Outdoor Air Required to Achieve Target CO2 Levels
Full Results
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How to Use This Calculator
Pick the space type to auto-load the ASHRAE 62.1-2022 breathing zone rates per person and per floor area (Residential uses ASHRAE 62.2), plus a typical metabolic rate. You can override any value manually for custom spaces.
Enter the number of occupants, floor area, and ceiling height. The calculator uses floor area with the per-area OA component and ceiling height for the room volume time constant.
Check the auto-filled OA per person and per area values match your actual system design. These are minimum breathing zone rates, so apply air distribution effectiveness and system-level ventilation efficiency factors from ASHRAE 62.1 separately.
The results show the predicted steady-state CO2 at your current OA rate, plus the OA flow required to hit 800, 1000, 1100, and 1500 ppm targets. Compare the result with 1,000 ppm (Health Canada 24-hour limit for homes) and with outdoor plus 700 ppm. Neither is an ASHRAE 62.1 compliance test, since compliance is based on ventilation rates.
Indoor CO2 Levels, Ventilation, and Cognitive Performance
CO2 gets more attention than any other IAQ indicator because it's easy to measure, tightly linked to ventilation adequacy, and directly connected to occupant productivity. In a Canadian school or office building, CO2 well above 1,000 ppm is a sign the ventilation system isn't delivering enough outdoor air for the number of people in the room. That's fixable. Knowing the numbers makes it actionable.
The Steady-State CO2 Formula
When a space reaches steady state, the CO2 removal rate by ventilation equals the CO2 generation rate by occupants. The steady-state formula is: indoor CO2 (ppm) equals outdoor CO2 plus the total CO2 generation rate divided by the outdoor air supply rate. CO2 generation per person scales with metabolic rate. A sedentary adult (1 MET) generates about 0.0039 L/s of CO2, the value this calculator uses. A student at 1.3 MET generates about 0.0050 L/s, and a fitness class participant at 2 MET about 0.0077 L/s. The time to reach steady state is roughly 3 room time constants, where the time constant equals room volume divided by outdoor air supply flow. A 200 m3 classroom with 50 L/s of outdoor air has a time constant of 4,000 seconds, or about 67 minutes, meaning full steady state takes roughly 3 hours of occupancy.
ASHRAE 62.1 and the 700 ppm Rule
ASHRAE Standard 62.1 sets minimum outdoor air by space type through its ventilation rate procedure, as used in this calculator: breathing zone airflow equals the people rate times occupancy plus the area rate times floor area. In the 2022 edition, Table 6-1 gives 2.5 L/s per person plus 0.3 L/s per m2 for office space and conference rooms, and 5 L/s per person plus 0.6 L/s per m2 for classrooms of age 9 and up. Health Canada cites a rule of thumb, attributed to ASHRAE, of keeping indoor CO2 within 700 ppm of the outdoor level, which is about 1,125 ppm with 425 ppm outdoors. It is a rule of thumb rather than a compliance test, because the standard's requirements are ventilation rates. Table 6-1, as revised by Addendum w to the 2022 edition, also carries a maximum CO2 above ambient value for many space types, for example 600 ppm for office space and classrooms and 1,500 ppm for conference and meeting rooms. ASHRAE's Indoor Air Quality Procedure allows ventilation rates to be traded against source control, so CO2 alone isn't the full picture for spaces with significant non-occupancy sources. The National Building Code of Canada refers to ASHRAE 62.1 in Part 6 for ventilation. See the ventilation effectiveness calculator to apply Ez factors for non-uniform air distribution.
CO2 and Cognitive Performance
Health Canada's residential indoor air quality guideline for carbon dioxide sets a long-term exposure limit of 1,000 ppm (1,800 mg/m3) as a 24-hour average. It is based on an increased risk of symptoms such as eye irritation, headaches, and tiredness, and of lower scores on tasks and tests, and it serves as an indicator of adequate ventilation rather than a poisoning threshold. Some controlled studies also report lower scores on decision-making tests at around 1,000 ppm, although the results are mixed. Health Canada recommends adequate ventilation and control of indoor sources when levels run above the limit. Our IAQ calculator incorporates CO2 alongside other IAQ indicators for a complete picture.
