CO2 Build-Up Calculator

CO₂ build-up calculator

Enter a room and see what its carbon dioxide does over an occupied day — the curve, the level it settles at, and how long it takes to cross the 1,000 ppm comfort target. Real single-zone mass balance, with the method published below.

Floor area (m²) The occupied room only

Ceiling height (m) Typical Indian room: 3.0 m

Number of people

Activity level Sets the CO₂ generation rate

Air changes per hour Sealed room ≈ 0.2–0.5 · window ajar ≈ 1–3 · fan on ≈ 3–8

Hours occupied

Outdoor CO₂ (ppm) India is typically 420–450 ppm

— ppm at the end of the period —

Room volume—

Steady state—

Crosses 1,000 ppm—

Outdoor air—

Per person—

How this is calculated

Single well-mixed zone, transient mass balance — the same model behind the ventilation-rate arithmetic in ASHRAE Standard 62.1:

V · dC/dt = G − Q · (C − C_out)

where V is room volume in m³, C is indoor CO₂, G is CO₂ generation from the occupants in m³/s, Q is outdoor airflow in m³/s, and C_out is the outdoor concentration. With constant G and Q this solves to C(t) = C_ss + (C₀ − C_ss) · e^−Qt/V, settling at a steady state of C_ss = C_out + 10⁶ · G / Q.

Per-person CO₂ generation is scaled from metabolic rate. The reference value is 0.0052 L/s (about 19 L/h) for a sedentary adult at roughly 1.2 MET — the figure the ventilation standards are built on. Other activity levels are scaled by MET ratio: sleeping about 0.9 MET, light activity 2.4 MET, hard exercise 6–8 MET. A school-age child is scaled by body surface area to roughly 0.6× an adult.

What this model assumes: the room is well mixed (no stratification or dead corners), generation and airflow are constant over the period, and there is no CO₂ source other than the occupants. Real rooms deviate — gas cooking, combustion heaters and fermentation all add CO₂; poor air distribution creates pockets that run higher than the average. Treat the output as a well-founded estimate of what a room like yours should do, and a measurement as the truth about what yours actually does.

Reading the result

Two numbers matter more than the curve itself.

The steady state is where the room ends up if the occupancy continues indefinitely. It is the honest verdict on the room, because it does not depend on how long you happened to model. If it sits above 1,000 ppm, the ventilation is inadequate for that occupancy no matter how long anyone stays.

Outdoor air per person is the number a ventilation engineer would look at. ASHRAE Standard 62.1 specifies a per-person outdoor-air rate plus a per-floor-area rate for each space type; for an office the per-person component is 2.5 L/s and for a classroom 3.8 L/s. If this calculator tells you the room delivers 1.2 L/s per person, that is the size of the gap.

What to do with a bad result

Raise the air-exchange rate — the only thing that lowers CO₂. Open something, or run a fan. Our fan size calculator works out how much air you actually need to move.
Do it on demand rather than continuously. Try setting the air-change rate to what your exhaust fan delivers and see how fast the room recovers — that recovery time is how long the fan needs to run per cycle, not all day. This is the basis of demand-controlled ventilation.
Then measure. A model tells you what a room like yours should do. Only a sensor tells you what yours does — see how to check and measure CO₂ in your room.

Questions about this calculator

How fast does CO₂ build up in a closed room?

It depends on room volume, occupancy and air-exchange rate. A closed 20 sq m room with a 3 m ceiling and four seated adults at 0.5 air changes per hour crosses 1,000 ppm in well under an hour and settles above 2,000 ppm. Enter your own figures above to get the curve for your room.

What air change rate should I enter?

If you do not know it, use these as rough guides: a modern sealed room with the windows and door shut is around 0.2–0.5 ACH; an older building with leaky frames is 0.5–1.5 ACH; a room with a window open a few centimetres is 1–3 ACH; a room with an exhaust fan running is typically 3–8 ACH. Air conditioning adds nothing — split and window units recirculate and supply no outdoor air.

Why does the calculator settle at a level instead of climbing forever?

Because as indoor CO2 rises, the difference between indoor and outdoor air grows, and each cubic metre of air exchanged removes more CO2. Generation and removal eventually balance at the steady-state level. Only with zero air exchange does the concentration climb without limit.

Is this the same model ventilation engineers use?

Yes — it is the standard single-zone transient mass balance that underlies the ASHRAE 62.1 ventilation-rate arithmetic. The full equation and the per-person generation rates are published on this page so you can check the working.

How this is calculated

Reading the result

What to do with a bad result

Stop modelling it and start holding it

Questions about this calculator

CO₂ build-up calculator