Estimate indoor CO2 ppm from the number of people, room size, time and ventilation, or convert a CO2 percent or gas volume to ppm.
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CO2 PPM Formula
The volume-fraction formula below applies to CO₂ gas volume divided by total mixture volume at matching conditions. Room mode separately includes occupant generation and outdoor-air exchange.
Vc is the CO₂ volume and Vt is the final mixture volume, measured in matching units and at the same temperature and pressure. Multiplying by 106 gives ppm on a volume-fraction basis. Conversion to mg/m³ additionally depends on temperature and absolute pressure; 20°C and 1 atm should not be called a universal standard state.
What Is CO2 PPM?
CO₂ ppm expresses a fraction on a stated basis. For an ideal gas, volume fraction equals mole fraction; 1,000 ppm is 0.1%. A fraction is dimensionless, while a mass-per-volume value changes with gas density.
Outdoor CO₂ varies with location and time. The calculator’s editable 420 ppm value is an illustrative baseline, not a current global measurement. Outdoor-air ventilation drives an indoor concentration toward the outdoor value when other sources and sinks are absent.
CO2 Concentration Reference Levels
CO₂ benchmarks have different purposes and averaging periods. They are not interchangeable safety thresholds, and a CO₂ reading alone does not establish overall indoor air quality.
| CO₂ level | Context | Meaning |
|---|---|---|
| 420 ppm | Editable calculator example | Use a measured local outdoor baseline. |
| 1,000 ppm | 0.1% volume fraction | No universal air-quality or safety pass is implied. |
| 5,000 ppm | US OSHA PEL and NIOSH REL | 8-hour time-weighted occupational limits; not an instantaneous ceiling or general indoor target. |
| 30,000 ppm | NIOSH short-term exposure limit | 15-minute occupational limit. |
| 40,000 ppm | NIOSH IDLH | Immediately dangerous to life or health. |
Occupational references: CDC/NIOSH carbon dioxide guide. These limits do not establish adequate building ventilation.
How Occupancy Drives Indoor CO2 Buildup
People generate CO₂ at rates that depend on activity, age, body size and other characteristics. The editable 0.005 L/s example equals 18 L/h; it is not a universal rate for every occupant. See NIST on occupant CO₂ generation.
For example, a 35 m³ room starting at 420 ppm with one person generating an assumed 9 L/h and no ventilation reaches about 1,449 ppm after four hours: 420 + (9 × 4 / 35,000) × 10⁶. Actual generation and ventilation must be measured or estimated for the situation.
Room mode uses the constant-source, well-mixed balance: C(t) = C_out + (S/A)(1 − exp(−At)), starting at C_out. Here S is the occupant source in ppm/hour and A is outdoor-air changes per hour. When A = 0, C(t) = C_out + St. This simplified model is not inherently conservative and does not represent stratification or changing occupancy.
Ventilation Standards and Monitoring Benchmarks
ASHRAE’s 2025 position document explains the limitations of using indoor CO₂ to infer ventilation or air quality. The often-cited 700 ppm above outdoor air is not a universal limit in Standard 62.1. Applicable ventilation requirements depend on occupancy and the standard’s specified procedures.
Building certification schemes and local rules use their own definitions, averaging periods and measurement conditions. Check the applicable current requirement rather than treating a single ppm number as a universal compliance test.
CO2 and Cognitive Performance: What the Research Shows
Allen and colleagues’ 2016 controlled exposure study reported associations between cognitive scores, CO₂, ventilation and volatile organic compounds under its tested office conditions. Those findings do not establish a universal cognitive-performance threshold for every room or person.
Studies of low-level CO₂ and cognitive performance have produced differing results, as discussed in the ASHRAE position document. CO₂ can help investigate occupant-related ventilation when its sources, mixing and timing are understood, but it does not capture all pollutants or prove a space is safe.
