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Fan Laws Calculator

Work out fan laws instantly with clear inputs, formula shown and shareable results.

About Fan Laws Calculator

The fan laws state that for a fixed system, airflow varies with speed, pressure with the square of speed and power with the cube of speed. The cube relationship is why trimming a fan by 20 percent saves nearly half its energy, and why speeding one up by 20 percent nearly doubles the motor load.

Formula

Fan laws

CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)²; P2 = P1 x (N2/N1)³

N
Fan rotational speed

Worked example

With the default values below, Fan Laws Calculator returns New airflow of 5,833. Change any input above and every figure here is recalculated in your browser.

Inputs used

Original fan speedminimum 1 rpm
1,200 rpm
New fan speedminimum 1 rpm
1,400 rpm
Original airflowminimum 1 CFM
5,000 CFM
Original static pressureminimum 0.01 in.wg
1.2 in.wg

Results

New airflow
5,833 CFM
New static pressure
1.633 in.wg
Speed ratio
1.1667
Power multiplier
1.5880

Step by step

  1. 1Values used
  2. 2Fan laws
  3. 3New airflow= 5,833 CFM
  4. 4New static pressure= 1.633 in.wg
  5. 5Speed ratio= 1.1667
  6. 6Power multiplier= 1.5880

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Fan Laws figure worked out?

CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)²; P2 = P1 x (N2/N1)³. The fan laws state that for a fixed system, airflow varies with speed, pressure with the square of speed and power with the cube of speed.

Do the fan laws hold if I change the ductwork?

No. They apply only when the system curve is unchanged. Adding dampers or duct alters the resistance and moves the operating point off the original curve.

What values do I need to enter?

This calculator takes 4 inputs: Original fan speed, New fan speed, Original airflow, Original static pressure. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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