If there is one number that decides whether a pump will work properly in your system, it is total dynamic head, or TDH. Get it right and the pump delivers the flow you need at good efficiency. Get it wrong and you end up with a pump that delivers a trickle, or one that runs so far out on its curve that it overheats, cavitates or burns out its motor. Many pump failures trace back not to the pump itself, but to a miscalculated TDH.
This guide explains what TDH is, the formula behind it and a clear step-by-step method with worked examples.
What Is Total Dynamic Head?
TDH is the total energy a pump must add to each unit weight of liquid to move it through the system at the required flow rate. It is expressed as a height of liquid column, in metres or feet. Head is used instead of pressure because it does not depend on the liquid’s density, so a pump curve in metres of head applies to water, light oils or other liquids, with power adjusted for density.
For water, approximate conversions are:
- 1 bar ≈ 10.2 metres of head
- 1 psi ≈ 0.703 metres of head (or 2.31 feet)
The TDH Formula
In simple terms:
TDH = Static Head + Pressure Head + Friction Head + Velocity Head
Each component represents a different source of resistance.
Static head is the vertical height difference the liquid must be lifted. It has two parts:
- Static suction head or lift: the vertical distance between the pump centreline and the liquid surface on the suction side. If the liquid is above the pump, it is a positive suction head. If it is below, it is a suction lift.
- Static discharge head: the vertical distance from the pump centreline to the point of discharge or to the liquid surface in the delivery tank.
For a borehole submersible pump, static head is measured from the pumping water level (the water level while pumping, which is lower than the resting level) up to the discharge point. This is a very common source of error, because using the resting water level underestimates the lift.
Pressure head is any pressure the pump must deliver at the discharge point, converted into metres of head. Examples include the pressure needed at a sprinkler, a spray nozzle, a pressurised tank or the minimum pressure at the highest tap in a building.
Friction head is the energy lost to friction as liquid flows through pipes, valves, bends, strainers, meters and other fittings. It increases rapidly with flow rate.
Velocity head is the kinetic energy of the moving liquid, calculated as v²/2g, where v is velocity and g is gravitational acceleration. It is usually small in typical water systems and often neglected, but it can matter in high-velocity or short pipe systems.
Step-by-Step Method
Step 1: Define the flow rate. TDH depends on flow because friction rises with it. Calculate TDH at the design flow, and check at minimum and maximum flows if demand varies.
Step 2: Draw the system. Sketch the pipework from source to discharge, noting elevations, pipe lengths, diameters, materials and every fitting.
Step 3: Determine static head. Measure or estimate the vertical lift from the lowest liquid level to the highest discharge point. Use the lowest expected water level for sources like tanks, rivers, wells and boreholes, because that creates the highest lift.
Step 4: Convert required pressure to head. If the system needs a pressure at delivery, multiply by 10.2 for pressure in bar to obtain metres of head for water. For example, 2 bar of required residual pressure equals about 20.4 metres.
Step 5: Calculate friction losses in straight pipe. Use a recognised method, such as the Hazen-Williams equation (common for water) or the Darcy-Weisbach equation (more general). Many engineers use friction loss tables or charts for typical pipe materials, such as PVC, HDPE or steel, which give head loss per 100 metres of pipe at a given flow and diameter.
Step 6: Calculate losses in fittings. Each valve, elbow, tee, reducer, strainer and non-return valve adds resistance. Two common methods are:
- Equivalent length method: each fitting is expressed as an equivalent length of straight pipe, which is added to the actual pipe length.
- K-factor method: each fitting has a resistance coefficient K, and the loss is K × v²/2g.
Step 7: Add velocity head if needed. For most cases, it is small and can be included for completeness.
Step 8: Sum all components. Add static, pressure, friction and velocity head to get TDH.
Step 9: Add a modest margin. A margin of about 5 to 10 percent accounts for pipe ageing, scaling, calculation uncertainty and future wear. Avoid piling on large safety factors, which lead to oversized pumps.
Friction Loss: The Part Most People Underestimate
Friction head is where most TDH errors occur. A few points help:
- Pipe diameter has a huge effect. Doubling the diameter reduces friction loss dramatically, often by a factor of more than 30 for the same flow. Using slightly larger pipe is often the cheapest way to cut energy costs.
- Velocity guidelines. For water, keep velocity in suction lines around 0.6 to 1.5 m/s and in discharge lines around 1.5 to 2.5 m/s as a general guide. Higher velocities increase friction, noise and water hammer risk.
