Calculate the internal water volume held inside a pipe of a given diameter and length.
How to Use the Pipe Volume Calculator
Enter the pipe "Diameter (mm)".
Enter the pipe "Length (m)".
Click "Calculate" to see the internal water volume held inside the pipe.
Worked Example
For the placeholder values, a 25 mm pipe that is 20 m long: the radius is 25 ÷ 2000 = 0.0125 m, so the volume is π × 0.0125² × 20 ≈ 0.009817 m³. Multiplying by 1,000 gives 9.82 litres, which is what the calculator displays.
A quick check without a calculator: the litres held per metre equal the diameter in millimetres squared × 0.000785. For 25 mm that is 625 × 0.000785 ≈ 0.49 litres per metre.
Water Held per Metre of Pipe
Inside diameter
Litres per metre
Litres per 10 m
15 mm
0.177
1.77
20 mm
0.314
3.14
25 mm
0.491
4.91
32 mm
0.804
8.04
40 mm
1.257
12.57
50 mm
1.963
19.63
75 mm
4.418
44.18
100 mm
7.854
78.54
150 mm
17.671
176.71
Volume grows with the square of the diameter, so doubling the bore from 25 mm to 50 mm holds four times as much water per metre.
Practical Uses of the Result
Waiting for hot water: if a geyser is 15 m of 20 mm pipe away from a tap, about 4.71 litres of cooled water sits in the line. At a tap flow of 6 L/min, it takes roughly 47 seconds before hot water arrives, and longer if the pipe itself has to warm up. Moving the geyser closer or using a smaller pipe reduces this wait.
Disinfecting or flushing a line: chlorine dose and flush volume are based on the water the pipe holds. Follow the dosing instructions of the product or your water authority.
Pipe supports and loads: water weighs about 1 kg per litre, so 30 m of 110 mm pipe running full carries roughly 285 kg of water in addition to the pipe's own weight.
Solar and heating circuits: knowing the water in the pipework helps when choosing an expansion vessel or the amount of antifreeze or additive to mix.
Use the inside diameter. Entering the outside diameter of a thick-walled pipe can overstate the volume noticeably, especially for high-pressure pipes with thicker walls.
Frequently Asked Questions
Volume = π × r² × Length, where r is the pipe's internal radius. This gives the internal water-holding volume of a straight pipe run.
It's useful for calculating how much water is needed to fill/flush a system, sizing expansion tanks, or estimating drain-down time and dead-leg volume in hot water systems.
No, it only calculates the volume of the straight pipe length entered. Fittings add a small additional volume not accounted for here.
Divide the litres by 1,000 for cubic metres. For US gallons, divide by about 3.785; for imperial gallons, divide by about 4.546.
Run the calculator once for each diameter with the total length of that size, then add the results. For example, 10 m of 25 mm (4.91 L) plus 15 m of 20 mm (4.71 L) holds about 9.62 litres.