Total dynamic head (TDH): how to calculate pump head.
Size a pump on flow alone and you either burn energy or fail to reach the tank. Total dynamic head (TDH) is the total resistance a pump must overcome — static lift, friction, pressure, and velocity. Get TDH right and pump selection becomes straightforward. Here is the full breakdown with a worked example.
What is total dynamic head?
Total dynamic head is the equivalent height, in metres of liquid, that a pump must add to move fluid from the source to the destination at the required flow rate. It is called dynamic because part of it — friction — only exists when the fluid is moving. A pump curve plots head against flow; you select the pump where its curve meets your system TDH at the design flow.
The four components of TDH
| Component | What it is |
|---|---|
| Static head | Vertical lift from source liquid level to discharge point |
| Friction head | Losses in pipe, fittings, and valves as fluid flows |
| Pressure head | Any pressure difference between source and destination vessels |
| Velocity head | Energy to accelerate the fluid to pipe velocity (usually small) |
For most open-tank transfer systems, static head and friction head dominate; pressure and velocity head are often negligible but must be checked for closed or pressurised systems.
The TDH formula
Friction head is the term engineers most often underestimate. It rises with the square of velocity, so a slightly undersized pipe can double your friction losses and starve the system.
Worked example: transfer pump
A plant transfers water from a ground sump to an overhead tank:
- Vertical lift (static head) = 18 m
- Pipe: 50 mm bore, 60 m total length, flow 400 L/min (0.0067 m³/s)
- Friction loss (from pipe + fittings) = 6.5 m
- Both sump and tank open to atmosphere → pressure head = 0
- Velocity head at ~3.4 m/s ≈ 0.6 m
TDH = 18 + 6.5 + 0 + 0.6 = 25.1 m.
So you select a pump that delivers 400 L/min at roughly 25 m of head at its best-efficiency point. Picking a pump rated for only the 18 m static lift would leave it unable to reach the tank once friction is added.
From TDH to pump power
Once you have TDH and flow, hydraulic power follows directly, and shaft/motor power after dividing by pump and motor efficiency. Run the numbers with the pump power calculator, and see the full derivation in our pump power calculation guide. Motor sizing then flows from the shaft power figure.
Common mistakes
- Ignoring friction head. On long or small-bore lines it can exceed the static lift.
- Mixing pressure and head units. Convert bar/psi to metres of the actual liquid before adding.
- Using nominal flow, not design flow. Friction scales with velocity squared, so the flow assumption matters a lot.
- Forgetting fittings. Elbows, valves, and reducers add equivalent length that must be in the friction term.