Demand-Controlled Ventilation
Demand-controlled ventilation (DCV) uses CO2 sensors to modulate outdoor air dampers based on actual occupancy rather than design maximum. When a conference room is half-full, DCV supplies about half the people-based outdoor air and saves the heating and cooling energy that would have gone to conditioning the excess. In a Canadian winter that saving is large: heating outdoor air from -25 C to 21 C takes roughly 55 W for every L/s. The setpoint is normally derived from the design ventilation rate and expected occupancy, and this calculator's 800, 1,000, and 1,100 ppm targets show the outdoor air each one would need. The demand-controlled ventilation calculator covers DCV sizing and setpoints. See the dilution air calculator for general ventilation sizing by contaminant type.
CO2 in Canadian Residential Buildings
Residential ventilation in Canada is covered by ASHRAE 62.2 and the mechanical ventilation provisions of the National Building Code, and HRVs and ERVs are the usual way to meet them. Standard 62.2 sets the total ventilation rate as 0.15 L/s per m2 of floor area plus 3.5 L/s for each bedroom plus one, which comes to 44 L/s for a 200 m2 three-bedroom house. The whole house ventilation calculator covers sizing to that rate, and this calculator's Residential preset uses the same two terms, so enter bedrooms plus one as the occupant count. Bedrooms are the tight spot: in this calculator, two adults at 1 MET need about 13.5 L/s of outdoor air reaching the room to hold 1,000 ppm overnight, which is why a closed bedroom with no ventilation drifts above that level. Running the HRV on high speed during gatherings raises outdoor air when occupancy is high.
Frequently Asked Questions
Outdoor CO2 is about 425 ppm. Health Canada's long-term exposure limit for homes is 1,000 ppm as a 24-hour average, set to reduce the risk of symptoms such as headaches and tiredness, and it works as an indicator of adequate ventilation rather than a poisoning threshold. A common rule of thumb, which Health Canada attributes to ASHRAE, keeps CO2 within 700 ppm of outdoor levels, roughly 1,125 ppm today. Standard 62.1 sets its requirements as ventilation rates by space type, so a reading above these numbers means the space is under-ventilated for its occupancy, not that it fails the standard. For bedrooms during sleep, staying below 1,000 ppm is worth targeting. Use this calculator to verify your ventilation system delivers enough outdoor air to stay within these ranges at design occupancy.
Steady-state CO2 equals outdoor CO2 plus CO2 generation rate divided by outdoor air supply rate, with both flows in L/s and the ratio multiplied by 1,000,000 to give ppm. A sedentary adult (1 MET) generates about 0.0039 L/s of CO2 and a student at 1.3 MET about 0.0050 L/s. For 30 students at 0.005 L/s each with 180 L/s of outdoor air, the generation rate is 0.15 L/s, and 0.15 divided by 180 times 1,000,000 gives 833 ppm above outdoor, so steady-state CO2 is about 1,258 ppm with 425 ppm outdoors. That is above both the 1,000 ppm and 1,125 ppm reference points, meaning more outdoor air is needed. This calculator does the math automatically for any space type and occupancy.
ASHRAE 62.1-2022 Table 6-1 gives breathing zone rates by space type. Office space needs 2.5 L/s per person plus 0.3 L/s per m². Classrooms (age 9 plus) need 5 L/s per person plus 0.6 L/s per m². Conference and meeting rooms need 2.5 L/s per person plus 0.3 L/s per m². These are minimums before air distribution effectiveness is applied, and the combined per-person rate depends on how many people share the floor area, so the CO2 you get at full design occupancy varies by space. Use this calculator to see the steady-state CO2 and the outdoor air needed for a target. See the ventilation effectiveness calculator to apply distribution efficiency corrections.
CO2 reliably indicates under-ventilation relative to occupancy and correlates with human-generated pollutants. High CO2 means the space is under-ventilated for the number of people in it. However, CO2 doesn't capture outdoor-sourced pollutants like PM2.5 or ozone, and it doesn't reflect material-sourced VOCs, mold, or radon. A classroom with high CO2 is under-ventilated. A newly renovated office with low CO2 might still have high formaldehyde from new materials. Use CO2 as a ventilation adequacy indicator and pair it with the IAQ calculator for a complete picture.
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