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Pump Affinity Law: Flow vs Impeller Diameter

Q2Q1=D2D1\frac{Q_2}{Q_1} = \frac{D_2}{D_1}
sig figs

Worked example: 250 mm impeller at 60 m3/h; 48 m3/h needs a 200 mm trim. Press Try an example to run it live, then adjust anything.

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Pump Affinity Law: Head vs Impeller Diameter: Both use impeller diameter.
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Affinity with impeller trim →

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Pump Affinity Law: Flow vs Impeller Diameter explained

D1D2Q1Q2

When a pump is chronically oversized and the speed is fixed by a across-the-line motor, the shop answer is to put the impeller on a lathe. Take a 10 in impeller passing 400 gpm down to 9.5 in and you get 400 × 0.95 = 380 gpm at the same rpm. Machinists have been doing this since the 1920s, and manufacturer curves still print the trim range as a family of nested lines on one chart.

The trap: the diameter laws are an approximation, not physics. They hold well for trims within about 10–15% of full diameter; cut deeper and the vane tips no longer match the volute, efficiency falls several points, and the real flow lands short of prediction. Most manufacturers void the efficiency guarantee below roughly 80% of maximum diameter, and a trim is irreversible: you cannot weld the metal back on.

Pump Affinity Law: Flow vs Impeller Diameter formula

Q2Q1=D2D1\frac{Q_2}{Q_1} = \frac{D_2}{D_1}
Where
  • Q1Q_1= Flow at diameter 1 (L/min)
  • D1D_1= Impeller diameter 1 (mm)
  • Q2Q_2= Flow at diameter 2 (L/min)
  • D2D_2= Impeller diameter 2 (mm)
In this toolkit

The pump and fan affinity laws

7 formulasHow flow, head and power scale with impeller speed and diameter — the first, second and third power relationships behind every VFD retrofit.

Revised 16 October 2026
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formula.expert. (2026, October 16). Pump Affinity Law: Flow vs Impeller Diameter Calculator. https://formula.expert/solvers/pump-affinity-flow-diameter

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