A pump is chosen by two numbers together: the flow you need (GPM) and the headit must produce to deliver that flow. Head is not just the height you lift the water — it is the height plus the friction the pipe adds plus any pressure needed at the outlet. Add those up and you have the total dynamic head.
Total dynamic head = lift + friction + pressure
Static lift is the vertical distance from the source water surface to the discharge point. Friction loss comes from the flow, pipe bore, length and material (calculated with the Hazen–Williams formula, plus an allowance for fittings). Pressure head converts any required outlet pressure at about 2.31 ft per psi.
Example: 40 ft (12 m) of lift plus 10 ft (3 m) of friction, feeding an open tank at no pressure, is a 50 ft (15 m) total head. Take “50 ft at your GPM” to the pump curve.
Why it’s called dynamic
The friction part grows with flow, so the same installation has a higher total head at 16 GPM than at 8. The head the pump feels while actually pumping — total dynamic head (TDH) — is what pump curves are drawn against, which is why you size with it rather than the static lift alone.
Pump Head Calculator
Enter the lift, pipe size, length and flow in GPM — it returns the total dynamic head in feet, the duty point and an estimated motor size.
Choosing from the curve
Find your duty point — the required flow at the calculated head — and pick a pump whose curve passes at or just above it, ideally near the middle where efficiency peaks. Never buy on the max head printed on the box: that is the head at zero flow. A pump advertising 130 ft (40 m) max might deliver only a trickle at 100 ft (30 m).
Pipe size changes everything
Friction loss rises with roughly the fifth power of shrinking diameter — going one pipe size down can triple or quadruple the friction head at the same flow. If friction dominates your total, upsizing the delivery pipe is almost always cheaper than buying a bigger pump and paying its running cost forever.
Suction lift and boreholes
A surface pump can only draw water up about 25 ft (7 m) of suction lift in practice — atmospheric pressure sets the limit — so deeper sources need a submersible or a pump lowered to the water. For wells and boreholes, measure the lift from the drawdown level while pumping, not the pump’s installed depth, and have the NPSH requirement checked on deep or hot installations.