Work out blower speed instantly with clear inputs, formula shown and shareable results.
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.
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
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.
Check the multiplier against the motor service factor. A ratio above about 1.1 usually pushes a correctly sized motor past its nameplate rating.
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.