Air Changes Per Hour (ACH) Calculator

How this is worked out

Air changes per hour (ACH) is the ventilation airflow rate divided by the room's volume: ACH = airflow (m³/h) ÷ volume (m³). It's a simple bulk-flow measure — it tells you how many room-volumes of air move through per hour, not whether the air is actually well mixed (poor air distribution can leave stagnant pockets even at a high ACH figure). Target ACH figures vary hugely by room type: a bedroom might aim for 1–2, a kitchen or bathroom with an extractor fan considerably more, and specialist spaces like laboratories or hospital rooms much more still — this tool doesn't recommend a target, since the right figure depends on the room's purpose and local building regulations.

Frequently asked questions

Does a higher ACH always mean better air quality?

Generally yes for diluting contaminants and moisture, but it comes with a cost in heating/cooling energy (more air exchange means more conditioned air lost) and, for mechanical ventilation, fan noise and running cost — real designs balance ACH against those factors rather than maximising it.

Can I use this for natural ventilation (open windows)?

Only roughly — natural ventilation's airflow rate depends on wind and temperature differences and isn't a fixed number the way a fan's rated output is, so this tool is most accurate for mechanical ventilation systems with a known airflow rating.

Related tools: HVAC / BTU Calculator, Surface Area & Volume Calculator, Wind Chill & Dew Point Calculator.

Source / Last verified: standard ventilation-engineering definition (ACH = airflow ÷ room volume) — general HVAC practice, read/checked 1 October 2026. This tool doesn't recommend a target ACH for any specific room type, since that depends on use and local regulations.