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A CFM calculator works out the airflow — in cubic feet per minute — a room needs for proper ventilation, from its size and how many times an hour the air should change. It is how you size a bathroom exhaust fan, a kitchen or workshop extractor, or a supply register.
Under-sized fans leave moisture and odours behind; this gives the target CFM from the room volume and a sensible air-change rate for the room type, so you buy a fan that actually clears the air.
CFM = room volume (length × width × height) × air changes per hour ÷ 60. Air-change targets vary by room — roughly 6/hour for living areas, 8 for bathrooms, 10-15+ for kitchens and laundries, 20+ for workshops. Size the fan at or above the result and keep duct runs short and smooth.
A 12 × 10 ft bathroom with an 8 ft ceiling (960 ft³) at 8 air changes per hour needs about 128 CFM.