CFM Calculator

Calculate room airflow in cubic feet per minute and estimate duct area from air changes and velocity.

Calculate ventilation airflow

Enter room dimensions, required air changes per hour, and target duct velocity.

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Air Changes per Hour (ACH)

Target Air Velocity (ft/min)

About CFM and ventilation estimates

CFM means cubic feet per minute, a common measure of air volume moving through a fan, duct, or room. This calculator estimates ventilation airflow from room volume and air changes per hour. Room volume is length multiplied by width multiplied by ceiling height. Multiplying that volume by the desired hourly air-change rate and dividing by 60 converts the requirement to cubic feet per minute. A 2,400-cubic-foot room at six air changes per hour therefore needs 240 CFM under this simplified method. Air changes per hour describe how many room volumes are theoretically replaced in one hour. The appropriate rate depends on occupancy, contaminants, moisture, heat, process loads, and applicable mechanical or health standards. Storage spaces may need relatively little ventilation, while kitchens, workshops, restrooms, laboratories, and crowded rooms can need substantially more. Outdoor-air requirements based on people and floor area are not always equivalent to an ACH target, so use the method required for the project. The calculator also provides a preliminary duct area by dividing CFM by target air velocity. It converts that area into the diameter of a round duct with equal cross-sectional area. This is a geometric estimate only. Actual duct sizing considers friction loss, equivalent length, fittings, filters, coils, branch balancing, sound, leakage, static pressure, fan curves, and acceptable velocity ranges. A nominal duct size should be selected from available products only after pressure-loss calculations confirm system performance. Measured fan airflow is rarely identical to a nameplate free-air rating. Resistance in the installed system reduces delivery, while supply and exhaust imbalance can pressurize or depressurize a building. Heat gain and cooling load calculations may also produce an airflow requirement greater than the ventilation estimate. Use the highest applicable design requirement and ensure conditioned makeup air is considered. This tool is useful for early comparisons, exhaust-fan screening, and rough duct checks, but it does not replace HVAC design. Verify rates against current codes and standards, account for local climate and occupancy, and have a qualified professional specify equipment where life safety, combustion appliances, hazardous contaminants, or complex air distribution are involved.

CFM calculation examples

Room and air changes	Required airflow	Duct estimate
20 ft by 15 ft by 8 ft at 6 ACH	240 CFM	0.30 square foot duct area at 800 ft/min
30 ft by 20 ft by 10 ft at 8 ACH	800 CFM	1.00 square foot duct area at 800 ft/min
12 ft by 10 ft by 8 ft at 5 ACH	80 CFM	0.10 square foot duct area at 800 ft/min

How to calculate CFM

Measure the room length, width, and average ceiling height in feet.
Choose the air-change rate required for the occupancy and applicable standard.
Enter a preliminary duct velocity appropriate for the system.
Calculate airflow, then verify duct size and fan performance with a pressure-loss design.

Frequently asked questions

What does CFM measure?

CFM measures cubic feet of air moved each minute. It describes volumetric airflow rather than air speed or pressure.

How do I choose air changes per hour?

Use the rate required by the applicable building, mechanical, health, or process standard. The correct value depends on occupancy, pollutants, moisture, and room use.

Is more CFM always better?

No, excess airflow can waste energy, create noise, upset pressure balance, and cause drafts. Airflow should satisfy ventilation or conditioning needs while remaining balanced and controllable.

Can the equivalent diameter be used as the final duct size?

It is only a preliminary area conversion. Final sizing must account for friction, fittings, available static pressure, sound limits, leakage, and standard duct dimensions.

Why does installed fan airflow differ from its rating?

Many ratings are measured at a stated static pressure or under free-air conditions. Ducts, grilles, filters, dampers, and coils add resistance that changes the operating point.