TDH: What Total Dynamic Head Is and How to Calculate It

Executive summary

Total dynamic head (TDH) is the real head a pump must overcome to move fluid through its installation. It is not the simple level difference: it is the sum of static head, friction losses and velocity head. Getting the TDH right is the step that decides whether a pump delivers the expected flow or falls short.

This guide covers what makes up TDH, the formula, how to calculate it step by step, a worked example with real numbers and the most common mistakes. At the end you can get your system's TDH in seconds with the free calculator.

Who this is for

• Project engineers about to select a pump who need the correct operating head.
• Plant and process engineers who want to verify whether their pump is properly sized.
• Maintenance managers looking at pumps that "don't give the flow" and hunting for the cause.

The real plant problem

The most common mistake is sizing a pump on static head alone (the level difference) and ignoring friction. As soon as fluid moves through pipe, elbows and valves, losses appear that grow with flow. If those losses are left out, the pump is chosen "short": it delivers less flow than planned, or must be pushed with more pressure than expected. TDH exists precisely to avoid that mistake.

Engineering fundamentals

TDH is the energy per unit weight the pump adds to the fluid. It breaks down into three terms:

TDH = H_static + h_friction + h_velocity

1) Static head (H_static). The level difference between suction and discharge, plus any pressure difference. It is constant: it does not change with flow.

2) Friction losses (h_friction). Energy lost to friction in pipe, fittings and valves. They grow with flow (roughly with the square) and depend on diameter, length, material and fittings. They are estimated with Hazen-Williams or Darcy-Weisbach.

3) Velocity head (h_velocity = v²/2g). The kinetic energy of the moving fluid. It is usually small compared with the other two, except at high velocities.

More generally, the Bernoulli-based equation between suction (1) and discharge (2) is:

H = (P₂ − P₁)/(ρg) + (v₂² − v₁²)/2g + (z₂ − z₁) + h_f

In practice, the TDH of most water systems is dominated by static + friction; velocity head is often folded in because of its low value.

How to calculate it step by step

1. Static head: measure the level difference between source and destination and add any pressure difference (for example, a pressurized tank).
2. Pipe data: total length, diameter, material (Hazen-Williams C factor) and fittings (elbows, valves).
3. Friction losses: calculate h_friction at the operating flow; fittings are added as equivalent length.
4. Velocity head: v²/2g using the pipe velocity (usually small).
5. Add them up: TDH = static + friction + velocity. That is the head to look up on the pump curve.
6. Verify that velocity stays in a healthy range (≈ 1–2.5 m/s) and that the available NPSH has margin.

Worked example with numbers

System: static head 20 m; 120 m of 4" steel pipe (C = 120); 4 elbows and 2 valves; elevation 1500 m above sea level. Pump curve: (0 m³/h, 40 m), (50 m³/h, 34 m), (100 m³/h, 16 m).

Solving the system, the operating point lands at 68.04 m³/h, and the TDH at that flow breaks down as follows:

Component	Value
Static head	20.00 m
Friction losses (Hazen-Williams, with fittings: +13.8 m equivalent)	8.89 m
Velocity head (v = 2.33 m/s → v²/2g)	≈ 0.28 m
Operating TDH	≈ 28.89 m
Available NPSH (1500 m above sea level, 20 °C)	8.43 m
Estimated brake power	10.26 HP (7.65 kW)

Engineering read: friction (8.89 m) contributes nearly a third of the TDH; ignoring it would have left the pump short by that same margin. Velocity head (~0.28 m) is minor and usually folded in. The 2.33 m/s velocity is acceptable, and the available NPSH of 8.43 m gives margin against cavitation.

When it applies and when it does not

It applies to any liquid pumping system, whether to size the pump or to verify its operating point. It is the number you take to the manufacturer's curve.

Be careful with: viscous fluids or fluids carrying solids (they change the losses); systems with high discharge pressure (the pressure term outweighs the level term); and suction runs with lift, which also affect NPSH.

Common mistakes

• Using static head only. Without friction, TDH is underestimated and the pump comes up short.
• Forgetting fittings. Elbows and valves add equivalent length; in the example, +13.8 m.
• Confusing pressure with head. Convert both to the same units (m of water column) before adding.
• Calculating TDH at a flow that is not the operating flow. Friction changes with flow; use the real operating flow.
• Ignoring elevation in NPSH. The higher the elevation, the lower the available NPSH.

Checklist / decision criteria

• Did I clearly separate static, friction and velocity?
• Did I include the equivalent length of all fittings?
• Did I calculate friction at the real operating flow?
• Did velocity land in ≈ 1–2.5 m/s?
• Did I convert every pressure to m of water column before adding?
• Does the available NPSH exceed the required NPSH with margin?

Frequently asked questions

What is total dynamic head (TDH)?

It is the total head a pump must overcome in its installation: the sum of static head, friction losses and velocity head. It is the value you take to the pump curve to select the pump.

How do you calculate a pump's TDH?

By adding static head + friction losses (in pipe and fittings, at the operating flow) + velocity head (v²/2g). You can get it in seconds with the calculator below.

What is the difference between static head and TDH?

Static head is only the level/pressure difference and it is constant. TDH adds the friction losses (which grow with flow) and the velocity head; that is why TDH is always greater than static head whenever there is flow.

SEMHYS tools

Enter the static head, your pipe data and 2–3 points of your pump curve in the free SEMHYS pump calculator and you will instantly get the TDH, the operating point, the available NPSH, the velocity and the estimated power, along with the curve chart.

For multiple scenarios or a diagnostic report of your plant, see the options in the SEMHYS store.

References

1. Hydraulic Institute. ANSI/HI 14.3 — Rotodynamic Pumps for Design and Application (system head = static + friction).
2. Total Dynamic Head — Bernoulli-based pump head equation (components: static, friction, velocity and pressure).
3. ISO 9906:2012 — Rotodynamic pumps — Hydraulic performance acceptance tests.