How this calculator works
People exhale carbon dioxide at a rate set by their metabolism: roughly 0.011 cubic metres an hour asleep, 0.018 at rest, 0.022 doing light work and 0.047 during hard exercise. A room removes it by exchanging air with the outside, at a ventilation rate equal to the room volume multiplied by the number of air changes an hour. When the removal matches the production, the concentration stops rising at a steady level: the outdoor concentration plus the production divided by the ventilation rate. The calculator also integrates the approach to that level, which is exponential with a time constant equal to the room volume divided by the ventilation rate. A room can take an hour or more to reach its steady state, which is why a classroom becomes stuffy before the end of a lesson. Carbon dioxide at these levels is not toxic, but above about 1,000 ppm people report poorer concentration and more headaches, which is why 1,000 ppm is a common target for classrooms and offices.
Formula
room volume (m3) = length x width x height. ventilation (m3/h) = volume x air changes per hour. CO2 added (m3/h) = people x generation per person. steady concentration (ppm) = (outdoor ppm + people x generation / ventilation) x 1,000,000. concentration at time t = steady + (outdoor - steady) x exp(-ventilation x t / volume). The most people who stay below 1,000 ppm = (0.001 - outdoor) x ventilation / generation.
Worked example
A 4 m by 4 m room with a 2.5 m ceiling has a volume of 40 m3. At 1.5 air changes an hour the ventilation is 60 m3/h. Four people sitting exhale 4 x 0.018 = 0.072 m3/h, so the steady concentration is 420 + 0.072 / 60 x 1,000,000 = 420 + 1,200 = 1,620 ppm. To stay below 1,000 ppm the room would hold (0.001 - 0.00042) x 60 / 0.018 = 1.93 people, so three people are too many without more ventilation.
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Frequently asked questions
Is 1,000 ppm dangerous?
No. Carbon dioxide at 1,000 to 2,000 ppm is not toxic, but it is a good proxy for stale air: if CO2 is high, other human-generated contaminants and airborne pathogens are also concentrated. The 1,000 ppm target is a ventilation marker, not a health limit.
Why does my bedroom reach high levels overnight?
Bedrooms usually have low ventilation when the door and window are closed, and sleeping people still exhale CO2. A small window cracked open or a trickle vent makes a large difference because it adds air changes hour after hour.
Does a CO2 meter measure ventilation?
It measures the result. A meter in a full room shows the concentration, and you can watch it fall when a window opens. The steady level tells you the effective air change rate for that occupancy.
Does this replace ASHRAE or building code calculations?
No. The air change rates here are typical values for understanding a room. A design calculation must use the ventilation rates in ASHRAE 62.1 or the local building code, which are set per person and per floor area and account for the intended use.
Sources
- ASHRAE Standard 62.1: ventilation and acceptable indoor air quality
- Persily and de Jonge (2017): carbon dioxide generation rates for building occupants, Indoor Air
- Environment and Climate Change Canada: emission factors and reference values for greenhouse gas quantification (national inventory edition, updated annually)