Pipe Volume & Flow Calculator NZ 2026

This calculator works out how much water a pipe holds and how fast it can move, two figures that matter for plumbing, irrigation, drainage and water-storage jobs. The volume tells you the capacity of a length of pipe in litres, which is useful for sizing pumps and pressure tanks, working out how long it takes to fill or flush a line, dosing a pipe with chemicals, or estimating how much water sits in a system. The flow rate tells you how much water passes a point each second or minute at a given velocity, which helps you check whether a pipe is big enough for the demand it serves, since a pipe that is too small chokes the flow and a pipe that is too large wastes money and can cause other problems. You enter the internal diameter of the pipe in millimetres, which is the bore the water actually flows through rather than the outside diameter, the length of the run in metres, and a flow velocity in metres per second, and the calculator returns the volume in litres and cubic metres, the cross-sectional area, and the flow rate in litres per second and litres per minute. As a guide, water in household and irrigation pipes typically flows at around 1 to 2 metres per second, with lower speeds preferred to reduce noise, friction loss and wear. Remember to use the internal diameter, because pipe is often described by its nominal or outside size, which is larger than the bore. This is a calculation tool for planning and learning; for pressurised systems and compliance work, follow the relevant standards and a plumber's or engineer's advice.

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19.6 L
water held in the pipe
Volume0.0196 m³
Cross-section19.6 cm²
Flow rate176.7 L/min

Use the internal diameter (the bore), not the outside size. Real flow is reduced by friction and fittings. An estimate only.

How it works

The cross-sectional area is pi times the radius squared, using the internal radius. The volume is that area times the length, converted to litres by multiplying cubic metres by 1,000. The flow rate is the area times the velocity, giving cubic metres per second, converted to litres per second and per minute.

Worked example

A 50mm internal diameter pipe has a radius of 25mm, giving a cross-section of about 19.6 square centimetres. Over a 10 metre run that holds about 0.0196 cubic metres, or 19.6 litres. At a flow velocity of 1.5 metres per second the flow rate is about 2.9 litres per second, which is roughly 177 litres per minute.

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Frequently asked questions

How do I find the volume of water in a pipe?

Treat the pipe as a cylinder: the cross-sectional area is pi times the internal radius squared, and the volume is that area times the length. Multiply cubic metres by 1,000 for litres. This calculator does it from the internal diameter and length you enter.

Should I use internal or external pipe diameter?

Use the internal diameter, the bore the water flows through. Pipe is often sold by a nominal or outside size that is larger than the bore, so using the outside diameter overstates both the volume and the flow. Check the pipe's specifications for the internal size.

What is a normal water flow velocity?

Water in household and irrigation pipes commonly flows at around 1 to 2 metres per second. Lower speeds reduce noise, friction loss and wear, while higher speeds can cause water hammer and erosion. Designers pick a velocity to balance pipe size against pressure loss.