A CFM calculator figures out how much airflow, in cubic feet per minute (CFM), you need for a room, bathroom fan, range hood, grow tent or HVAC system. Enter the room dimensions and the desired air changes per hour (ACH), and it calculates the required CFM.
The formula is CFM = (room volume in cubic feet × ACH) ÷ 60. A bedroom that’s 12 × 14 × 8 feet has a volume of 1,344 cubic feet. For 6 air changes per hour, CFM = 1,344 × 6 ÷ 60 = 134.4 CFM. A common guideline for bathroom exhaust fans is about 1 CFM per square foot of floor space for standard bathrooms, with larger bathrooms often sized by fixtures or volume.
Range hoods are often sized by the stove’s heat output or width; a frequently used rule is about 100 CFM per 10,000 BTUs for gas cooktops, or about 100 CFM per linear foot of cooktop width for wall-mounted hoods. Grow tents and workshops may need higher air exchange rates to manage heat, humidity and fumes.
Tips: longer duct runs, elbows and small ducts reduce actual airflow, so choose a fan with some extra capacity. Check local building codes, especially for range hoods, since very powerful hoods may require makeup air. Quiet fans are rated in sones; lower sones mean less noise.
Why use an online CFM Calculator: it saves time, avoids simple arithmetic mistakes and shows each step, so you can check your homework, plan a project or make a decision with confidence. Bookmark the page so it’s ready the next time you need it.
Tips for getting accurate results from the CFM Calculator: double-check that every input uses the right units, enter numbers without commas or symbols, and run the calculation a second time with a quick estimate in your head to confirm the answer looks reasonable.
Learn more on Wikipedia’s cubic feet per minute article. Find BTU, watt and square footage calculators on Minedit.