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CO₂ Ventilation Rate Calculator

Estimate how much outdoor air each person is getting from a steady indoor CO₂ reading — or the CO₂ level a given ventilation rate would settle at.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

NIST (Persily & de Jonge 2017): about 0.0046 L/s for a man aged 30–40 and 0.0035 L/s for a woman aged 30–40 at 1.2 met (sitting, light activity). Higher for more active people.

Measure it, or use a recent local value.

Result

Outdoor air per person
16.8CFM
Total outdoor air
67CFM

About 16.8 CFM of outdoor air per person (67 CFM for 4 people).

  • Valid only at steady state (constant occupancy and ventilation for several hours). A rising CO₂ reading underestimates the true ventilation.
Outdoor air per person (L/s)
7.93L/s
Show the working
  1. Steady-state mass balance: V = G ÷ (C_in − C_out)
    0.0046 L/s ÷ (1,000 − 420) × 10⁶ = 7.931 L/s per person
  2. Convert: 1 L/s = 2.119 CFM
    16.8 CFM per person

CO₂ as a ventilation tracer

People breathe out carbon dioxide at a fairly predictable rate. When occupancy and ventilation have been steady for long enough, the indoor CO₂ settles where the CO₂ people add is carried away by outdoor air:

Ventilation per person = generation rate ÷ (indoor CO₂ − outdoor CO₂).

The generation rate depends on body size, age and activity. NIST researchers published rates by age, sex and activity level; the default here is for an adult man aged 30–40 doing light seated work.

Limits of the method

The steady-state assumption is the big one: a room whose CO₂ is still rising has more ventilation than this method suggests. CO₂ also tells you about ventilation per person, not about other pollutants, filtration or air cleaning.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Ventilation from a CO₂ reading
Number of people
4
CO₂ generation per person (L/s)
0.005
Outdoor CO₂
420 ppm
Indoor CO₂ (steady)
1,000 ppm

Result

Outdoor air per person
16.8 CFM
Total outdoor air
67 CFM
Outdoor air per person (L/s)
7.93 L/s

About 16.8 CFM of outdoor air per person (67 CFM for 4 people).

Assumptions

  • Well-mixed space at steady state; outdoor air is the only dilution; CO₂ comes only from occupants.

Limitations

  • CO₂ indicates ventilation per person, not overall air quality; it does not measure other pollutants.
  • Generation rates depend on body size, age and activity (see the NIST paper).

Frequently asked questions

What indoor CO₂ level is good?

CO₂ itself is not usually the concern at typical indoor levels; it is an indicator of how much outdoor air there is per person. Lower readings mean more outdoor air per person.

What outdoor CO₂ value should I use?

A measurement taken outside at the same time is best. Global background levels are currently a little above 420 ppm, and cities can be higher.

Why does the activity level matter?

More active people produce more CO₂, so the same reading means more ventilation per person in a gym than in an office.

Sources

  1. Carbon Dioxide Generation Rates for Building Occupants (Persily & de Jonge, Indoor Air 2017) — NIST. CO₂ generation per person by age, sex and activity (e.g. about 0.0046 L/s for men and 0.0035 L/s for women aged 30–40 at 1.2 met); steady-state relation between generation, ventilation and the indoor–outdoor CO₂ difference.
  2. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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