Ventilation gets specified two ways. Sometimes it is a flow — so many litres per second per person. Sometimes it is a rate — so many times an hour the whole room's air is replaced. Codes use both, often on the same page, and converting between them is the skill.

$\mathrm{ACH} = \dfrac{3600\,\dot{V}}{V_{room}}$ — read aloud A-C-H equals thirty-six hundred V-dot over V-room. $\mathrm{ACH}$ is the air changes per hour, a plain count with no units beyond “per hour”; $\dot{V}$ is the ventilation airflow in cubic metres per second; and $V_{room}$ is the room volume in cubic metres — length times width times height, and it does mean the whole room, ceiling void excluded unless it is open to the space. The 3600 is simply the seconds in an hour.

Working from a schedule in litres per second, two conversions have to happen and only one of them is obvious. Seconds climb to hours — that is the 3600 — and litres shrink to cubic metres, which is a thousand. Do both and the working shortcut is $\mathrm{ACH} = \dfrac{3.6\,\dot{V}}{V_{room}}$ with $\dot{V}$ in L/s. The classic slip is running only one ×60 — that answer comes out sixteen times too big and looks perfectly plausible on a drawing, which is exactly what makes it dangerous.

Numbers worth carrying as sanity rails: an office runs 4–6 ACH, an operating theatre 20 or more, a domestic bedroom under 1. If your arithmetic gives a bedroom 40 air changes an hour, the arithmetic is wrong, not the bedroom.