Pump selection is one of the most critical tasks in plumbing and MEP design for buildings. Select a pump with too little head and water never reaches the top floors. Select one with too much head and you waste energy, cause water hammer, and shorten valve and fitting life. Getting it right requires calculating the Total Dynamic Head (TDH) — the sum of all resistances the pump must overcome to deliver water at the required flow rate and pressure.
What is Total Dynamic Head (TDH)?
Total Dynamic Head is the total equivalent height (in metres of water column) that the pump must lift and push water through. It accounts for three components:
- Static Head (Hs): The vertical elevation difference between the pump centre line and the highest delivery point.
- Friction Head Loss (Hf): Pressure lost overcoming pipe wall friction and fittings resistance along the entire pipe route.
- Velocity Head (Hv): Kinetic energy added to water as it accelerates through the pipe (usually small, often 0.1–0.5m).
- Residual Pressure Head (Hr): Minimum required pressure at the delivery point (e.g., 10m at highest floor outlet).
Step 1 — Calculate Static Head
Measure the vertical distance from the pump's centreline (or the sump water level if suction is from below) to the highest water delivery point in the system — typically the overhead tank or the top-floor outlet.
Step 2 — Calculate Friction Head Loss
Friction losses occur in straight pipe runs and at every fitting — elbows, tees, valves, reducers. Use the Darcy-Weisbach equation for straight pipes:
Hazen-Williams Formula (Alternative)
For water supply design, the Hazen-Williams formula is often simpler and widely used in India:
Step 3 — Determine Residual Pressure
The system must deliver water at a minimum residual pressure at the point of use. NBC (National Building Code) India and IS 2065 specify minimum pressures at fixtures:
| Fixture Type | Min. Residual Pressure |
|---|---|
| Ordinary tap / wash basin | 7 kPa (0.7 m WC) |
| Flush valve (WC) | 70 kPa (7 m WC) |
| Shower head | 30 kPa (3 m WC) |
| Fire hydrant (IS 3844) | 350 kPa (35 m WC) |
| Overhead tank inlet (general) | 0.5–1.0 m WC above tank rim |
Worked Example — 10-Storey Building Water Supply Pump
Use our Pump Head Calculator to compute TDH and motor power instantly for any building water supply system. Enter your pipe details and the calculator handles the Darcy-Weisbach friction loss automatically.
Common Pump Sizing Mistakes
- Ignoring suction head: If the pump draws from a sump below ground level, the suction static head adds to the total. Also check that the Net Positive Suction Head available (NPSHa) exceeds the pump's NPSHr to prevent cavitation.
- Underestimating fittings losses: In a building with many bends, valves, and tees, fitting losses can equal 30–50% of straight pipe friction. Always add an equivalent length for fittings.
- Not accounting for pressure drop across the overhead tank float valve: Ball-float valves have a typical pressure drop of 3–5m WC at design flow. Add this to the required residual head.
- Selecting at a single duty point: Check the pump curve at minimum, normal, and maximum flow conditions. Ensure the pump operates within its Preferred Operating Region (POR) — 70–120% of Best Efficiency Point (BEP) flow.
Conclusion
Pump head calculation is straightforward once you break TDH into its three components: static head (elevation), friction losses (pipe and fittings), and residual delivery pressure. A properly sized pump delivers water reliably to every floor at adequate pressure, runs efficiently at its design point, and has a long service life.
Use the MEPMate Pump Head Calculator to solve TDH and motor power for your next water supply design in seconds. Also explore the Water Storage Tank Calculator for sizing the sump and overhead tank capacity.