Formula & worked example
r=N2/N1; Q2=Q1r; ΔP2=ΔP1r²; P2=P1r³.
Reducing a fan from 1,500 to 1,200 rpm gives 80% airflow, 64% pressure and about 51.2% ideal shaft power.
HVAC design calculator · Updated Sep 17, 2026
Fan Affinity Laws Calculator — RPM, CFM, Pressure, Power & VFD Savings
Predict fan airflow, pressure and power after an RPM change, or reverse-solve RPM from target airflow. Includes percent changes, ideal energy savings and optional annual electricity-cost impact.
Inputs
Live result
Speed ratio 0.8 (-20% change)
New airflow 8,000 CFM
New pressure 320 Pa
New ideal power 2.56 kW
Ideal power change -48.8%
Annual energy difference 9,760 kWh
Annual cost difference 1,171.2 currency units
Assumptions & limits
- Base mode assumes the same fan diameter, air density and similar operating point.
- Verify large changes against the manufacturer fan curve and motor/VFD limits.
Frequently asked questions
What does this Fan Affinity Laws Calculator calculate?
Predict fan airflow, pressure and power after an RPM change, or reverse-solve RPM from target airflow. Includes percent changes, ideal energy savings and optional annual electricity-cost impact.
Can I use the result for final HVAC selection?
Use the result as a transparent engineering cross-check. Confirm final selections against measured design conditions, manufacturer data, applicable standards and local code.
Why are the assumptions shown?
HVAC shortcuts can look precise while hiding density, efficiency, free-area or pressure assumptions. MAXScanner keeps the governing inputs visible so the result can be audited.