Use the free Pipe Volume Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Anyone who has drained a heating system, filled a long garden pipe, or dosed a water treatment system has faced the same question: how much water is actually inside this pipe? Guessing is risky. Too little treatment chemical does nothing, too much can damage the system, and draining a long run takes far longer than people expect. The Pipe Volume Calculator answers the question exactly. You enter the inside diameter of the pipe and its length, and it tells you the water volume in litres, US gallons, and cubic metres.
Using the tool is simple, but one detail matters more than any other: use the inside diameter, not the outside diameter. Pipe sizes are often sold by their nominal size, which is not the same as the bore you can measure. If you can, measure the inside of the pipe directly, or look up the bore for your pipe type and schedule. Enter that figure in inches, then enter the total length in feet, including bends and vertical runs, because every foot of pipe holds water. The tool does the rest in a second.
The maths is the volume of a cylinder. The tool halves the inside diameter to get the radius, converts inches and feet into metres, and multiplies pi times the radius squared times the length. That gives cubic metres, which it converts into litres by multiplying by 1,000, and into US gallons by dividing by 3.785. A one-inch pipe that is 100 feet long holds about 15.4 litres, or just over 4 gallons. Double the diameter, and the volume quadruples, because volume grows with the square of the radius, not in a straight line.
That square relationship catches many people out. A small increase in pipe size makes a big difference to the water inside, and to the weight of a full pipe. Water weighs one kilogram per litre, so a long run of large pipe can weigh hundreds of kilograms when full, which matters for supports and fixings. The same maths matters when you winterise a system, flush a new installation, or work out how long a drain-down will take. Knowing the volume turns a vague job into a timed, planned one.
This tool is an educational estimator only, not professional plumbing advice. Its results are estimates based on the numbers you enter, and they assume a straight pipe of constant diameter. Real systems have fittings, valves, bends, and sections of different sizes, so the true volume can differ. Check your local plumbing codes, and consult a licensed plumber before any real installation, repair, or change to a pressurised system. Never open a pressurised pipe, and never guess at the pressure inside a system you do not understand.
To get the best result, break a mixed system into sections. Work out each section separately, one diameter at a time, and add the volumes together. Include vertical runs, because a pipe that goes up two floors holds just as much water as one that runs sideways. If you are dosing chemicals, use the total volume and follow the product label exactly, because overdosing is just as harmful as underdosing. Write the volume on the system or in your notes, so the next person does not have to measure it all again.
The Pipe Volume Calculator is free, it works entirely in your browser, and your numbers never leave your device. Use it before you drain, fill, dose, or plan any pipe job, and you will avoid the most common mistake in pipe work: guessing. Measure the bore, measure the length, and let the tool do the arithmetic. A two-minute calculation now can save an hour of mess, a wasted chemical dose, or a support that was never built to carry the weight of the water inside.
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