- Pipe material matters. Roughness differs between new and old pipes. Corroded steel or scaled pipes have much higher friction than smooth plastic.
- Fittings add up. In compact systems with many valves and bends, fitting losses can exceed straight pipe friction.
- Use realistic flow. Calculate at the actual flow the pump will deliver, not at an arbitrary rating.
Worked Example 1: Simple Surface Pump to a Tank
A surface pump moves water from a ground-level tank to an elevated storage tank.
- Flow rate: 15 m³/h (about 4.2 L/s)
- Suction side: liquid surface is 1.5 m above pump centreline (flooded suction), so static suction head = +1.5 m
- Discharge: water outlet in the elevated tank is 22 m above the pump centreline, discharging at atmospheric pressure
- Pipe: 50 mm HDPE, total length of 80 m with several fittings
Static head: discharge elevation minus suction head = 22 − 1.5 = 20.5 m
Pressure head: open discharge to atmosphere, so 0 m
Friction head: Suppose, from friction tables for this pipe at 15 m³/h, that straight-pipe loss is 3.0 m per 100 m. For 80 m, loss = 2.4 m. Fittings (valves, elbows and a non-return valve) are estimated at 1.2 m. Total friction ≈ 3.6 m.
Velocity head: small, say 0.2 m
TDH = 20.5 + 0 + 3.6 + 0.2 = 24.3 m
Add a 10 percent margin to give a selection value of about 26.7 m. The pump chosen should deliver 15 m³/h at roughly 27 m, near its best efficiency point.
Worked Example 2: Borehole Submersible Pump
A borehole supplies a farm storage tank.
- Flow rate: 8 m³/h
- Resting water level: 30 m below ground
- Drawdown at 8 m³/h: 12 m, so pumping water level = 42 m below ground
- Tank inlet: 6 m above ground level
- Rising main: 50 mm, length of 60 m from pump to surface, plus 120 m of horizontal pipe to the tank
Static head: 42 m (pumping level to ground) + 6 m (tank height) = 48 m
Pressure head: open tank inlet = 0 m
Friction head: Total pipe length is 180 m. If friction loss is 1.0 m per 100 m at this flow, straight-pipe loss is 1.8 m. Fittings and valves add 1.0 m. Total friction ≈ 2.8 m.
TDH ≈ 48 + 0 + 2.8 + 0.1 (velocity) ≈ 51 m
With a 5 to 10 percent margin, the selection target is about 54 to 56 m at 8 m³/h. Note how using the resting water level of 30 m would have underestimated static head by 12 m, a serious error that would have produced a pump delivering far less water than expected.
Special Cases to Watch
Pressurised delivery. Where water is delivered into a pressurised tank or distribution network, convert the required pressure to head and add it directly. For sprinkler or drip irrigation, include the operating pressure of emitters, filters and valves.
Variable water levels. Where source or tank levels vary, calculate TDH for both extremes. The pump must handle the highest TDH, but should also run acceptably at the lowest.
Long pipelines. Friction dominates. Consider larger pipe sizes to cut head and operating cost, and check surge risk.
Altitude and temperature. These affect suction performance and NPSH rather than TDH itself, but they should be considered in the same design review.
Liquids other than water. TDH in metres is still valid, but pump power and pressure vary with density, and viscosity may reduce pump performance and increase friction.
Solar pumping. TDH is the critical sizing parameter for solar pumps, and varies with pumping water level through the day and across seasons. Calculate conservatively.
Common TDH Mistakes
- Using resting instead of pumping water level in boreholes.
- Ignoring fitting losses, particularly in plant rooms with many valves.
- Using the wrong flow rate for friction calculations.
- Forgetting required delivery pressure.
- Using nominal pipe diameter instead of actual internal diameter, which differs by material and pressure class.
- Applying large safety factors repeatedly, creating oversized pumps.
- Neglecting pipe ageing and scaling in long-life systems.
- Mixing units, such as metres and feet or bar and psi.
Conclusion
TDH is the foundation of good pump selection. It combines the vertical lift, the pressure required at the delivery point, the energy lost to friction and a small velocity term into a single value that determines how hard the pump must work. By defining flow first, sketching the system, measuring static head from the correct water level, calculating friction carefully and adding a sensible margin, you can specify a pump that meets your needs and runs efficiently. Take the time to calculate TDH properly, and every step that follows, from pump curves to energy cost, becomes easier and more reliable.
