Fan Affinity Speed Calculator
CFM, pressure and power from a fan speed change.
Enter your values
ⓘ Live results
Results
- New airflow
- 9,333 CFM
- New static pressure
- 2.042 in wc
- New brake horsepower
- 7.94 hp
- Speed ratio
- 1.167
- Power change
- 58.8%
Quick answer
Airflow scales linearly with speed, pressure with the square of speed, and power with the cube of speed.
CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)^2; BHP2 = BHP1 x (N2/N1)^3
How to use the Fan Affinity Speed Calculator
- 1Enter the fan's current CFM, static pressure and brake horsepower at its current speed.
- 2Enter the current and new RPM.
- 3Read the new CFM, pressure and power computed from the affinity laws.
Worked example
Using the values the calculator loads with:
Inputs
- Current CFM8000 CFM
- Current static pressure1.5 in wc
- Current brake HP5 hp
- Current RPM1200 rpm
- New RPM1400 rpm
Results
- New airflow9,333 CFM
- New static pressure2.042 in wc
- New brake horsepower7.94 hp
- Speed ratio1.167
- Power change58.8%
Frequently asked questions
Why does power rise so fast?
Power follows the cube law, so a modest speed increase produces a much larger power increase.
Do the fan laws always apply?
They assume the system curve and air density stay the same and the fan operates in its normal range.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.