Ventilation Rate Calculator
The Ventilation Rate Calculator works out the required outdoor (fresh) air supply for a space from the number of occupants, an air rate per person, and optionally a floor area with a per-area rate. It reports the rate in litres per second and cubic metres per hour, and — if you give the room volume — the air changes per hour (ACH).
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the number of occupants and, if needed, change the per-person outdoor air rate (default 10 L/s per person).
- Optionally add a floor area and a per-area rate (L/s/m²) if your ventilation standard includes a building-fabric term.
- Optionally enter the room volume in cubic metres to also get the air changes per hour.
How it works
Required outdoor air (L/s) = occupants × per-person rate + floor area × per-area rate. Multiplying by 3.6 converts L/s to m³/h. Air changes per hour = (L/s × 3.6) ÷ room volume, which tells you how many times the whole room's air is replaced each hour.
Worked example
Worked example. For 20 occupants at 10 L/s per person: 20 × 10 = 200 L/s, which is 200 × 3.6 = 720 m³/h. In a 300 m³ room that is 720 ÷ 300 = 2.4 air changes per hour.
Common mistakes
- Mixing up L/s per person with m³/h per person — this tool takes the per-person rate in litres per second.
- Forgetting the per-area term when the applicable standard requires it, which understates the required fresh air.
- Confusing supply air (total flow through the system) with outdoor air — this calculator sizes the outdoor/fresh-air component only.
Frequently asked questions
What per-person rate should I use?
10 L/s per person is a common general-purpose figure, but the correct value depends on the space type and the standard in force (for example AS 1668.2 or ASHRAE 62.1), which may specify anything from about 7.5 to 15+ L/s per person plus a per-area allowance.
What is a good number of air changes per hour?
It depends entirely on the use: offices might target a few ACH, while kitchens, labs or clean spaces need many more. ACH is an outcome of the required flow and room volume, so use your standard's flow requirement rather than picking an ACH target in isolation.
Related tools
- Air Changes Per Hour Calculator
- Air Conditioner BTU Calculator
- Heat Load Rough Estimator
- Extractor Fan Size Calculator
- BTU to kW Converter
- Duct Velocity Calculator
Explore more in HVAC, Plumbing, Water & Building Services.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.