CFM – Cubic Feet per Minute Calculator

How to use the calculator

1. Set Calculation Mode to Calculate CFM.
2. Enter the average air velocity in FPM (Feet per Minute).
3. Enter the cross-sectional opening size in Area, measured in ft².
4. Read the airflow under Result, in cfm. The result is rounded to two decimal places.

The Calculation Mode menu also lists Calculate FPM and Calculate Area. Both calculations require a known CFM value, but the current interface hides the CFM field. It also hides another required input in each of those modes. Consequently, the reverse modes may not produce a usable result from the visible fields. To work backward, use the rearranged equations in the formula block with your known values.

What CFM tells you about airflow

CFM is a volume flow rate: it describes how many cubic feet of air pass through a cross-section each minute. FPM describes speed, not volume. At the same average velocity, a larger opening carries more air; through the same opening, a higher average velocity carries more air. That is why both velocity and area are needed to calculate CFM.

The area in this equation is the cross-section perpendicular to airflow, such as the open face of a duct or vent. It is not the floor area of a room. For a rectangular opening, multiply its inside width by its inside height. For a circular opening, use \(\text{Area}=\pi d^2/4\), where \(d\) is the inside diameter. Measure dimensions in feet to obtain ft²; if dimensions are in inches, calculate the area in square inches and divide by 144.

A calculated CFM is the flow through the opening at the velocity you entered. It does not, by itself, establish how much ventilation a room needs. Required airflow depends on the application and may be governed by occupancy, equipment specifications, ventilation standards, or local requirements. Likewise, an airflow measured at one duct section should not automatically be treated as the flow delivered at every outlet.

Measurement tips and limitations

• Use average velocity. Air often moves faster near the center of a duct than near its walls. A single spot reading may not represent the full cross-section; several measurements across the opening can give a better average.
• Use the effective opening. For a duct, use its internal cross-sectional dimensions. A grille’s outside dimensions may overstate the space air can pass through because the frame and blades occupy part of the opening.
• Keep units consistent. The calculator expects fpm and ft². If you measured a length in meters, convert it to feet before calculating an area in ft². The meters-to-feet converter can help with that step.
• Do not divide by zero. Calculating FPM requires an area greater than zero; calculating area requires an FPM greater than zero. Zero velocity or zero area gives zero CFM in the forward equation.
• Allow for real-system losses. This equation relates velocity and area at the point of measurement. It does not predict fan performance or account for duct resistance, leaks, filters, or changes in air density. If you need to express a measured volume in another unit, use the volume unit converter; remember that CFM is a rate, so the time unit must be converted as well.