Fan Laws (Flow/Head/Power/Speed) Calculator

Scale fans confidently using affinity laws linking flow head power and speed across systems. Compare two operating points by changing speed or diameter with consistent units and conversions. See real time results tables charts and exportable summaries instantly. Built for HVAC pumps blowers labs and plants. Trust accurate math with clear guidance every time.

Inputs

Base Point (1)

Volumetric flow rate at point 1.
Static pressure rise or head at point 1.
Shaft/absorbed power at point 1.
Rotational speed at point 1.
Impeller / fan wheel diameter at point 1.

Target Point (2)

New rotational speed.
New diameter (same as D1 if unchanged).

Results

Quantity Point 1 Point 2 Units Scaling
Flow (Q) — — — Q₂ = Q₁ · (N₂/N₁) · (D₂/D₁)³
Head / ΔP (H) — — — H₂ = H₁ · (N₂/N₁)² · (D₂/D₁)²
Power (P) — — — P₂ = P₁ · (N₂/N₁)³ · (D₂/D₁)⁵
Speed (N) — — rpm N₂/N₁ ratio drives all scaled outputs
Diameter (D) — — m D₂/D₁ enters with exponents 3, 2, 5 respectively
Results maintain the same units as inputs. Affinity laws assume similar geometry and similar efficiency.

Example Data

Q1H1P1N1D1N2D2Q2H2P2Units
2.05002.212000.5015000.50 ———m³/s, Pa, kW, rpm, m
45001.83.09000.4012000.45 ———CFM, inH₂O, kW, rpm, m
1206008.017500.3514500.35 ———L/s, Pa, kW, rpm, m

Formulas Used (Fan Affinity Laws)

For geometrically similar fans and comparable efficiency at the same fluid density:

  • Q₂ = Q₁ · (N₂/N₁) · (D₂/D₁)³   (Volumetric flow)
  • H₂ = H₁ · (N₂/N₁)² · (D₂/D₁)²   (Head / static pressure rise)
  • P₂ = P₁ · (N₂/N₁)³ · (D₂/D₁)⁵   (Power)

If diameter is unchanged (D₂=D₁), these reduce to: Q ∝ N, H ∝ N², P ∝ N³.

How to Use This Calculator

  1. Enter the base point values: Q1, H1, P1, N1, D1 and choose units.
  2. Enter the target speed N2. Toggle diameter change and set D2 if different.
  3. Click Compute. Scaled Q2, H2, P2 appear with the same units as inputs.
  4. Export your results using Download CSV or Download PDF.
  5. Use the example data to validate and learn expected scaling behavior.

FAQs

They assume similar efficiency between points. Real fans may shift efficiency with speed or diameter. Treat results as ideal scaling and verify against manufacturer curves.

These relations hold at the same fluid density. For large density changes, adjust head/power with density corrections or consult fan curves at actual conditions.

Set D2 = D1 or disable diameter change. Then the laws simplify to Q ∝ N, H ∝ N², and P ∝ N³ for quick speed-only scaling.

Accuracy declines far from the original operating region, near stall, surge, or choking, or when Reynolds number and geometry similarity assumptions break down.

No. Use it for preliminary estimates and “what‑if” comparisons. Final selection should reference certified fan curves and performance data at actual duty conditions.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.