How Much Water Can PE Pipe Carry? Flow Calculations for 20–63 mm Pipe

Are tonnes per hour and cubic metres per hour equivalent?

For practical water calculations, 1 m³ is approximately 1 tonne. A water flow of 5 tonnes per hour therefore corresponds to approximately 5 m³/h in many irrigation calculations. This approximation applies to water; it must not be carried over to fluids with a different density.

Pressure and flow rate are different quantities. ATÜ or bar is used to describe pressure or a pressure class, while flow rate is the volume of water passing in a given time. A higher pressure rating on a PE100 polyethylene pipe does not, by itself, guarantee a higher flow rate.

Why do sizes of 20, 25, 32, 40, 50 and 63 mm not specify a flow rate?

PE pipe is usually sold by outside diameter. Pipes marked 6 ATÜ, 10 ATÜ or 16 ATÜ can have different wall thicknesses at the same 32 mm outside diameter, leaving different internal flow areas. A single tonnes-per-hour figure for every 32 mm pipe would therefore be misleading.

Theoretical Flow by Internal Diameter

The values below are theoretical examples for a full pipe at a water velocity of 1 m/s, without accounting for losses. In a real system, elbows, couplings, valves, filters, pipe length, elevation difference and pump capacity can reduce the flow achieved. Use the actual internal diameter, not the product outside diameter, when reading this table.

Formula: flow rate (m³/h) = internal cross-sectional area (m²) × water velocity (m/s) × 3,600. Doubling velocity increases the theoretical flow, but friction losses and the risk of water hammer can also increase sharply.

How do you choose between 32, 40, 50 and 63 mm pipe?

A short garden line, a field irrigation line hundreds of metres long and a pump serving a large elevation difference require different assessments. First calculate the combined flow of the outlets that will operate at the same time. Then check the actual internal diameter and total head loss along the line.

• 32 mm PE100 pipe: may suit low-flow branches or short runs where the project calculations support its use.
• 40 mm PE100 pipe: can provide a larger flow area than 32 mm pipe; the actual benefit depends on wall thickness and line length.
• 50 mm PE100 pipe: may be considered for higher target flow rates or longer runs.
• 63 mm PE100 pipe: is commonly considered for mains, longer runs or lines serving several outlets.

These examples do not constitute a completed design. Use the pipe diameter calculator for a preliminary assessment, then verify the result against the pump curve.

How do length, elevation and fittings affect flow?

Longer pipe runs increase friction loss. An upward elevation change adds static head. Restrictive couplings, unnecessary elbows, partly closed valves and dirty filters also reduce the available pressure. A flow measured on a 10 metre test line should therefore not be assumed for a 300 metre field installation.

A pump’s maximum catalogue flow is not enough to size the system. Flow varies with the head against which the pump operates. The duty point depends on the pump curve, suction conditions and total dynamic head. ATÜ pressure class selection and flow calculation are separate steps, but both must be checked for the same project.

7 Inputs for a Useful Site Calculation

1. Measure the total pipe length and the highest elevation difference.
2. Find the total flow rate of drippers, sprinklers or outlets that will operate simultaneously.
3. Get the make-model and operating curve of the pump.
4. Verify the outside diameter, ATÜ/PN and SDR or wall thickness of the pipe.
5. List elbows, valves, filters, couplings and constrictions.
6. Note the targeted run time and water temperature.
7. Check the selection against the SDR, PN and ATÜ guide before finalising it.