Indoor CO2 Ventilation Calculator
ASHRAE ventilation math is written for HVAC engineers. Enter your room size, how many people are in it, and how much fresh air gets in, and get the same answer in plain numbers. Nothing uploaded.
Result
Good — fresh airWhat this means
This room maintains good air quality at the entered occupancy and ventilation level.
Learn more: CO2 as a stand-in for stale air
The balance behind the number
The calculator assumes 420 ppm of CO2 outdoors. Each person adds CO2 by breathing, and ventilation removes it. The steady-state level is 420 ppm plus the people's CO2 output divided by the airflow in cubic meters per second. The airflow is air changes per hour times the room volume, divided by 3,600.
The output per person depends on activity: 5.2 millionths of a cubic meter per second awake and lightly active, and 3.1 millionths asleep. In a 50 cubic meter room with two awake people, a sealed room at 0.3 air changes an hour settles near 2,900 ppm. A cracked window at 3 changes an hour brings it to about 670 ppm, and good mechanical ventilation at 6 changes gives about 545 ppm.
What the bands mean
The calculator labels up to 800 ppm as good, 800 to 1,000 as acceptable, 1,000 to 1,500 as reduced focus likely, 1,500 to 2,500 as poor and anything above that as well above recommended limits. The ventilation presets run from 0.3 changes an hour for a sealed room to 10 for an excellent fresh-air system. The bands are the calculator's own guidance.
Room size matters as much as the number of people. The same two people in a room twice as big, with the same air changes per hour, settle at about half the CO2 above the outdoor level. Our bedroom CO2 post works through a sleeping example.
FAQ
Does CO2 itself cause the stuffy feeling?
CO2 is used as a marker of shared, stale air. A high reading tracks low ventilation, which also lets other pollutants and moisture build up in the room.
How much does opening a window help?
In the calculator, going from 0.3 to 3 air changes an hour is a tenfold increase in ventilation. For the same people and room, the CO2 above the outdoor level drops to a tenth.
Why does room volume matter?
The same CO2 output is spread through more air in a bigger room, so it reaches a lower steady level than in a small room with the same people and ventilation.