Feet Per Minute to CFM Converter

Convert FPM to CFM

Feet Per Minute to CFM Formula

Formula

\[Q=v\times A_{\mathrm{ft}^2}\]

Where:

\(Q\) — volumetric airflow in cubic feet per minute (CFM)
\(v\) — air velocity in feet per minute (ft/min)
\(A_{\mathrm{ft}^2}\) — cross-sectional area in square feet (ft²)

Airflow equals the distance air travels per minute multiplied by the area it passes through. If you select another area unit, the calculator converts the area to square feet before multiplying.

Feet Per Minute to CFM – Calculation Example

Example: air moves at 300 ft/min through an opening with an area of 1 ft², matching the calculator’s default inputs.

\[Q=300\ \mathrm{ft/min}\times1\ \mathrm{ft}^2=300\ \mathrm{ft}^3/\mathrm{min}\]

Result: the airflow is 300 ft³/min, or 300 CFM.

How to use the calculator

Enter the measured or specified air speed in Velocity. The field uses feet per minute (ft/min).
Enter the area perpendicular to the airflow in Area. For a rectangular duct, multiply its inside width by its inside height; for a round duct, use the area of its inside opening.
Choose the matching Unit for the area: ft², in², m², cm², or mm². The selected unit applies to the number you entered in Area.
Read Cubic Feet per Minute (CFM). The displayed result is rounded to two decimal places.

What feet per minute and CFM tell you

Feet per minute (FPM) measures air velocity: how far the air travels in one minute. Cubic feet per minute (CFM) measures volumetric flow: how much space the moving air occupies as it passes through an opening each minute. Velocity alone cannot determine CFM because the same air speed can move different volumes through different-sized openings.

For example, at 300 ft/min, a 1 ft² opening carries 300 CFM, while a 2 ft² opening carries 600 CFM. At a fixed area, doubling the velocity also doubles the calculated flow. This relationship helps when interpreting duct measurements, checking ventilation airflow, or comparing velocities at openings of different sizes.

The area must be the cross section through which the measured air is flowing, not the floor area of the room being served. A duct’s inside dimensions may differ from its nominal dimensions. If your measurements are metric, you can use the corresponding area unit directly; for a linear dimension that needs converting, the meters to feet converter may help. Convert each length before calculating an area, or calculate the area in metric units and select the matching unit here.

Area conversions used by the calculator

The formula requires square feet. The calculator applies the following conversions to the entered Area before multiplying by Velocity:

Selected unit	Area in square feet
ft²	Entered area
in²	Entered area ÷ 144
m²	Entered area × 10.7639
cm²	Entered area ÷ 929.03
mm²	Entered area ÷ 92,903

For instance, 72 in² is 0.5 ft². At 300 ft/min, the calculated flow is \(300\times0.5=150\) CFM. If you are working with other area units, convert them to one of the five supported choices before entering the value. The land area unit converter can help with an area expressed in another unit, although duct and vent openings are usually much smaller than land areas.

Assumptions and practical limits

This calculation treats the entered velocity as representative of the entire entered area. In a real duct, air velocity often varies across the cross section and near the walls. A single reading at the center may not equal the average velocity. For a more reliable airflow estimate, use an appropriate measurement method to determine average velocity across the opening.

For a grille, screen, or other partly blocked opening, distinguish its overall face area from its effective open area. Use the area that corresponds to where and how the velocity was measured; mixing a velocity measured through the openings with the grille’s full face area can overstate airflow. An air-handling system’s actual flow can also change with filters, dampers, and pressure losses, so this result is not a prediction of fan performance.

Both inputs must be zero or greater. Entering zero for either velocity or area produces zero CFM. The calculator reports volumetric airflow at the conditions of the measurement; it does not calculate air mass flow or adjust for changes in air density, temperature, or pressure.

Frequently Asked Questions

How do I calculate CFM for a round duct using its diameter?

Use the duct’s inside diameter to find its cross-sectional area, then multiply by air velocity. If diameter \(d\) is in inches and velocity \(v\) is in ft/min, the shortcut is \(\mathrm{CFM}=v\pi d^2/576\). For a 12-inch duct at 500 ft/min, this gives about 393 CFM. Measure the inside diameter rather than using a nominal duct size.

How can I calculate air velocity if I know CFM?

Divide CFM by the opening’s area in square feet: \(v=Q/A\). For example, 600 CFM through a 0.75 ft² opening gives 800 ft/min. This is the average velocity implied by those values, not necessarily the speed at every point in the opening. Use the same cross section for both the airflow and area figures.

What is a good air velocity in feet per minute?

There is no single good FPM value for every duct or vent. An acceptable velocity depends on the system design, duct size, noise limits, pressure loss, and whether you are measuring a duct or a room outlet. Higher velocity can increase noise and resistance. Compare your reading with the equipment specifications or the ventilation design target rather than treating one number as universal.

How do I convert CFM to cubic meters per hour?

Multiply CFM by approximately 1.699 to get cubic meters per hour: \(1\ \mathrm{CFM}\approx1.699\ \mathrm{m^3/h}\). Thus, 300 CFM is about 510 m³/h. To convert ft/min to m/s instead, multiply by 0.00508. These are different conversions: a velocity value cannot become a volumetric airflow value without an opening area.

Can I use CFM to calculate air changes per hour?

Yes, if the CFM represents airflow into or out of the space being evaluated. Divide the room’s volume in cubic feet into the hourly airflow: \(\mathrm{ACH}=60\times\mathrm{CFM}/\mathrm{room\ volume}\). A 1,000 ft³ room receiving 100 CFM has a nominal 6 ACH. Actual ventilation effectiveness also depends on where air enters and leaves and whether the airflow is outdoor air or recirculated air.

How do I find total CFM across several vents?

Calculate CFM for each vent separately, then add the results. A vent carrying 120 CFM and another carrying 180 CFM provide 300 CFM together. Do not add their velocities and multiply by their combined area unless the velocities are the same. If you need one representative velocity, divide the total CFM by the combined cross-sectional area.

What if air flows in the opposite direction?

This calculator accepts nonnegative velocity, so its result describes the amount of airflow, not its direction. If you are tracking supply and exhaust together, label each measured flow by direction before combining them. For a signed net flow, choose a convention—for example, supply positive and exhaust negative—and subtract exhaust CFM from supply CFM outside the calculator.

Feet Per Minute to Cubic Feet Per Minute Converter