Pump Flow Rate Calculator – Free Online Tool

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About this calculator

This calculator rearranges the pump power formula to estimate the flow rate a pump can deliver, given its shaft power, the head it must work against, and its efficiency — useful for a quick check of whether an existing pump and motor can meet a target flow at a given head.

Formula

Q (m³/hr) = (P × 367.3 × η) ÷ H

PShaft power — kW
HTotal dynamic head — metres
ηPump efficiency — 0–1
QFlow rate — m³/hr

Worked Example

Given: A pump has 5.83 kW shaft power, 70% efficiency, working against a 30m head.

Result: Q = (5.83 × 367.3 × 0.7) ÷ 30 = 49.9 m³/hr.

How to Use

Enter the pump's shaft power in kW.
Enter the total dynamic head it must pump against.
Enter its efficiency at that operating point.
Click Calculate to get the resulting flow rate.

Important Assumptions

Assumes water; for other liquids the actual flow will differ because power also depends on liquid density.
Real pump flow at a given head follows the manufacturer's performance curve — this calculator gives an estimate, not the exact curve value.

Common Mistakes

Treating this as equivalent to reading the manufacturer's pump curve — always check the actual curve for final selection, since efficiency itself varies with flow and head.

Frequently Asked Questions

Why might this differ from the pump's actual rated flow?

Efficiency changes across a pump's operating range, so a single efficiency value gives only an estimate — the manufacturer's performance curve is the authoritative source for exact flow at a given head.

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.