CFM Calculator

Ventilation moves air through a space measured in cubic feet per minute (CFM). Air changes per hour (ACH) describes how many times the room volume is replaced each hour. The formula CFM = volume (cu ft) × ACH ÷ 60 converts ACH targets into fan or duct flow rates for sizing exhaust fans, makeup air, or rough fresh-air estimates.

Residential guidance often targets 0.35–0.5 ACH for general whole-house ventilation, higher for kitchens and baths during exhaust operation. Bath fans commonly use 6 ACH as a sizing example while running; whole-house energy recovery ventilators use lower continuous rates. This is a room-level estimate — central systems use different metrics and controls.

Room volume is length × width × height. Standard 8-foot ceilings make a 12 × 14 ft bedroom 1,344 cubic feet. Vaulted ceilings need the actual average height, not floor area alone. Metric mode accepts meters and converts volume to cubic feet before applying ACH. Open floor plans share air with adjacent rooms — sizing a bath fan may still use the enclosed volume even when the door stays open during showers.

Worked example with defaults: 12 ft × 14 ft × 8 ft = 1,344 cu ft at 6 ACH → 1,344 × 6 ÷ 60 = 134.4 CFM. A 110 or 130 CFM bath fan bracket fits that ballpark. Kitchen range hoods often need 100–400+ CFM by cooktop type and duct run — check appliance manuals and local mechanical code.

Duct length, bends, and wall caps reduce delivered CFM versus fan nameplate rating. Straight, short duct runs with proper diameter hit rated flow; long flexible ducts do not. If measured airflow falls short, upsize duct or fan — not just the ACH assumption. For intermittent exhaust, run the fan long enough to complete the ACH target — a 130 CFM fan for 15 minutes clears a different volume than running continuously.

CFM applies to ventilation, heating, and cooling airflow — but HVAC duct sizing for conditioned air also depends on BTU load and velocity limits. Use this calculator for ventilation-oriented ACH math; pair with the duct size calculator when you know target CFM and velocity. Occupancy, pets, and humidity sources raise effective ventilation need even when floor area stays the same.

Limits: not a replacement for ASHRAE 62.2 whole-building ventilation calculations or local mechanical code. Commercial spaces, labs, and garages have separate exhaust requirements. Combustion appliances need dedicated makeup air analysis beyond this room formula.