and radiators and a boiler holding about 70 litres:
For each pipe size you enter, the calculator takes the bore, works out the litres per metre as π × bore² ÷ 4,000 and multiplies by the length. It adds the sizes together for the water in the pipes and, for central heating, adds the radiator and boiler water for the whole system. The inhibitor is 1% of the system water and the weight is about 1 kg per litre. Copper bores use EN 1057 Table X walls, plastic bores use polybutylene push-fit walls and MDPE bores are average figures for SDR11 pipe. Fittings, valves and bends add a little water that is not included.
Multiply π by the bore in millimetres squared, divide by 4,000 for litres per metre, then multiply by the length in metres.
About 0.145 litres, from a 13.6 mm bore. Seven metres hold about one litre.
About 0.32 litres, from a 20.2 mm bore.
Yes. Its wall is thicker, so 15 mm push-fit pipe has a bore of about 11.3 mm and holds about 0.10 litres a metre.
A typical home with 8 to 10 radiators holds about 100 litres, most of it in the radiators. Add the radiator data sheet figures to the pipe volume.
Standard inhibitor is 1 litre per 100 litres of system water. Concentrates use less, so check the label.
The inside diameter, the bore. UK pipe sizes are outside diameters, so take off two wall thicknesses.
About 0.32 litres a metre, from an average bore of about 20 mm.
Copper bores from EN 1057 Table X wall thicknesses, plastic from polybutylene push-fit wall thicknesses, MDPE from average SDR11 bores. Inhibitor dose for standard 1% products. Reviewed November 2026.