cfm to rpm Converter

Convert CFM to RPM or RPM to CFM for a fan from its rated airflow at a known speed using the fan law, or from a known volume per revolution.

• CFM To Voltage Calculator
• m3/h to cfm Converter
• mph to rpm Converter
• Flow to Velocity Calculator
• All Unit Converters

Calculator Functions

Function	What it does	What it requires
Rotational speed (RPM)	Finds the fan speed needed for a target airflow	Requires the actual airflow in CFM and fan data
Airflow (CFM)	Finds the airflow a fan delivers at a given speed	Requires the rotational speed in RPM and fan data
Effective volume per revolution	Finds how much air a fan moves in one turn	Requires the actual airflow in CFM and the rotational speed in RPM

Fan data can be given four ways: the fan’s rated CFM at a rated RPM (selected at first), a known volume per revolution, an illustrative example value from 0.02 to 1.00 ft³ per revolution, or an ideal model from a diameter and an advance per revolution. Answers can be rounded to 0 through 4 decimal places.

CFM to RPM Formulas

CFM and RPM are different kinds of quantity, so a conversion needs to know the fan. The first fan law supplies the link: for one fan, airflow changes in direct proportion to speed, so the new airflow is the old airflow times the new RPM divided by the old RPM [1]. Dividing a fan’s rated CFM by its rated RPM gives the volume it moves per revolution, and that ratio converts either way.

Rotational speed (RPM)

The main formula applies directly. A fan rated at 1,000 CFM at 1,200 RPM moves 0.8333 ft³ per revolution, so 500 CFM needs 500 ÷ 0.8333 = 600 RPM.

Airflow (CFM)

A fan rated at 2,000 CFM at 1,725 RPM delivers 2,000 × 1,440 ÷ 1,725 = 1,669.57 CFM when it turns at 1,440 RPM. With a known volume of 0.1 ft³ per revolution, 3,000 RPM is 300 CFM.

The ideal diameter × pitch choice works V out as a cylinder of air as wide as the fan, pushed forward by the advance per revolution, with both lengths in feet. A 12 in diameter and a 6 in advance give 0.3927 ft³ per revolution, or 565.49 CFM at 1,440 RPM. A real fan moves less than this, because the hub blocks part of the circle and air slips past the blades.

Effective volume per revolution

A fan measured at 500 CFM while turning at 1,000 RPM moves 0.500 ft³ per revolution at that operating point.

How Many CFM Is 1440 RPM or 3000 RPM?

A speed has no CFM value of its own. 1440 RPM is 1,669.57 CFM for a fan rated at 2,000 CFM at 1,725 RPM, and 3000 RPM is only 125 CFM for a small fan rated at 100 CFM at 2,400 RPM. The answer comes from the rated point on the fan’s nameplate or data sheet, entered under Airflow (CFM). The same holds in the other direction: 55 CFM is 550 RPM for a fan that moves 0.10 ft³ per revolution and 1,100 RPM for one that moves 0.05.

Limits of This Estimate

• The fan law scales one fan from a known operating point. It does not compare different fans, and it assumes the ducts, filters and air density stay the same.
• Speed changes more than airflow. Pressure rises with the square of the speed ratio and power with its cube [1], so a fan run faster may exceed its motor or its rated speed.
• The illustrative values from 0.02 to 1.00 ft³ per revolution are arithmetic examples, not ratings of any fan or blower type.
• The diameter × pitch model is an upper bound from geometry. Blade pitch alone does not set real airflow.

Sources