Skip to content
Calculation Machine

Fan Laws Calculator

New CFM, pressure and power from a speed change.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
New airflow
1,200 CFM
New static pressure
0.72 in w.c.
New power
1.296 hp
Speed ratio
1.2
Power change
73%

Working

  1. 1ratio 1.2: flow x1.2, pressure x1.44, power x1.728

Quick answer

Airflow scales with speed, pressure with speed squared and power with speed cubed.

The formula

Q2 = Q1 x (N2/N1); P2 = P1 x (N2/N1)²; W2 = W1 x (N2/N1)³

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Current airflow1000 CFM
  • Current speed1000 RPM
  • New speed1200 RPM
  • Current static0.5 in w.c.
  • Current power0.75 hp

Result

  • New airflow1,200 CFM
  • New static pressure0.72 in w.c.
  • New power1.296 hp
  • Speed ratio1.2
  • Power change73%

How to use the Fan Laws Calculator

  1. 1Enter the current operating point — airflow, speed, static and power.
  2. 2Enter the new fan speed or pulley RPM.
  3. 3Check the new power against the motor's rating before making the change.

Frequently asked questions

Why does 20% more air cost 73% more power?

Power follows the cube of speed, so small airflow gains are expensive.

Do the laws work for slowing a fan?

Yes, and that is where they shine — dropping 10% speed cuts power by about 27%.

About this calculator