// tool 02 · carbon dioxide
CO₂ Build-up Calculator
People breathe out CO₂; ventilation clears it. The race between the two decides how fast a closed room turns stuffy. Set the room, the crowd and the airflow to see how many minutes it takes to cross 1,000 ppm, where the air finally settles, and how much fresh air it would take to keep up.
0.5 air changes / hour ≈ trickle vents, door shut
// 1000 ppm is a ventilation proxy, not a health limit (Persily 2021)
The physics
A room fills up like a leaky bucket. Occupants pour CO₂ in at a steady rate while ventilation drains it, so the level rises quickly at first and then eases toward a ceiling. How fast it climbs is set by the air changes per hour:
Where it finally stops is the steady state, the level at which fresh air removes CO₂ exactly as fast as people produce it:
Generation rate G comes from Persily's work: an office worker at 1.2 met produces about 0.0052 L/s of CO₂, and the harder you breathe, the more you make. The cognitive-impact research is why people care: a 2023 meta-analysis found complex task performance drops as CO₂ climbs through the 1,000 to 1,500 ppm range that ordinary meeting rooms reach easily.
The 1000 ppm myth: no current ASHRAE standard contains an indoor CO₂ limit, and hasn't for nearly 30 years. The 1,000 ppm figure is a useful ventilation proxy, a sign you're rebreathing air, not a health threshold for CO₂ itself (Persily 2021).
Frequently asked questions
How long does it take for CO₂ to build up in a room?+
It depends on how many people are in the room, how big it is, and how much fresh air comes in. A small, sealed meeting room with several people can climb past 1,000 ppm in well under half an hour, while the same room with a window open may never get there. This calculator estimates the time to 1,000 ppm from a fresh start so you can compare scenarios.
Is 1,000 ppm of CO₂ dangerous?+
No. 1,000 ppm is not a health limit for CO₂ itself. No current ASHRAE standard sets an indoor CO₂ limit, and hasn't for nearly 30 years. The figure is a useful ventilation proxy: when indoor CO₂ is well above outdoor levels, it means you are rebreathing air and fresh-air supply is low. CO₂ only becomes a direct health concern at far higher concentrations.
What is a good CO₂ level indoors?+
Outdoor air sits around 420 ppm. Indoors, below roughly 800 ppm is generally considered well ventilated, 800 to 1,000 ppm is typical for an occupied room, and sustained readings above about 1,400 ppm suggest ventilation is not keeping up. These are comfort and ventilation guides, not strict safety thresholds.
How much ventilation do I need to keep CO₂ down?+
ASHRAE 62.1 suggests on the order of 7.5 litres per second of outdoor air per person for offices. The calculator shows the fresh air per person your settings provide so you can compare. In practice, the more people and the smaller the room, the more air changes per hour you need to hold CO₂ steady.
Does opening a window actually lower CO₂?+
Yes. Opening a window or door increases the air changes per hour, which both slows the build-up and lowers the level CO₂ settles at. The effect depends on the size of the opening, indoor and outdoor temperature difference, and wind, so a cracked window helps less than one wide open. You can see the difference by raising the ventilation slider.
Sources
- Persily & de Jonge: Carbon dioxide generation rates for building occupants, Indoor Air (2017)
- ASHRAE Standard 62.1-2022: Ventilation for Acceptable Indoor Air Quality
- Persily: Quit Blaming ASHRAE Standard 62.1 for 1000 ppm CO₂ (2021), PMC8596488
- Meta-analysis: CO₂ exposure and cognitive performance (2023), ScienceDirect
- ASTM D6245: Standard Guide for Using Indoor CO₂ Concentrations to Evaluate Ventilation
Stop guessing, measure your actual CO₂.
This calculator assumes a perfectly mixed room and a steady crowd. Real rooms drift. Partycle reads true NDIR CO₂ wherever you are, so you catch the stuffy room before the 3pm fog sets in.