Air changes per hour (ACH) from the supply airflow and the room volume, with l/s per person and l/s per m² checks against a target.
Tender-stage only. Verify final design to project spec and CIBSE Guide A / B2.
For design guidance only. Always verify with a qualified engineer.
About this air change rate calculator
This free air change rate calculator works out how many times the air in a room is replaced each hour, the air change rate, or ACH, from the supply airflow into the space and the room volume. It also reports the ventilation rate as litres per second per person and litres per second per m² of floor, and back-calculates the airflow a chosen target ACH would demand. It is aimed at UK building-services engineers, ventilation designers and installers checking a design or a commissioning measurement against the regulations.
How the air change rate is calculated
The room volume is the floor area multiplied by the ceiling height, V = A × H. The air change rate is then the supply airflow divided by that volume, ACH = Q ÷ V (with Q in m³/h). Airflow converts to litres per second by dividing m³/h by 3.6 (1000 L ÷ 3600 s), and the required airflow for a target rate is the target ACH multiplied by the room volume. This follows the standard ventilation method in CIBSE Guide A and Guide B2. For occupiable office spaces, Approved Document F sets outdoor air at 10 l/s per person or 1 l/s per m² (whichever is greater) and a purge capability of at least 4 air changes per hour; for teaching spaces, Building Bulletin 101 requires a minimum daily average of 5 l/s per person.
Frequently asked questions
What is a good air change rate for a room?
It depends on use. Offices typically target around 4 to 6 air changes per hour, meeting rooms and classrooms more, and toilets or kitchens far higher. The l/s per person figure is usually the governing check for occupied rooms. Compare it against the Approved Document F or BB101 rate for your space type.
How do I convert m³/h to l/s?
Divide the airflow in cubic metres per hour by 3.6. This comes from 1 m³ being 1000 litres and one hour being 3600 seconds, so 1 m³/h = 1000 ÷ 3600 = 0.2778 l/s. The calculator does this conversion automatically.
How accurate is the result?
It is an indicative tender-stage check using the published CIBSE method. It assumes the supply airflow you enter is the actual rate reaching the room. Verify against measured air volumes, duct leakage and the as-built room dimensions before relying on it for compliance.