CO₂ Breathing Emission Calculator
Estimate how occupant breathing and ventilation change indoor carbon dioxide concentration over time using a single-zone, well-mixed mass-balance model.
Room function
People and time
Room size
CO₂ concentration results
This modeled level is below the 5,000 ppm occupational 8-hour limit, but indoor CO₂ is best interpreted as a ventilation indicator rather than a universal health cutoff.
Calculation breakdown
CO₂ concentration over time
The line shows the same well-mixed concentration values listed in the timeline table, from the starting level to the end of the stay.
Timeline detail
| Elapsed time | CO₂ concentration | Rise above outdoors |
|---|
How to use this CO₂ breathing emission calculator
What this calculator does
This calculator estimates how carbon dioxide from human breathing can accumulate in a single indoor room while outdoor air replaces part of the room air. It uses the standard single-zone mass-balance relationship between an indoor source, room volume, outdoor concentration, air changes per hour, and time. The result is useful for exploring how occupancy and ventilation interact, but it is not a substitute for a calibrated CO₂ monitor, a ventilation inspection, or a health assessment. The U.S. EPA notes that indoor CO₂ can provide information about ventilation but must be interpreted carefully; see its guidance on using indoor CO₂ as a ventilation indicator.
When to use it
Use the model to compare a small office before and after increasing ventilation, estimate how a classroom occupancy change affects the end-of-session concentration, test whether a long meeting approaches a steady value, or explore why the same number of people produces a larger concentration rise in a smaller room. For safety-critical spaces, unusual sources, or compliance work, measure the actual air and use applicable codes and professional guidance.
How to calculate
- The calculator opens with a complete demonstration: Office, 1.50 ACH, previously empty, 4 people, 8 hours, office work/light activity, 0.04% outdoor CO₂, and a 6 m × 4 m × 2.8 m room. The example workbook is already validated, so Download Excel is available immediately.
- Choose Room type. A typical ACH value is inserted automatically. Choose Custom ACH when you have a measured or design value, then edit Air changes per hour.
- Set This room..., Average room population, People stay here for..., and People usually stay here for.... The activity setting changes the per-person CO₂ generation assumption.
- Enter Outdoor CO₂ concentration as a percentage using a decimal point. For example, 0.04% equals 400 ppm.
- Select Room size entry and Room units. With Dimensions, enter Length, Width, and Ceiling height. With Direct volume, enter Volume. Switching metric/imperial converts the current room-size values rather than merely changing labels.
- Read the live results, time profile, and timeline table. Download Excel exports the current validated model. Reset clears the demonstration/input data, removes calculated content, and disables Download Excel until a complete valid state is entered again.
Input guide
Room type is required and selects a typical ventilation rate: Bedroom 0.50 ACH, Living room 1.00, Kitchen 2.00, Bathroom 2.50, Office 1.50, Classroom 2.00, Conference room 1.80, or Gym 4.00. Custom ACH makes Air changes per hour editable; it accepts a plain nonnegative decimal from 0 through 60. Higher ACH generally pushes indoor CO₂ closer to the outdoor level. A common mistake is treating ACH as air speed; it means room-volume replacements per hour.
This room... is required. “Was previously empty” starts the model at the outdoor concentration. “Was previously occupied” uses a fixed 1,000 ppm carryover because the source workflow distinguishes prior occupancy but does not expose a measured starting concentration. Treat this as an explicit modeling assumption, not a measurement. Average room population is a required whole-number count from 1 to 10,000; more people increase the source rate proportionally. People stay here for... is required in hours, greater than 0 and no more than 168; longer stays move the concentration closer to its steady state.
People usually stay here for... is required and selects a per-person generation rate: Sleeping 0.30 L/min, Resting/sedentary 0.35 L/min, Office work/light activity 0.50 L/min, or Intensive work/exercise 1.20 L/min. These are representative physiological assumptions; real emission rates vary among people. Outdoor CO₂ concentration is required, entered as percent from 0 to 2%; 0.04% is 400 ppm. Do not type “400” when the field is in percent, and decimal-comma text such as 0,04 is rejected instead of being silently reinterpreted.
