Parallel Pump Operation Calculator
Two pumps in parallel never deliver twice the flow of one — the system curve pushes back. Enter the pump point and system curve to find both operating points and the real gain.
Two Pumps in Parallel
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How the calculation works
Parabolic pump curve + quadratic system curve, identical pumps at constant speed
Pump curve: H = H0 − a·Q², with a = (H0 − Hr) ÷ Qr²
System curve: H = Hs + K·Q², with K = friction loss at rated flow ÷ Qr²
One pump: Q1 = √[(H0 − Hs) ÷ (a + K)], H1 = Hs + K·Q1²
Two pumps in parallel (flow doubles at each head): H = H0 − (a/4)·Q²
Two pumps: Q2 = √[(H0 − Hs) ÷ (a/4 + K)], H2 = Hs + K·Q2²
Flow gain = Q2 ÷ Q1 (always below 2.0)
Pump curve: H = H0 − a·Q², with a = (H0 − Hr) ÷ Qr²
System curve: H = Hs + K·Q², with K = friction loss at rated flow ÷ Qr²
One pump: Q1 = √[(H0 − Hs) ÷ (a + K)], H1 = Hs + K·Q1²
Two pumps in parallel (flow doubles at each head): H = H0 − (a/4)·Q²
Two pumps: Q2 = √[(H0 − Hs) ÷ (a/4 + K)], H2 = Hs + K·Q2²
Flow gain = Q2 ÷ Q1 (always below 2.0)
Notes & limitations
Parallel-pump curve addition (flows add at equal head) and the quadratic system curve H = Hs + KQ² are standard pump-engineering practice (Hydraulic Institute; Karassik’s Pump Handbook). The parabolic pump curve is an approximation — real curves are flatter near shutoff and steeper near runout.
- Shutoff head must exceed static head, or the pumps cannot move water at all. Typical centrifugal shutoff is 1.10–1.25 × rated head — confirm against the manufacturer’s curve.
- This assumes identical pumps at the same speed. Mismatched pumps in parallel can drive the weaker pump to shutoff or reverse flow — each pump needs its own check valve.
- A steep system curve (high K) gives small gains from paralleling; a flat, static-head-dominated system gives gains closer to 2×. The gain ratio tells you which case you have.
- Check NPSH available, motor loading, and runout at the two-pump point — the higher flow moves each pump right on its curve.
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