CO2 in Room Calculator
Estimate CO2 levels in a room based on volume, occupants, and ventilation rate.
Check if your indoor air quality is healthy.
CO₂ concentration in a room rises when people breathe and falls when fresh air is supplied.
Indoor CO₂ tracks closely with cognitive performance and alertness, which is why it matters in offices, classrooms and bedrooms far more than most people realize.
CO₂ accumulation formula: ΔCO₂ (ppm) = (Occupants × CO₂ Generation Rate × Time) / Room Volume × 1,000,000
CO₂ generation rates per person (roughly):
- Resting / sleeping: ~0.2 L/min
- Seated work / light activity: ~0.3 L/min
- Standing / light exercise: ~0.5–0.8 L/min
- Vigorous exercise: ~1.5–2.5 L/min
This calculator uses about 0.3 L/min, the standard figure for a seated adult.
Room CO₂ level formula (with ventilation): Steady-State CO₂ = Outdoor CO₂ + (Occupants × Generation Rate) / Ventilation Rate × 1,000,000
Where Ventilation Rate = air changes per hour (ACH) × Room Volume, converted to the same units as the generation rate.
Air changes per hour, and what the four settings mean
ACH is how many times per hour the room’s entire air volume is replaced. It is the number ventilation engineers actually quote, and it is why room volume matters here: the same fan in a bigger room is a lower ACH.
| Setting | ACH | What that looks like |
|---|---|---|
| No ventilation | sealed | Doors and windows shut, treated as pure accumulation with no fresh air at all |
| Poor | 0.5 | An old building’s leakage, or a closed room with a door cracked now and then |
| Moderate | 2.0 | A window open, or a basic HVAC system doing its job |
| Good | 5.0 | A well-designed mechanical system with dedicated fresh-air supply |
The room does not jump to steady state, it approaches it. The calculator uses the standard exponential fill, so a short meeting in a poorly ventilated room lands well below that room’s eventual ceiling, and a long one gets close to it.
Steady-state CO₂, worked through for a 90 m³ room with 8 people:
| ACH | Fresh air | Steady-state CO₂ |
|---|---|---|
| 0.5 | 750 L/min | 3,620 ppm |
| 2.0 | 3,000 L/min | 1,220 ppm |
| 5.0 | 7,500 L/min | 740 ppm |
That is the whole argument for ventilation in one table. Going from a cracked door to an open window takes a room out of the headache band; going from an open window to real mechanical ventilation takes it under 1,000.
CO₂ thresholds and effects:
| CO₂ Level (ppm) | Classification | Effect |
|---|---|---|
| 400–600 | Outdoor / fresh air baseline | Optimal cognitive performance |
| 600–1,000 | Acceptable (ASHRAE standard) | Minor drowsiness in some people |
| 1,000–1,500 | Elevated | Measurable decline in decision-making (Harvard study: ~15% impairment) |
| 1,500–2,500 | High | Headaches, fatigue, difficulty concentrating |
| 2,500–5,000 | Very high | Significant cognitive impairment, possible nausea |
| > 5,000 | Dangerous | Workplace limit; oxygen displacement risk |
Worked example: Conference room: 6 m × 5 m × 3 m = 90 m³ (90,000 L). 8 people seated. No ventilation. Outdoor CO₂: 420 ppm. Generation per person: about 0.3 L/min. After 60 minutes:
- CO₂ added = 8 people × 0.3 L/min × 60 min = 144 L
- Rise = 144 L ÷ 90,000 L × 1,000,000 = 1,600 ppm
- Room CO₂ = 420 + 1,600 = about 2,020 ppm
That is well into the “high” band, where headaches and poor concentration set in. A packed room with no fresh air climbs fast.
Solution: Open a window or run mechanical ventilation. Even a few air changes per hour pulls a room like this back toward 800–1,000 ppm.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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