Room Ventilation (air changes per hour) Calculator

Understanding the room ventilation calculator — concept and principle

This calculator works out how much air a fan must move to replace the air in a room a given number of times per hour. Enter the room volume, the target air changes per hour (ACH) and the rated flow of a fan, and it returns the required airflow in m³/h and L/s, plus the ACH your chosen fan would deliver. People use it when sizing a bathroom or kitchen extractor, ventilating a workshop or checking whether an existing fan is big enough.

Six roomfuls of 40 m³ an hour is 240 m³/h; the 150 m³/h fan manages only 3.75.

How it is calculated

Airflow needed Q = V × ACH, with V the room volume in m³ and ACH the number of air changes per hour. The result is in m³/h.
L/s = Q ÷ 3.6, because 1 m³/h = 1,000 L ÷ 3,600 s.
ACH with this fan = fan flow ÷ V.

Room volume is length × width × ceiling height. A 4 m × 4 m room with a 2.5 m ceiling is 40 m³.

The principle

ACH describes ventilation as a rate of dilution. If a fan moves a volume of air equal to the room every hour, that is one air change per hour. Because fresh air mixes with the existing air instead of pushing it out like a piston, one air change does not remove all contaminants; with good mixing it removes roughly 63% (1 − e⁻¹). This is why moisture-heavy or odour-heavy rooms such as bathrooms and kitchens need several air changes per hour, while living spaces need far fewer. Typical design ranges are about 6–10 ACH for bathrooms, 10–15 ACH for kitchens while cooking and 2–4 ACH for living rooms.

Worked example

Inputs: room volume 40 m³, target 6 ACH, fan rated 150 m³/h.
Airflow needed: 40 × 6 = 240 m³/h.
In litres per second: 240 ÷ 3.6 = 66.7 L/s.
ACH that the fan provides: 150 ÷ 40 = 3.75 ACH.
The fan falls short of 6 ACH; you would need at least 240 m³/h.

Each point is the calculator re-run with only “Room volume (m³)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Room volume (m³)	Airflow needed (m³/h)
10	60
20	120
30	180
40	240
50	300
60	360
80	480

How to read the result

Compare the fan's ACH with your target. If it is lower, the room will clear moisture and smells slowly; condensation and mould are common signs of under-ventilation. Remember that the catalogue rating is usually measured with no ducting. Long ducts, bends, grilles and filters create resistance, and the real flow can be much lower, so choose a fan with some margin and check its performance curve at the expected pressure.

Common mistakes

Using floor area instead of volume, which ignores ceiling height.
Mixing units: m³/h, L/s and CFM differ (1 L/s = 3.6 m³/h; 1 CFM ≈ 1.70 m³/h).
Forgetting that extracted air must be replaced; without an inlet such as a door gap or vent, the fan cannot reach its rated flow.

Keep in mind: ACH targets are rules of thumb. Building regulations in many countries set minimum extract rates in L/s per room instead, and fuel-burning appliances have separate air-supply rules. Real designs should follow local codes and, where needed, a qualified ventilation engineer.

Frequently asked questions

How do I convert CFM to m³/h?

Multiply CFM by about 1.699. A 100 CFM fan moves roughly 170 m³/h.

Is a higher ACH always better?

Not necessarily. Very high rates can cause draughts, noise and heat loss in winter; heat-recovery ventilation helps when high rates are needed.

Sources: ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings · ASHRAE Handbook — Fundamentals, chapter on ventilation and infiltration

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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