Classroom CO₂ Ventilation Calculator

How classroom ventilation affects indoor CO₂

Classroom ventilation determines how quickly outdoor air replaces the CO₂ exhaled by students, teachers, and other occupants. A tightly enclosed room with a full class can accumulate CO₂ during a lesson when its fresh-air supply cannot keep pace. At typical indoor concentrations, CO₂ is useful as an indicator of how effectively occupied classroom air is being replaced with outdoor air. This calculator estimates that relationship from the room’s air volume, the number of people, their CO₂ generation rate, and the ventilation rate.

The classroom CO₂ calculator reports two related quantities. First, it estimates the steady-state concentration: the level the room approaches if occupancy and ventilation remain unchanged for a long time. Second, it estimates the time required to reach a selected CO₂ threshold when the classroom begins at the outdoor concentration. The steady-state result helps assess whether the stated ventilation rate can support the stated class size, while the time result helps show how quickly readings could rise during a lesson.

Classroom CO₂ inputs in plain language

For this classroom CO₂ estimate, each form field describes a part of the room’s air balance. Room volume is the amount of air available for dilution and can be found from floor area and ceiling height. Occupants are the people producing CO₂. CO₂ generation per person varies with age and activity; a quiet seated lesson generally differs from physical activity, singing, or a busy lab. Ventilation rate in air changes per hour, or ACH, describes how many room volumes of air are effectively replaced each hour. Outdoor CO₂ is the concentration in incoming air, and threshold CO₂ is the reading you want to evaluate.

• Room Volume (m³): the size of the classroom air reservoir.
• Number of Occupants: the students, teachers, and other people adding CO₂.
• CO₂ Generation per Person (L/min): a breathing-related emission rate for each person.
• Ventilation Rate (ACH): how aggressively outdoor air flushes the room.
• Outdoor CO₂ (ppm): the background level in the air entering the building.
• Threshold CO₂ (ppm): the target or limit you want to test against.

When classroom ventilation inputs are uncertain, use the calculator to compare plausible scenarios rather than treating one output as a precise measurement. Test a lower and higher ACH, compare different class sizes, or examine the effect of increasing effective air changes by opening windows. The direction of the change is especially useful: more ACH lowers indoor CO₂, while more occupants or higher per-person generation raises it.

How the classroom CO₂ formula works

The classroom CO₂ calculation uses a well-mixed mass-balance model: people add CO₂ through respiration and ventilation removes indoor air while supplying outdoor air. Let be the room volume, the number of occupants, and the generation rate per person. The total generation is . ACH multiplied by room volume gives the ventilation flow per hour. The steady-state concentration is then

where is the outdoor CO₂ concentration in parts per million. The factor of one million converts the volumetric ratio to ppm. The form’s generation input is in liters per minute per person, and the script converts it to cubic meters per hour before using this equation. Larger classroom volume and higher ACH lower the calculated concentration; more occupants and greater generation raise it.

Classroom CO₂ does not jump to its steady-state value immediately after a class enters. With constant occupancy and ventilation, it rises fastest at first and then slows as it approaches equilibrium. The time to reach a threshold concentration from outdoor conditions follows first-order kinetics:

This estimates the time for a classroom beginning at outdoor CO₂ to reach the selected threshold when occupancy and ventilation stay constant. If the threshold is above the calculated steady-state value, the room does not reach it under those assumptions, so the calculator reports that the threshold is above steady state.

Interpreting classroom CO₂ readings

Classroom CO₂ readings are commonly used as a practical ventilation indicator because occupants are the source and outdoor-air delivery is the principal removal mechanism in this model. A sensor trend can reveal whether CO₂ rises throughout a full lesson, levels off, or falls after occupancy decreases or outdoor air increases. The appropriate target for a particular school may depend on its policies, equipment, climate, occupancy pattern, and the purpose of its indoor-air program.

For classroom ventilation planning, the important comparison is between the CO₂ produced by the occupants and the outdoor air actually delivered to the room. Portable air cleaners can be valuable for particulate control, but ordinary filtration does not remove CO₂. Lowering calculated classroom CO₂ therefore requires more outdoor-air exchange, a verified improvement in HVAC delivery, a larger effective room volume, or fewer occupants.

Worked example: a 200 m³ classroom at 3 ACH

Consider the calculator’s default classroom conditions: a 200 m³ room, 25 occupants, CO₂ generation of 0.3 L/min per person, ventilation of 3 ACH, and outdoor CO₂ of 420 ppm. The script converts the total generation to 0.45 m³ per hour. Dividing that by the 600 m³-per-hour ventilation flow produces a 750 ppm indoor increment, so the calculated steady-state concentration is 1170 ppm. With a 1000 ppm threshold, the calculated time from outdoor conditions is about 29.7 minutes.

This classroom CO₂ example shows why both outputs matter. A steady-state result of 1170 ppm means the room approaches a value above the selected 1000 ppm threshold if the class and ACH remain unchanged. The threshold-time result shows that the crossing occurs during a typical lesson rather than only after many hours. Increasing ACH, increasing effective room volume, reducing occupancy, or reducing generation per person lowers the eventual plateau and delays the threshold crossing.

Classroom CO₂ assumptions and result interpretation

The classroom CO₂ model deliberately simplifies indoor conditions so the ventilation relationship remains clear. It assumes well-mixed room air, even though real rooms can have stagnant corners, drafts, or occupied zones that differ from the room average. It also assumes constant occupancy and ventilation. Late arrivals, open doors, changing window positions, cycling equipment, and variable outdoor-air delivery can all make real sensor data depart from the estimate.

Classroom CO₂ input quality also affects the result. Generation rates vary with age, body size, and activity. Room volume should represent the actual air space rather than only floor area, and an ACH value from a drawing or report may not match field delivery if controls, dampers, filters, or equipment operation differ. Comparing calculator scenarios with observed classroom sensor trends can help assess whether an assumed ventilation rate is plausible.

Use the classroom CO₂ output comparatively. If one scenario produces a lower steady-state concentration or a longer time to reach the same threshold, it represents a more favorable balance between occupant generation and outdoor-air removal. Comparing class schedules, occupancy plans, window-opening routines, or possible HVAC changes can be more useful than relying on one modeled number as a measurement.

In short, the calculator models a simple classroom air-balance story: occupants add CO₂, ventilation removes it, and room volume determines how much air is available for dilution. Use it to explore ventilation breaks, interpret CO₂ sensor patterns, compare room-use scenarios, or demonstrate how a first-order model applies to an occupied classroom.