Ventilation Calculator (ACH)

It works out how much airflow a ventilation system must move to refresh a room’s air often enough. Enter the room size and an air-changes-per-hour figure — or pick a room type for a typical one — and it returns the required airflow in CFM (L/s, m³/h), ready to size a duct or pick a fan.

Airflow (CFM) = room volume (cu ft) × air changes per hour ÷ 60. So a 13 × 10 × 8 ft bathroom (about 1,040 cu ft) at 8 ACH needs roughly 140 CFM — about 66 L/s or 240 m³/h.

It depends on what the room is used for. Bedrooms sit around 4 ACH, living rooms and offices around 6, kitchens and bathrooms about 8, restaurants and gyms about 10, and warehouses as low as 2. These are rules of thumb — the calculator presets a typical figure per room type, but your local building regulations set the number that actually applies.

Occupied spaces are often sized by fresh air per person instead — commonly around 20 CFM per person (roughly 10 L/s). For a meeting room or classroom, work out both the ACH figure and the per-person figure and design for the larger of the two, since a packed small room needs more air than its volume alone suggests.

The airflow figure is the same either way — what differs is the direction. Wet and smelly rooms (bathrooms, toilets, kitchens) are extract-ventilated so odours and moisture don’t spread; living and working spaces are supplied with fresh air. Balanced systems do both, and the fan or duct still needs to carry the airflow this calculator gives you.

Air-change rates are a sizing convention, not a law of physics — the right rate for your building comes from local regulations or standards such as ASHRAE 62.1, which also account for occupancy and pollutant sources. Use this to get the airflow in the right range and confirm the required rate before ordering fans or ductwork.