Room size entry chooses Dimensions or Direct volume. Room units chooses Metric or Imperial. In Dimensions mode, Length, Width, and Ceiling height are required positive decimals. The startup examples are 6 m, 4 m, and 2.8 m. In Direct volume mode, Volume is required and must be positive. Larger room volume dilutes the same source more strongly. Inputs use ordinary decimal notation; scientific notation and ambiguous grouping formats are rejected.
Output guide
Final CO₂ level is the estimated end-of-stay concentration in ppm, while the percentage below it is the same concentration by volume. CO₂ rise above outdoors is the final concentration minus the outdoor baseline. Steady-state CO₂ is the concentration the constant model approaches if occupancy and ventilation continue indefinitely; with zero ACH it is shown as “No finite steady state.” Room volume is the canonical internal volume in cubic meters, and CO₂ generation is the combined occupant source in liters per hour.
The summary pills repeat Room, Ventilation, Occupancy, and Duration from the same canonical state. The breakdown adds Initial CO₂, Air changes per hour, CO₂ produced during stay, and Floor area when dimensions are available. The CO₂ concentration over time line and the timeline columns Elapsed time, CO₂ concentration, and Rise above outdoors all use the identical modeled series. A high modeled level should prompt attention to ventilation and real measurement, not a medical conclusion.
Worked example
With 4 people doing light office work at 0.50 L/min each, the combined source is 120 L/h, or 0.12 m³/h. A 6 m × 4 m × 2.8 m room has a 67.2 m³ volume. At 1.50 ACH and an outdoor/initial concentration of 400 ppm, the mass-balance solution after 8 hours is about 1,590 ppm, or 0.159%. The modeled rise is about 1,190 ppm. Because 8 hours is much longer than the 1/1.5-hour ventilation time constant, the result is already very close to the same model's steady-state value of about 1,590 ppm.
Learn more
The relationship between indoor CO₂ and ventilation is a single-zone mass-balance problem; a NIST review explains why CO₂ can be informative without being a universal indoor-air-quality threshold. Read the NIST review of indoor CO₂ and ventilation standards. For occupational exposure context, the NIOSH Pocket Guide for carbon dioxide lists a 5,000 ppm time-weighted recommended exposure limit and a 40,000 ppm IDLH value. These occupational limits are not design targets for ordinary rooms.
How the model works and what changes the result
For a well-mixed room with constant outdoor concentration, occupancy, source rate, and air exchange, the indoor concentration follows an exponential approach toward equilibrium. In practical terms, occupant breathing adds CO₂ while ventilation removes indoor air and replaces it with outdoor air. When ACH is greater than zero, the model is:
where C is ppm, Cₒ is outdoor ppm, C₀ is starting ppm, n is ACH in h⁻¹, t is hours, G is total CO₂ generation in m³/h, and V is room volume in m³. With zero ACH, the model becomes a linear accumulation from the starting concentration.
The strongest levers are usually occupancy, source rate, room volume, and outdoor-air exchange. Doubling occupancy doubles the modeled generation source. Increasing room volume lowers the concentration rise for the same source. Increasing ACH lowers the steady-state excess above outdoors and normally reduces how quickly a crowded room reaches high concentrations. EPA's current residential ventilation guidance describes air exchange as the rate at which outdoor air replaces indoor air and gives context for air-change rates and indoor air quality.
Interpretation cautions
- Indoor CO₂ is useful as a ventilation and occupancy indicator, but it does not measure particles, volatile compounds, humidity, combustion products, or infection risk by itself.
- The model assumes uniform mixing. A real sensor near a person, doorway, supply vent, or return grille can read differently from a room average.
- Room-type ACH values are representative assumptions for comparison, not code requirements or measured ventilation rates. Use Custom ACH when you know the actual value.
- Human CO₂ generation varies with body size, age, metabolism, and activity. The four activity rates are intentionally transparent approximations so the result can be reproduced.
- The occupational 5,000 ppm and 40,000 ppm values are exposure-reference context. Ordinary comfort, ventilation, and building-design decisions often use much lower observed CO₂ values for different purposes.