Pipe Volume Calculator

Volume = π × (inner diameter ÷ 2)² × length

d

Inner diameter of the pipe

L

Length of the pipe run

Q

Flow rate, in liters per minute

Use it for the volume of the pipe itself: draining a line down, filling a heating loop, working out how much water a repair will release. If you want the volume of water moving through the pipe per minute, that is a flow rate question rather than a capacity one.

1. A 20 mm bore pipe, 10 m long, on a 20 L/min supply

   1. Radius in meters: 20 mm ÷ 2 = 0.01 m
   2. Cross-section: π × 0.01² = 0.000314 m²
   3. Volume: 0.000314 × 10 = 0.00314 m³, which is 3.14 L
   4. Water is 1 kg per liter, so it weighs 3.14 kg
   5. Fill time: 3.14 ÷ 20 = 0.157 minutes = 9.4 seconds

   0.83 US gallons. Under four liters is the whole reason a short run of small pipe barely matters when you drain a system down.

2. The same pipe over a 100 m run

   1. One meter of this pipe holds 0.3142 L
   2. Multiply by the length: 0.3142 × 100 = 31.42 L
   3. Water weight: 31.42 kg — no longer something to ignore
   4. Fill time: 31.42 ÷ 20 = 1.571 minutes, which is 1 minute 34 seconds

   Volume goes up in a straight line with length but with the square of the diameter: doubling the bore to 40 mm quadruples this to 125.7 L.

This is the volume of the pipe bore and nothing else. Fittings, valves, a manifold, the cylinder or boiler at the end of the run and the water already in the rest of the system are all additional. Nominal pipe sizes are labels rather than measurements, so use an inner diameter you have measured or looked up for the actual material. The fill time assumes the flow rate you typed in holds throughout, which it will not if the run is long, the pipe is narrow, or a valve is partly closed.

How do I calculate the volume of a pipe?

Halve the inner diameter to get the radius, then use the cylinder formula π × r² × length. A 20 mm bore, 10 m long, is π × 0.01² × 10 = 0.00314 m³ — 3.14 liters. Work entirely in meters and the answer arrives in cubic meters, which converts to liters by multiplying by 1,000.

Should I use the inner or the outer diameter?

The inner diameter, always — that is the space the water actually occupies. A pipe called half inch has an outside diameter near 21 mm in some materials and under 16 mm in others, and the bore can range from 12 to 18 mm depending on wall thickness. Using the outside diameter overstates the volume of a small pipe by 40% or more.

How many liters are in a meter of pipe?

It depends entirely on the bore: 15 mm holds 0.177 L per meter, 20 mm holds 0.314, 25 mm holds 0.491 and 50 mm holds 1.963. Because the radius is squared, doubling the diameter quadruples the volume in a meter of pipe.

Why is the fill time only an estimate?

Because the real flow rate falls as pressure is lost to friction along the pipe and through fittings. A supply that delivers 20 liters per minute at the tap delivers less at the end of a long, narrow run. The figure here is the time at the rate you enter, which is what you need for sizing a pump or a drain-down.

How much does the water in a pipe weigh?

One liter of water weighs one kilogram, so the water weight in kilograms is the same as the water capacity in liters. A 20 mm pipe 10 m long holds 3.14 kg, which is nothing; a 50 mm main 100 m long holds 196 kg, which is a structural question.

• Copper Tube Handbook — Copper Development Association
• WaterSense labeled showerheads — US Environmental Protection Agency