Calculating CFM: Formula, Examples, and Units

To calculate cubic feet per minute (CFM), divide the volume of air moved, in cubic feet, by the time taken, in minutes: CFM = cubic feet ÷ minutes. Enter the volume and elapsed time to get an airflow rate; this calculator uses cubic feet and minutes and also shows the equivalent rate in cubic metres per hour.

Formula

CFM = V ÷ t, where V is air volume in cubic feet (ft³) and t is elapsed time in minutes (min). The result is cubic feet per minute (ft³/min, commonly written CFM). To convert that result to cubic metres per hour, multiply CFM by 1.6990108: 1 CFM is approximately 1.699 m³/h.

The calculation assumes the stated volume of air moves during the stated time. It is a flow-rate calculation, not a measurement of pressure or a guarantee that a fan or ventilation system will deliver that flow after installation.

Worked Example

Using the calculator’s defaults, suppose 600 ft³ of air moves in 2 minutes.

1. Divide volume by time: 600 ft³ ÷ 2 min = 300 ft³/min.
2. Convert to metric: 300 × 1.6990108 = 509.70324 m³/h.
3. Round each displayed result to two decimal places: 300.00 CFM and 509.70 m³/h.

What CFM Measures

CFM is a volumetric flow rate: it describes how much air volume passes a point, or is moved, per minute. It does not state the air’s mass, its speed at a particular spot, or the pressure needed to move it. Those properties may matter when selecting or evaluating equipment, but they are different measurements.

For example, 300 CFM means a flow of 300 cubic feet of air each minute under the conditions being considered. It does not mean that a room necessarily receives 300 CFM of fresh outdoor air: the flow could be recirculated, reduced by duct resistance, or measured at a different point in the system.

How to Calculate CFM in Different Situations

The basic formula works whenever you know a volume and the time required to move it. Make sure the volume and time describe the same flow event.

• From volume and time: Divide cubic feet by minutes. If a container holding 120 ft³ is filled or emptied in 30 seconds, convert 30 seconds to 0.5 minutes first; 120 ÷ 0.5 = 240 CFM.
• From cubic feet per hour: Divide by 60 to convert the hourly rate to CFM. For example, 18,000 ft³/h ÷ 60 = 300 CFM.
• From cubic metres per hour: Divide by approximately 1.699 to get CFM. For instance, 500 m³/h ÷ 1.699 ≈ 294.29 CFM.
• From a measured air speed: Multiply average speed by the opening’s area, using compatible units. With speed in feet per minute and area in square feet, the product is CFM. A single point reading may not represent the average speed across the opening.

For the speed-and-area method, CFM = average velocity (ft/min) × area (ft²). For a rectangular opening, area is width × height; for a circular opening, area is π × radius². Use the opening’s actual clear area, not an outside dimension that includes a frame or obstruction.

Choosing and Interpreting a CFM Figure

What airflow is appropriate depends on the task and the system. A room’s dimensions alone do not establish a universal CFM requirement: the intended ventilation rate, equipment, duct layout, pressure losses, and applicable project criteria can all affect the figure. Use the relevant design specification or equipment data rather than treating a rough calculation as a universal sizing rule.

When comparing a fan’s listed airflow with a calculated need, check whether the figures use the same units and represent comparable operating conditions. A free-air rating may not equal delivered airflow once the fan is connected to ducts, filters, grilles, or other resistance. The operating pressure and installation details matter, so a rated CFM should not automatically be assumed to be the installed system’s actual CFM.

Common Errors and Limits

• Mixing time units: The formula needs minutes for a CFM result. Convert seconds to minutes by dividing by 60, or hours to minutes by multiplying by 60.
• Mixing volume units: Do not divide cubic metres by minutes and label the result CFM. Convert the volume to cubic feet first, or report the result in metric units.
• Confusing area and volume: Square feet measure area; cubic feet measure volume. For a speed-and-area calculation, their product gives volume per time only when speed is in feet per minute.
• Rounding too early: Keep extra digits during conversion and round the final result to a practical precision. Small differences may simply reflect rounding.
• Assuming the flow is constant: If the rate changes over the measurement period, volume divided by total time gives the average flow, not the peak or instantaneous flow.

The simple volume-over-time formula cannot account on its own for changing air density, temperature, pressure, leaks, measurement error, or system resistance. For a reliable field measurement, the method and instrument should suit the duct or opening and the required accuracy.

Frequently Asked Questions

What is the formula for CFM?
CFM = air volume in cubic feet ÷ time in minutes. For example, 600 ft³ moved in 2 minutes is 300 CFM.

How do I convert CFM to m³/h?
Multiply CFM by approximately 1.699. To convert m³/h to CFM, divide by approximately 1.699.

Can I calculate CFM from air velocity?
Yes. Multiply average velocity in feet per minute by the opening area in square feet. The result is CFM; use an average velocity across the opening rather than assuming one point is representative.

Is CFM the same as air speed?
No. CFM is volume per unit time, while air speed is distance per unit time. To relate them, you also need the cross-sectional area through which the air moves.

Does a fan’s rated CFM equal its installed airflow?
Not necessarily. Ducts, filters, grilles, and other resistance can affect delivered airflow. Check the rating conditions and system information, or measure the installed flow when an accurate result is needed.

Related

• BPM and Semitone Calculator: Convert Tempo and Pitch
• Bowl Segmenting Calculator: Divide a Bowl Evenly
• Boiling Point Elevation Calculator and Formula

in v | Air volume | ft3 | 600 | 0
in t | Time | min | 2 | 0
out cfm | Airflow | CFM | v / t | 2
out m3h | Airflow | m³/h | cfm * 1.6990108 | 2