Pipe Volume Calculator
Work out how much liquid a pipe holds from its inner diameter and length. The calculator treats the bore as a cylinder, returns the volume in litres, US gallons and cubic feet, and weighs the contents for water or any fluid density you set. Turn on the flow option to get how long the pipe takes to fill and the velocity of the liquid inside. Switch between metric and imperial units to match your measurements.
Formula
Worked example
A 10 m pipe with a 50 mm bore: radius = 0.025 m, area = π × 0.025² = 0.0019635 m², volume = 0.0019635 × 10 = 0.019635 m³ = 19.63 L (5.19 US gal, 0.6934 ft³), weighing about 19.58 kg of water. At 0.5 L/s it fills in about 39 seconds, with water moving at roughly 0.25 m/s.
How pipe volume is calculated
A length of pipe is a cylinder, so its internal volume equals the area of the circular bore multiplied by the length. The bore area is π times the inner radius squared, and the radius is half of the inner diameter. Writing it out, volume = π × (diameter ÷ 2)² × length. The single most important detail is that the diameter must be the inner (internal) diameter, the open bore the liquid actually flows through, not the outside diameter. Pipe is usually sold by nominal size and outside diameter, and the wall thickness can shrink the usable bore noticeably, so a pipe labelled by its outer size will hold less than the label might suggest. Once the volume is known in cubic metres it converts cleanly to litres by multiplying by 1000, to US gallons by dividing litres by 3.785411784, and to cubic feet by dividing by 0.0283168.
Weight of any liquid inside
Knowing the volume lets you estimate how much the full pipe weighs, which matters when sizing brackets, hangers, and pumps or when planning to drain a system. Weight is volume multiplied by the density of the liquid. The calculator defaults to pure water at about 25 degrees Celsius, with a density of 997 kilograms per cubic metre, so one litre weighs almost exactly one kilogram. To weigh a different fluid, change the liquid density value: seawater is roughly 1025 kg/m³, milk about 1030 kg/m³, and many oils sit between 850 and 950 kg/m³. The weight, the velocity and the cost all update from that one density figure. The pipe material itself is not included here, since this figure is the contents only. For long horizontal runs, remember that the liquid weight is distributed along the pipe, so support spacing should account for the load per metre rather than the total alone.
Fill time and flow velocity
Turn on the flow option to enter a flow rate, in litres per second for metric or US gallons per minute for imperial, and the calculator returns two extra figures. The time to fill is simply the pipe volume divided by the flow rate, which tells you how long a pump or supply line takes to charge the run or how long a purge or flush will take. The average flow velocity is the flow rate divided by the bore cross-section area, a key plumbing and HVAC number because water moving faster than about 1.5 to 2.4 m/s (5 to 8 ft/s) tends to be noisy and can erode fittings over time, while very low velocities can let sediment settle. If your velocity comes out high, a larger bore lowers it sharply because area grows with the square of the diameter.
Costing the contents and accurate measuring
The optional cost estimate prices a full pipe of liquid at your unit rate, billed the way utilities usually charge: per 1000 litres in metric or per 1000 US gallons in imperial. It is handy for water-loss and leak estimates or for valuing the product sitting in a process line. On measurement: small errors in diameter have an outsized effect because volume depends on the radius squared, so doubling the bore quadruples the capacity. Measure the inner diameter carefully, ideally with calipers across the open end, and keep a consistent unit system. For schedule and class pipe, manufacturer tables list the exact inner diameter for each nominal size and wall thickness, which is more reliable than measuring a fitting by eye. If a run changes diameter, calculate each section separately and add the volumes.
Approximate water held per metre of pipe
| Inner diameter | Litres per metre | US gallons per metre |
|---|---|---|
| 15 mm | 0.18 | 0.05 |
| 25 mm | 0.49 | 0.13 |
| 50 mm | 1.96 | 0.52 |
| 100 mm | 7.85 | 2.07 |
| 150 mm | 17.67 | 4.67 |
| 200 mm | 31.42 | 8.3 |
Litres per metre of straight pipe by inner diameter (volume = π × r² × length).
Frequently asked questions
Should I use the inner or outer diameter?
Always use the inner (internal) diameter, the open bore the liquid flows through. Pipe is often labelled by its nominal or outside size, but the wall thickness reduces the usable bore, so using the outside diameter overstates the volume. Check the manufacturer table for the exact inner diameter of your pipe size and wall thickness.
How do I find the weight of a liquid other than water?
Change the liquid density value in the calculator. Water defaults to 997 kg/m³, so one litre is about one kilogram. Seawater is roughly 1025 kg/m³, milk around 1030 kg/m³, and many oils are 850 to 950 kg/m³. The weight, flow velocity and cost all recalculate from the density you enter, so the same pipe holding oil weighs a little less than when full of water.
How long does a pipe take to fill, and how fast is the water moving?
Turn on the flow rate option. The fill time is the pipe volume divided by the flow rate, and the average velocity is the flow rate divided by the bore area. Plumbing and HVAC design usually keeps water below about 1.5 to 2.4 m/s (5 to 8 ft/s) to limit noise and erosion, so if the velocity is high consider a larger bore.
Does this account for elbows, valves, and fittings?
The calculator models a straight cylinder of constant bore, which is an excellent approximation for ordinary pipe runs. Bends and elbows add a tiny amount of volume that is usually negligible against a long straight length. If a run changes diameter, calculate each section separately and add the volumes together.