Office Ventilation Calculator: Air Change Rate and CO2

CO2 in a meeting room: build-up calculation and airflow rules

The tool estimates CO2 concentration at the end of a meeting with a well-mixed room model: each seated person emits 0.018 m³ of CO2 per hour, and ventilation renews the air at a fixed rate, except standard mechanical ventilation, sized at 30 m³/h per occupant. For 6 people over one hour in 65 m³ with that ventilation (180 m³/h), it gives 982 ppm, above the 800 ppm benchmark.

The model

Fresh-air flow Q = volume × air change rate (0.2 per hour for a closed room without ventilation, 3 for a tilted window, 4 for an air handling unit); for standard mechanical ventilation, Q = occupants × 30 m³/h and rate = Q ÷ volume. Steady-state concentration = outdoor CO2 + (occupants × 0.018 ÷ Q) × 1,000,000. Concentration after time t = outdoor + (steady state − outdoor) × (1 − e^{−rate × t}), starting from outdoor air (420 ppm by default).

Example

25 m² × 2.6 m = 65 m³, 6 people, 60 minutes, standard ventilation: Q = 6 × 30 = 180 m³/h (2.77 air changes per hour), steady state 1,020 ppm, end of meeting 982 ppm, recommended airing 6 minutes. Without ventilation, door and windows closed: 1,926 ppm.

Comparison with French labour law

For mechanical ventilation, the French Labour Code sets a minimum fresh-air flow per occupant: 25 m³/h in an office, 30 m³/h in a meeting room. The tool's standard ventilation option applies this meeting-room minimum, 30 m³/h per person, and so assumes a compliant system. If yours is undersized, the real concentration will be higher: check its actual airflow, which the tool does not measure; other countries set their own rules (ASHRAE 62.1 in the United States, for instance).

Limits

• The cognitive impact percentages shown (−15% from 1,000 to 1,499 ppm, −50% from 1,500 ppm) follow the order of magnitude of a single controlled study (Allen et al., Environmental Health Perspectives, 2016, 24 participants) and are not a diagnosis.
• Air assumed perfectly mixed, occupants seated, nothing measured: the tool does not replace a CO2 sensor or advice from an occupational physician.

Sources and dates: article R4222-6 of the French Labour Code, minimum fresh-air flow per occupant, created by decree no. 2008-244 of 7 March 2008, in force since 1 May 2008. 800 ppm benchmark: opinion of France's High Council for Public Health of 28 April 2021 on venues open to the public, issued during the Covid-19 epidemic (checked 14/10/2026).

Frequently Asked Questions

How much fresh air per person does an office need?

For mechanical ventilation, article R4222-6 of the French Labour Code sets 25 m³/h per occupant in an office and 30 m³/h in a meeting, dining or sales room. Six people in a meeting therefore need at least 180 m³/h of fresh air; other countries apply their own standards, such as ASHRAE 62.1 in the United States.

At what CO2 level should a room be aired?

The benchmark most used in France is 800 ppm, from the High Council for Public Health's opinion of 28 April 2021 on venues open to the public, issued during the Covid-19 epidemic; the tool shows "ideal working level: below 800 ppm" and turns amber from 1,000 ppm and red from 1,500 ppm. Outdoor air is around 420 ppm.

How long should windows stay open after a meeting?

The tool suggests 5 minutes plus one minute per 250 ppm above 800 ppm, with a minimum of 5 minutes. For a meeting ending at 982 ppm, that gives 6 minutes with windows wide open.

How does CO2 build up during a meeting?

It rises along an exponential curve towards a ceiling equal to outdoor CO2 plus emissions divided by the fresh-air flow. With 6 people in 65 m³ and ventilation of 180 m³/h (30 m³/h per person), the ceiling is 1,020 ppm and the room reaches 870 ppm after 30 minutes and 982 ppm after one hour.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.