Fan performance

HVAC Fan Law Calculator

Estimate CFM, pressure, and brake-power changes for the same fan on the same system when RPM changes.

Created · Updated

Scale a known fan operating point

Use measured or manufacturer-supported baseline values. Keep all airflow, pressure, and power inputs in one displayed unit system.

Enter values and calculate.

Estimated new airflowCFM or m³/h
Estimated new pressurein. w.g. or Pa
Estimated new brake powerhp or kW
Fan speed change%
Brake power change%

How to use this calculator

  1. Enter a known operating point for the same fan and system.
  2. Enter the proposed fan RPM after a sheave or speed-control change.
  3. Review all three predicted values and check the fan curve, maximum RPM, and motor capacity.

Formula

Q2 = Q1(N2/N1)
P2 = P1(N2/N1)²
BHP2 = BHP1(N2/N1)³

Worked example

Increasing the same fan from 800 to 880 RPM is a 10% speed increase. A 5,000 CFM, 1.5 in. w.g., 5 hp baseline scales to about 5,500 CFM, 1.815 in. w.g., and 6.655 hp.

Result interpretation

The 10% example shows the main field risk: airflow rises 10%, but estimated brake power rises about 33.1%. The estimate is useful for planning, not permission to change a sheave. A different system curve, density, fan efficiency, damper position, filter condition, or operating region can change the actual result.

Frequently asked questions

Do fan laws work for any speed change?

They are scaling relationships for the same fan under similar conditions. Larger changes demand especially careful review against actual fan curves and limits.

Can I use motor RPM instead of fan RPM?

Only for a direct-drive fan or when the drive ratio is unchanged. For a belt drive, calculate or measure fan shaft RPM.

Does static pressure always follow the square law in the field?

The relationship assumes the same system resistance curve. Changed dampers, filters, ductwork, or control behavior can move the operating point.

Related resources

Sources