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Maximum CO2 Concentration Calculator

This tool was developed to support the use of real-time carbon dioxide (CO2) sensors indoors as a way to help evaluate ventilation rates in classrooms. Users input their target air changes per hour through ventilation and information about the classroom, and the calculator returns the estimated CO2 concentration.

Important: sufficient clean air delivery can be achieved through any combination of outdoor air ventilation and filtration, but this tool is only for assessing outdoor air ventilation. It is possible to exceed these target CO2 concentrations and still be meeting targets for clean air through filtration.

This guidance does not supercede state or local guidance or guidance from CDC, WHO, or other bodies. It is provided to support efforts to understand ventilation rates in indoor spaces.

1. Room Size

sq. ft.

2. Airflow

per hour
Air changes per hour
Clean air rate (cfm)
Room volume (ft3)
Cubic feet per min. × 60 min. per hour
Room length × width × height (ft.)
Note: This tool considers outdoor air ventilation only. If additional impacts from filtration and air cleaners are needed, a combination of ventilation and filtration can be used.
Ideal (6 ACH) Excellent (5-6 ACH) Good (4-5 ACH) Minimum (3-4 ACH) Low (< 3 ACH)
Ideal (6 ACH) Excellent (5-6 ACH) Good (4-5 ACH)
Minimum (3-4 ACH) Low (< 3 ACH)

3. Occupancy

4. Outdoor CO2 Levels

Note: If outdoor CO2 measurements were NOT taken, use a default value of 400 parts-per-million.


Configure the settings and then click the button below to calculate.

Disclaimer and terms of use

Strategies to Reduce Infection Risk

If classroom CO2 consistently exceeds the above calculation:

  1. Consider increasing outdoor air ventilation either by increasing mechanical system operation and/or introducing natural ventilation (e.g., opening windows)
  2. Consider upgrading media filters to higher efficiency versions (e.g., MERV 13), if the HVAC system can accomodate this*
  3. Consider implementing (additional) portable air cleaners.*

This air cleaner calculator can be used to explore portable air cleaner options. Since portable air cleaners increase total effective ACH, but do NOT reduce indoor CO2 concentrations, use the linked portable air cleaner calculator, and NOT indoor CO2 measurements, to assess total effective ACH when using portable air cleaners.

*Strategies two and three increase total effective ACH, but do not change the indoor CO2 concentration. Only strategy one (increasing outdoor air ventilation) reduces the indoor CO2 concentration while also increasing total ACH. Implementing strategy two and/or three will reduce the portion of the total target ACH that will be provided by outdoor air ventilation, which can be helpful if the ventilation system has operational limitations.

About the Calculation

This estimate of the steady-state CO2 concentration is based on the number of people, an estimate of their ages and activity level, the room dimensions, and the outdoor air ventilation only. This calculator makes the following necessary assumptions to perform this simplified calculation:

  • Benefits from filtration and portable air cleaners are not captured in this calculation.
  • Classroom air is perfectly mixed (i.e., indoor CO2 concentration is uniform).
  • Indoor CO2 concentrations remain constant over time (i.e., at steady-state).

References and Assumed Calculation Inputs

CO2 Generation Rates (cfm) for Different Metabolic Rates by Age: Averages Using 50% Males and 50% Females

Sitting/reading Standing/light effort Age
Elementary School 0.0076279752 0.0128192361 6 to < 11
Middle School 0.0100646895 0.016951056 11 to < 16
High School 0.0107003541 0.0182223852 16 to < 21
Adult 0.0107003541 0.0183283293 30 to < 40

Source: Table 4 in Persily & de Jonge 2017

Activity Levels (met) for Relevant School Activities

Activity Metabolic Rate
Sitting/reading 1.5 met
Standing/light effort 3 met

Source: Table 3 in Persily & de Jonge 2017

Default Occupant Densities by Age

Age bracket Occupant Density Note
5 to 8 years old 25 people/1000 sq. ft. Used for elementary school ages in this calculator
>= 9 years old 35 people/1000 sq. ft. Used for students older than elementary school ages in this calculator

Source: ASHRAE Standard 62.1-2019: Ventilation for Acceptable Indoor Air Quality