A customer says, “The roof is 40 metres above the water. Please quote a 40-metre pump.” That 40 metres describes the elevation change, but water also loses pressure as it moves through the pipe, bends, valves and fittings.
Total dynamic head (TDH) is the head the pump must provide at the required flow. For an open source and an open receiving tank, a useful starting expression is:
If the receiving system requires pressure, add that required pressure head. Account for pressure differences between source and destination where applicable.
A worked example
Assume an open water source, an open rooftop tank, and these illustrative estimated values after a piping calculation: static rise 40 m, straight-pipe friction 9 m, and fittings/valves 5 m. Then TDH is 40 + 9 + 5 = 54 m at the chosen flow. If the project team applies a 10% allowance to these estimates, the evaluation target becomes about 59.4 m at that flow. The allowance is a project choice, not a universal pump-sizing rule.
Suppose the tank holds 5 m³. Filling it in eight hours needs 0.625 m³/h on average; in two hours, 2.5 m³/h; in one hour, 5 m³/h. Friction loss changes when flow changes, so do not use the same 9 m estimate for all three cases. Recalculate the system losses and compare a pump curve for each required filling time.
What to measure before calculating
- Water level at the source during operation, and the discharge water level or required outlet pressure.
- Required delivered flow, not simply tank volume.
- Actual pipe inside diameter, length, material, bends, valves and filters.
- Any expected variation in water level, simultaneous use or future extension.
A 90-metre horizontal pipe run contributes friction loss; it does not automatically add 90 metres to the static head. Conversely, a long narrow pipe can create substantial friction even when the height difference is modest. Larger pipe diameter or fewer restrictive fittings can sometimes change the pump choice.
How to use the result
Plot the required flow and calculated TDH on the manufacturer’s flow–head curve. Confirm the proposed operating point, motor rating and acceptable operating range for the exact model and supply frequency. If the point falls outside the curve, move to another model or revisit the system design. Do not select a pump only because its listed maximum head is greater than 59.4 m.
Have a pipe route to size?
Send a sketch with source and destination levels, pipe sizes, lengths, fittings and desired delivery time to CHANGHONG. A curve-based recommendation needs both flow and head.
More water pump buying guides
Technical background: Grundfos, calculating total dynamic head; Grundfos, pump curves.