Home office, door shut
12 m², 2.4 m ceiling, 1 person at a desk, 0.5 ACH
Settles near 1,700 ppm · crosses 1,000 ppm in about 75 minutes
Free tool
How fast does your room fill up?
Enter a room's dimensions, how many people are in it and roughly how well it is ventilated. This works out where the CO₂ concentration settles and how long it takes to get there. No sign-up, and the model is written out below so you can check it.
How quickly does CO₂ build up in a closed room?
Faster than most people expect. One adult in a closed 12 m² office at half an air change per hour crosses 1,000 ppm in about 75 minutes and settles near 1,700 ppm. A four-person meeting room in the same building crosses 1,000 ppm in under 30 minutes. Opening a window reverses it in ten to fifteen.
Your room
12 m²
2.4 m
1 person
Air changes per hour. Real buildings vary enormously; these are starting points, not measurements of your space.
Room volume 29 m³
Settles at
1,675ppm
Impaired
- To 1,000 ppm
- 1 h 14 min
- To 1,400 ppm
- 3 h 2 min
- To 2,500 ppm
- never reaches
Single-zone mass balance assuming instant mixing and constant occupancy, starting from an outdoor baseline of 425 ppm. Real rooms mix imperfectly and their air change rate moves with wind and temperature — so treat this as the shape of the problem, not a measurement of your room.
Worked examples
Four rooms you will recognise.
Every figure quoted anywhere on this site comes out of this model, so you can reproduce them above.
Bedroom overnight
12 m², 2.4 m ceiling, 1 person sleeping, 0.2 ACH
Settles near 2,500 ppm · crosses 1,400 ppm in about 2 hours
Four-person meeting room
20 m², 2.7 m ceiling, 4 people seated, 0.5 ACH
Settles above 3,000 ppm · crosses 1,000 ppm in under 30 minutes
The same office, window open
12 m², 2.4 m ceiling, 1 person at a desk, 4 ACH
Settles near 580 ppm · never reaches 1,000 ppm
The model
Stated in full, because you should be able to check it.
This is the standard well-mixed single-zone mass balance. Concentration approaches a steady state set by how fast CO₂ is generated and how fast the room exchanges air with outside:
C(t) = Cout + (G / (n·V)) · (1 − e−n·t) + (C0 − Cout) · e−n·t
G is CO₂ generation in m³/h, n is air changes per hour, V is room volume in m³, and Cout is the outdoor baseline, taken as 425 ppm.
Generation scales with metabolic rate at roughly 0.015 m³/h per met — a seated adult at 1.2 met produces about 0.018 m³/h, which is the 0.005 L/s figure used in ventilation standards. Getting that unit conversion wrong by a factor of 3.6 is the most common way this calculation goes astray.
Two assumptions worth naming: the room is treated as perfectly mixed, and occupancy is constant. Real rooms have gradients, and their air change rate moves with wind, temperature and whether a door happens to be ajar. This gives you the shape and the order of magnitude — which is exactly why the honest conclusion is to measure rather than model.
A model tells you what should happen. An instrument tells you what did.
No payment, no card details — an email address and nothing else. Or read what each concentration means first.