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Plant Engineering Calculator

Updated Sep 18, 2026

Pump Impeller Trim Calculator — Flow, Head & Power Affinity

Estimate a preliminary trimmed-impeller operating point from diameter ratio using pump affinity relationships for flow, head and power.

Live calculationWorking shownPrivate in browser

Inputs

Results

Diameter ratio 0.900
Estimated flow 180.00 m³/h
Estimated head 48.60 m
Estimated power 32.81 kW
Diameter trim 10.0 %
Power change 27.1 % reduction
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Formula

r=D2/D1; Q2=Q1r; H2=H1r²; P2=P1r³.

Your calculation, step by step

These steps use the values currently entered above so the result can be checked independently.

  1. 1.Diameter ratio r = D₂ ÷ D₁ = 225.00 ÷ 250.00 = 0.9000.
  2. 2.Flow Q₂ = Q₁ × r = 200.00 × 0.9000 = 180.00 m³/h.
  3. 3.Head H₂ = H₁ × r² = 60.00 × 0.9000² = 48.60 m.
  4. 4.Power P₂ = P₁ × r³ = 45.00 × 0.9000³ = 32.81 kW.
  5. 5.Diameter reduction = (1 − 0.9000) × 100 = 10.0%; screening power reduction = 27.1%.

How to use this calculator

  1. Enter measured or design values using the units shown beside each field.
  2. Review the live results and the substituted calculation steps before using the estimate.
  3. Check the assumptions below, then continue to a related calculator when the next engineering decision needs another model.

Worked example

Trimming a 250 mm impeller to 225 mm gives r=0.90, screening flow at 90%, head at 81% and power at 72.9% of the reference point.

Assumptions and limits

  • Affinity scaling is a modest-trim approximation; actual trimmed curves must come from the pump manufacturer.
  • Large trims can materially change efficiency, NPSH and curve shape.
  • The affinity laws scale the pump curve; the actual operating point is still set by its intersection with the system curve, especially where static head is significant.

How to interpret the result

Treat the primary result as an engineering screening value, then use the secondary outputs to understand scale, energy or operating impact. Confirm site measurements, equipment limits and applicable standards before making a design or control change.

Questions engineers ask

Does an impeller trim calculation give the pump’s actual new operating point?

Not by itself. The diameter affinity relationships scale a known pump point or curve approximately. The actual operating point is where the trimmed pump curve intersects the system curve; static head can make that intersection depart materially from a simple scaled duty point.

How much impeller trimming can I estimate with affinity laws?

Treat diameter scaling as a modest-trim screening method. Current pump-calculator and engineering references commonly warn that accuracy degrades as the trim becomes large, often around the 10–20% diameter-reduction range depending on geometry. Use the manufacturer’s trim curve and minimum-diameter guidance for final work.

Why can power fall much faster than flow after a trim?

In the practical diameter approximation used here, flow scales with r, head with r² and power with r³. The cubic term makes the predicted power change larger than the flow change, but actual absorbed power also depends on efficiency and the real system operating point.

Continue the calculation

Use the next calculator in the same engineering workflow instead of restarting from search.

pump affinity lawspump system curvepump power
Methodology. MAXScanner shows the governing formula, entered values and calculation outputs so the estimate is auditable. The calculator is a screening aid; assumptions and equipment-specific limits remain part of the engineering review.