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Blower Speed Calculator

Work out blower speed instantly with clear inputs, formula shown and shareable results.

About Blower Speed Calculator

Because airflow is proportional to blower speed, the speed needed for a target airflow is simply the current speed scaled by the airflow ratio. On a belt drive the same ratio applies to the driving pulley diameter. The motor load, however, changes with the cube of the ratio, so a 20 percent airflow increase asks the motor for about 73 percent more power.

Formulas

Speed for target flow

New rpm = current rpm x (target CFM / current CFM)

Belt drive

New drive pulley diameter = current diameter x speed ratio; power multiplier = ratio³

Worked example

With the default values below, Blower Speed Calculator returns Blower speed required of 1,080. Change any input above and every figure here is recalculated in your browser.

Inputs used

Measured airflowminimum 1 CFM
1,000 CFM
Target airflowminimum 1 CFM
1,200 CFM
Current blower speedminimum 1 rpm
900 rpm
Current drive pulley diameterminimum 0.5 in
4 in

Results

Blower speed required
1,080 rpm
Speed change
1.2000
Motor load multiplier
1.728
Drive pulley diameter needed
4.80 in

Step by step

  1. 1Values used
  2. 2Speed for target flow
  3. 3Belt drive
  4. 4Blower speed required= 1,080 rpm
  5. 5Speed change= 1.2000
  6. 6Motor load multiplier= 1.728
  7. 7Drive pulley diameter needed= 4.80 in

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 Blower Speed figure worked out?

New rpm = current rpm x (target CFM / current CFM). Because airflow is proportional to blower speed, the speed needed for a target airflow is simply the current speed scaled by the airflow ratio.

Can the existing motor take the increase?

Check the multiplier against the motor service factor. A ratio above about 1.1 usually pushes a correctly sized motor past its nameplate rating.

What values do I need to enter?

This calculator takes 4 inputs: Measured airflow, Target airflow, Current blower speed, Current drive pulley diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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