CFM Calculator
A 12 by 15 ft room with an 8 ft ceiling needs 144 CFM to reach 6 air changes per hour under the simple ACH model. This CFM calculator multiplies inside room length, width, and height to find air volume, then converts an air-changes-per-hour target into cubic feet per minute. Inputs can use feet or meters. The result is a theoretical room airflow, not a fan selection or complete HVAC design. Delivered airflow can fall below a product's free-air rating because of ducts, grilles, filters, bends, dampers, and static pressure. Confirm the target ACH and final equipment with the applicable code, standard, plan, or qualified professional.
Quick answer
CFM means cubic feet per minute of airflow.
What this tells you
- •CFM means cubic feet per minute of airflow.
- •ACH means how many times the full room air volume is replaced each hour.
- •The calculator finds room volume first, then divides the hourly airflow target across 60 minutes.
- •This is a room-airflow estimate, not a full duct or equipment sizing method.
- •Metric dimensions are converted with 3.280839895 feet per meter before the CFM calculation.
- •The metric output uses 0.028316846592 cubic meters per cubic foot.
- •Doubling room volume or ACH doubles the theoretical CFM when other inputs stay fixed.
- •The model assumes a rectangular volume and evenly mixed room air.
- •Target ACH must come from an appropriate design source because the calculator does not choose it.
How to Use
- 1Choose feet or meters before entering dimensions, and keep all three dimensions in that selected unit.
- 2Measure the inside length and width of the air volume served rather than the outside building footprint.
- 3Enter the average ceiling height. For a sloped or irregular room, first determine a defensible volume by another method.
- 4Enter a positive target ACH from the design brief, applicable standard, code requirement, or professional recommendation.
- 5Calculate to see room volume, hourly air volume, CFM, and cubic meters per minute.
- 6Check that the result represents supply, exhaust, or balanced ventilation as intended by the system plan.
- 7Allow for system resistance and verify delivered airflow through commissioning or field measurement when performance matters.
How It Works
Formula
Room Volume = Length x Width x Height
CFM = Room Volume x ACH / 60
Metric inputs are converted to feet before the airflow stepThe calculator converts every dimension to feet, then multiplies length x width x height for cubic feet. It multiplies volume by ACH to find cubic feet per hour and divides by 60 for CFM. For 12 x 15 x 8 ft, volume is 1,440 ft3. At 6 ACH, hourly air is 8,640 ft3 and airflow is 144 CFM. Metric volume and airflow are derived with 0.028316846592 m3 per ft3. The equation assumes steady flow and ideal room mixing.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
12 by 15 ft bedroom at 6 ACH
Room volume = 12 x 15 x 8 = 1,440 cubic feet. Airflow = 1,440 x 6 / 60 = 144 CFM.
4 by 5 by 2.4 m studio at 6 ACH
A 4 x 5 x 2.4 meter room has 48 m3 of volume. At 6 ACH, that is 288 m3/hour or 4.8 m3/min. Converting produces 169.5107 CFM before display rounding. Small decimal differences can result from displayed conversion precision.
20 by 24 ft garage at 8 ACH
Room volume = 20 x 24 x 10 = 4,800 cubic feet. Airflow = 4,800 x 8 / 60 = 640 CFM.
10 by 12 ft office at 4 ACH
Volume is 10 x 12 x 9 = 1,080 ft3. Multiplying by 4 gives 4,320 ft3/hour, and dividing by 60 gives 72 CFM. This only states the ideal ACH airflow and does not select a specific fan.
6 by 8 ft utility room at 10 ACH
The room contains 384 ft3. At 10 replacements per hour, the target is 3,840 ft3/hour. Dividing by 60 gives 64 CFM under the rectangular-room model.
CFM sensitivity for a 1,000 ft3 room
How changing ACH changes theoretical airflow when room volume stays at 1,000 cubic feet.
| Target ACH | Air per hour | Required CFM |
|---|---|---|
| 2 | 2,000 ft3 | 33.33 |
| 4 | 4,000 ft3 | 66.67 |
| 6 | 6,000 ft3 | 100 |
| 8 | 8,000 ft3 | 133.33 |
| 10 | 10,000 ft3 | 166.67 |
These are formula outputs only. They do not establish a suitable ACH for a particular occupancy or hazard.
From calculated CFM to delivered airflow
A fan rating may be measured at little or no external static pressure. Once air moves through a real system, resistance from filters, coils, grilles, duct length, fittings, and dampers can reduce the delivered flow. Use a fan curve at the expected pressure rather than matching only the headline CFM.
Room mixing also affects effectiveness. Short-circuiting between supply and return, dead zones, furniture, partitions, and poorly located exhaust points can leave parts of a room under-ventilated even when the measured total airflow matches the equation.
ACH is only one design method. Heating and cooling loads, contaminant source capture, moisture, occupancy, combustion safety, pressure relationships, and local requirements may call for another calculation or a higher controlling airflow.
Common mistakes
- Using floor area instead of room volume. CFM from ACH needs length, width, and height.
- Entering metric dimensions while the calculator is still set to feet, or the reverse.
- Using outside building dimensions instead of the inside air volume you want to condition or ventilate.
- Treating the ACH result as final equipment sizing without checking static pressure, duct losses, filters, or heat load.
- Selecting an ACH from a generic list without confirming the occupancy, process, hazard, or local requirement
- Using a fan's free-air rating as the airflow that will be delivered through a restrictive installed system
- Assuming one supply or exhaust point will mix the full room volume evenly
- Using one rectangular box for a room with large open connections, slopes, alcoves, or excluded volumes
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