Last updated: October 14, 2026
Total dynamic head (TDH) is the energy, expressed as a height of liquid, that a pump must add to move the design flow through a system. It is the number you take to a pump curve. Get it wrong and the pump runs off its curve: too little head and you don't get the flow, too much and you waste energy or run into cavitation and wear.
The four parts of TDH
TDH = static head + friction loss + minor losses + pressure head1. Static head
The vertical distance the liquid is lifted. With a suction lift (liquid surface below the pump centreline), static head is the lift plus the height from the pump centreline to the discharge point. With a flooded suction (surface above the pump), subtract the submergence instead. Use the highest point the liquid must pass if the line climbs over a high point and then drops, unless the downstream leg runs full and siphons.
2. Friction loss (Hazen-Williams)
For water at ordinary temperatures, the Hazen-Williams equation is the common field method:
Metric: hf = 10.67 × L × Q1.852 / (C1.852 × D4.8704) (m; L in m, Q in m³/s, D in m) US: hf = 0.002083 × L × (100/C)1.852 × Q1.852 / d4.8655 (ft; L in ft, Q in USGPM, d in in)C is the roughness coefficient: about 150 for new HDPE and PVC, 120 for steel and lower for old or rough pipe. D is the actual inside diameter, not the nominal size. That matters for HDPE: an 8-inch DR17 pipe has an average inside diameter of about 7.55 in (192 mm), not 8 in.
3. Minor losses (K method)
Each fitting, valve, entrance and exit loses a multiple K of the velocity head:
hm = ΣK × V² / 2gTypical K values: sharp-edged entrance 0.5, standard 90° elbow 0.9, swing check valve 2.0, free discharge or exit into a tank 1.0. Use values from your fitting supplier where you have them.
4. Pressure head
If the discharge must arrive at a pressure (for example, tying into a pressurised main), convert that pressure to head: h = P / (ρg), or for water m = kPa / 9.81 and ft = psi × 2.31. For an open discharge, pressure head is zero. The unit converter does this conversion with a specific-gravity setting.
Worked example
A sewer bypass pump lifts water from a wet well and discharges into a downstream manhole.
| Input | Metric | US |
|---|---|---|
| Design flow | 40 L/s | 634 USGPM |
| Pipe (suction and discharge) | 8" HDPE DR17, ID 191.8 mm | ID 7.549 in |
| Hazen-Williams C | 150 | 150 |
| Suction lift (water surface to pump centreline) | 3.0 m | 9.84 ft |
| Discharge outlet above pump centreline | 6.0 m | 19.69 ft |
| Suction / discharge pipe length | 8 m / 150 m | 26.2 ft / 492.1 ft |
| Suction fittings: entrance 0.5 + one 90° elbow 0.9 | ΣK = 1.4 | |
| Discharge fittings: two 90° elbows 1.8 + check valve 2.0 + exit 1.0 | ΣK = 4.8 | |
Step 1: Velocity and velocity head
Metric: A = π × 0.1918² / 4 = 0.0289 m²; V = 0.040 / A = 1.39 m/s; V²/2g = 0.098 m.
US: V = 0.4085 × 634 / 7.549² = 4.54 ft/s; V²/2g = 0.321 ft.
Step 2: Friction
Metric: suction 0.064 m; discharge 1.198 m. US: suction 0.21 ft; discharge 4.01 ft.
Step 3: Minor losses
Metric: suction 1.4 × 0.098 = 0.137 m; discharge 4.8 × 0.098 = 0.470 m. US: suction 0.45 ft; discharge 1.54 ft.
Step 4: Add it up
| Component | Metric (m) | US (ft) |
|---|---|---|
| Static head (3.0 + 6.0 m) | 9.00 | 29.53 |
| Suction friction | 0.064 | 0.21 |
| Discharge friction | 1.198 | 4.01 |
| Suction minor losses | 0.137 | 0.45 |
| Discharge minor losses | 0.470 | 1.54 |
| Pressure head (open discharge) | 0 | 0 |
| TDH | 10.87 m | 35.74 ft |
The US result, 35.74 ft, equals 10.89 m. The small difference from 10.87 m comes from the rounded constants in the two published forms of Hazen-Williams; it is well inside the accuracy of C.
Variation: pressurised tie-in
If the discharge instead ties into a main that needs 100 kPa (14.5 psi) at the connection, add pressure head of 100 / 9.807 = 10.20 m (14.50 × 2.307 = 33.46 ft). TDH becomes 21.07 m (69.19 ft).
Sanity checks
- Static head usually dominates on short bypass runs; friction dominates on long ones. Here friction and fittings add only about 1.9 m to 9 m of static head.
- Line velocities of roughly 0.6–3 m/s (2–10 ft/s) are typical. Much lower and solids can settle; much higher and friction and surge rise quickly.
- Check the result at the lowest and highest liquid levels you expect, not just one point.
Next: check the suction side with NPSH available, convert units with the free converter, or learn the full method in the course.
Worked examples are for learning. Check your own designs against manufacturer data and have them reviewed by a qualified professional. See the disclaimer.