How to Calculate Total Dynamic Head (TDH) for Pump Sizing — Complete Guide 2026

What is Total Dynamic Head (TDH)?

Total Dynamic Head (TDH) is the total pressure a pump must overcome to move fluid from the source to the destination. It's the single most important parameter for pump sizing — get it wrong, and your pump either can't deliver enough flow or wastes energy for years.

TDH is measured in meters (m) or feet (ft) of fluid column and consists of 4 components:

TDH = Static Head + Friction Losses + Fitting Losses + Residual Pressure

Component 1: Static Head (Hs)

Static head is the vertical distance the pump must push the fluid upward.

Hs = Elevation(discharge) - Elevation(suction)

If your pump is at ground level and must push water to a tank 20 m above:

Hs = 20 m

Key point: static head is purely vertical. Horizontal distance does NOT add to static head (it adds to friction instead).

Component 2: Friction Losses (hf)

Friction losses are caused by the fluid rubbing against the pipe walls. Two methods are standard:

Darcy-Weisbach (more accurate)

hf = f × (L/D) × V²/(2g)

Where f is the friction factor from the Swamee-Jain approximation:

f = 0.25 / [log10(ε/(3.7D) + 5.74/Re^0.9)]²

Hazen-Williams (simpler, water only)

hf = 10.67 × L × Q^1.852 / (C^1.852 × D^4.87)

Both methods give similar results for water at 10-25°C and velocities of 0.6-3 m/s. Outside this range, prefer Darcy-Weisbach.

Component 3: Fitting Losses (hm)

Every elbow, valve, tee, and reducer creates turbulence that dissipates energy:

hm = ΣK × V²/(2g)

Common K values:

Component 4: Residual Pressure (Hr)

The pressure required at the delivery point. For example, a fire hydrant needs 15 m of pressure at the nozzle.

Hr = required pressure at destination (m)

If no specific pressure is required (e.g., filling an open tank), Hr = 0.

Worked Example

Problem: Size a pump to supply water from a ground-level cistern to a rooftop tank 25 m above. The piping system consists of 80 m of 3-inch PVC pipe, with 4 standard 90° elbows, 1 check valve, and 1 gate valve. Flow rate = 5 L/s. No residual pressure required.

Step 1: Static Head

Hs = 25 m

Step 2: Friction Losses (Hazen-Williams)

PVC pipe: C = 150, D = 0.0762 m, Q = 0.005 m³/s

hf = 10.67 × 80 × 0.005^1.852 / (150^1.852 × 0.0762^4.87)
hf = 853.6 × 5.28×10⁻⁵ / (10544.67 × 2.14×10⁻⁶)
hf = 0.04509 / 0.02257
hf = 2.0 m

Step 3: Fitting Losses

Velocity: V = Q/A = 0.005/0.00456 = 1.10 m/s

K_total = 4×0.30 + 2.50 + 0.20 + 0.50 + 1.00 = 5.40
hm = 5.40 × 1.10² / (2 × 9.81) = 0.33 m

Step 4: TDH

TDH = 25 + 2.0 + 0.33 + 0 = 27.33 m

Step 5: Select Pump

Look for a pump that delivers 5 L/s at 27.33 m head with adequate NPSH margin. Add a 10-15% safety factor:

TDH_design = 27.33 × 1.15 = 31.4 m

Select a pump rated for at least 5 L/s at 32 m.

Common Mistakes

1. Confusing static head with pipe length. A 100 m pipe running horizontally adds zero static head.
2. Forgetting fitting losses. In short piping systems, fittings can account for 30-50% of total losses.
3. Using old roughness values. Pipes age. Apply a 10-20% safety factor on friction losses.
4. Ignoring residual pressure. The pump doesn't just need to "reach" the destination — it must arrive with usable pressure.

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