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TDH & NPSHa Calculator

Total dynamic head with Hazen-Williams friction and K-value minor losses, plus NPSH available. For water.

Units & flow

Unit system

Design flow

Water temperature (°F)

Site elevation (ft)

Elevations & pressure

Static suction lift (ft)

Pump centerline above source water surface. Negative = flooded suction.

Static discharge head (ft)

Discharge point (or receiving water surface) above pump centerline.

Required pressure at discharge (psi)

Gauge pressure needed at the outlet. 0 for open discharge.

Pump NPSHr at duty (ft, optional)

From the pump curve. Enables margin check.

How it's calculated

All calculations run in US customary units internally and are converted for display. Water properties are taken at the entered temperature.

Velocity: V (ft/s) = 0.4085 × Q (gpm) / d², d = pipe inside diameter (in).
Friction (Hazen-Williams): h_f (ft) = 0.002083 × L × (100/C)^1.852 × Q^1.852 / d^4.8655. L includes any added equivalent length. Valid for water at ordinary temperatures, turbulent flow.
Minor losses: h_m = ΣK × V²/2g, g = 32.174 ft/s². Each line uses its own velocity. Include an exit (K = 1.0) on the discharge to account for the velocity head leaving the pipe.
Total static head: static suction lift + static discharge head (flooded suction enters as a negative lift).
Pressure head: h = P (psi) × 2.3067 / SG.
TDH = total static + friction (suction + discharge) + minor losses (suction + discharge) + pressure head. Shown in psi/kPa as TDH × SG / 2.3067.
NPSHa = h_atm − static suction lift − h_f,suction − h_m,suction − h_vap, referenced to the pump centerline for an open (atmospheric) source.
Atmospheric pressure: standard atmosphere, P = 14.696 × (1 − 2.25577×10⁻⁵ × z_m)^5.2559 psia.
Vapor pressure: Antoine equation for water (1–100 °C), log₁₀ P_mmHg = 8.07131 − 1730.63 / (233.426 + T_°C).
Water SG vs 4 °C from a standard density–temperature fit.
Pipe IDs: HDPE IPS sizes use ID = OD × (1 − 2/DR) (minimum-wall basis, slightly conservative vs average ID). Steel and PVC use published schedule IDs. Hose uses nominal ID.

Warnings flag line velocity outside ~2–10 ft/s (0.6–3 m/s), suction velocity over ~6 ft/s, suction lift over 25 ft, NPSHa at or below zero, and NPSH margin under 10% / 3 ft of NPSHr (or NPSHa under 10 ft if no NPSHr is entered). These are rules of thumb; always confirm against the selected pump's curve.

Results are estimates for planning and training. Verify critical designs against manufacturer data and applicable codes.