Indoor CO₂ Buildup Calculator

Why Indoor CO₂ Matters in Occupied Rooms

Indoor CO₂ rises when people exhale into a room faster than outdoor air replaces the room air. Carbon dioxide is naturally present in outdoor air, and the calculator uses 400 parts per million (ppm) as its outdoor-air baseline. In enclosed classrooms, meeting rooms, bedrooms, and workspaces, concentration can climb during occupancy when ventilation is limited. A higher CO₂ reading can be a useful signal that the amount of outdoor-air exchange may not be keeping pace with the number of occupants. This Indoor CO₂ Buildup Calculator estimates how the room volume, occupancy, ventilation rate, starting level, and time work together so you can explore likely indoor conditions before or during a period of use.

This CO₂ buildup estimate treats the room as one well-mixed volume. Each person is assigned a constant carbon-dioxide generation rate, while ventilation replaces indoor air with outdoor air at the entered air-changes-per-hour rate. Let $C$ be indoor CO₂ concentration in ppm, $C_0$ the initial concentration, $C_o$ the outdoor concentration, $G$ the generation rate in cubic meters per hour, $V$ the room volume, and $\mathrm{ACH}$ the air changes per hour. The calculator’s concentration change is represented by $$\frac{dC}{dt}=\frac{G}{V}\times {10}^6-\mathrm{ACH}(C-C_o)\text{.}$$ Solving this first-order relationship gives the concentration at time $t$.

$$C\left(t\right)=C_o+\frac{G}{\mathrm{ACH}V}\times {10}^6+(C_0-C_o-\frac{G}{\mathrm{ACH}V}\times {10}^6)e^{-\mathrm{ACH}t}$$. In this indoor CO₂ model, the first term is the outdoor baseline, the next term is the added equilibrium contribution from occupants, and the exponential term describes the approach toward that equilibrium as time passes.

Using the Indoor CO₂ Buildup Calculator

To estimate CO₂ buildup in a particular room, enter its length, width, and height so the calculator can determine the room volume. Then enter the number of people, the air changes per hour supplied by ventilation or open windows, the expected exposure time, and the initial CO₂ level. The script uses a generation rate of 0.018 m³ of CO₂ per person per hour. It calculates both the concentration after the selected time and the equilibrium concentration that the same occupancy and ventilation conditions approach if they continue.

The indoor CO₂ result is shown in ppm with a Good, Moderate, or Poor label. On this page, a result at or below 1000 ppm is labeled Good, a result above 1000 ppm through 2000 ppm is labeled Moderate, and a result above 2000 ppm is labeled Poor. These labels are a quick way to compare scenarios rather than a substitute for measuring a specific room. Try changing only one input at a time: increasing ACH lowers the predicted concentration, while adding people, extending the time, or reducing room volume generally raises it.

Indoor CO₂ Buildup Example Scenario

A useful way to use this indoor CO₂ calculator is to compare the same room under different ventilation and occupancy conditions. Start with the room’s measured dimensions and a realistic starting CO₂ value, then estimate the number of people who will remain inside for the selected period. The equilibrium result is especially helpful for identifying whether a meeting, class, or gathering would continue to accumulate CO₂ if nobody leaves and the ventilation setting stays unchanged. If the result is higher than you want, test a higher ACH value, fewer occupants, or a shorter continuous occupancy period to see which change has the largest effect.

Indoor CO₂ Risk Classification

These indoor CO₂ categories match the labels used by the calculator’s result display. A predicted or measured concentration above 2000 ppm is a prompt to review outdoor-air delivery, occupancy, and how long the room is continuously occupied. The result does not identify the cause of a ventilation problem by itself: a low ACH setting, an unexpectedly crowded room, an incorrect room volume, or air that does not mix evenly can all make conditions differ from the estimate. Use the classification to prioritize a closer look at the room’s actual ventilation and occupancy pattern.

Improving Indoor Air Quality for CO₂ Control

For indoor CO₂ control, increasing the supply of outdoor air is the direct lever represented by the ACH input. Mechanical ventilation, outdoor-air systems, and open windows or doors can increase air exchange when conditions allow. Reducing the number of people in the room or breaking up a long occupancy period also reduces the amount of CO₂ generated during the scenario. The calculator is useful for comparing these choices because it separates the effects of room size, people, and ventilation rather than treating a single CO₂ reading as a fixed property of the space.

Use the CO₂ estimate as a planning aid for rooms with changing schedules. For example, enter a typical classroom, meeting, or living-space configuration and compare it with a fuller-than-usual occupancy level. You can also model a longer session to see whether the time-based result is already close to the equilibrium level. The output is based on the values entered in the form, so it is most useful when room dimensions, likely attendance, and ventilation assumptions are checked carefully. A portable fan may improve air circulation within a room, but the model’s ACH value specifically represents replacement with outdoor air.

Indoor CO₂ Model Limitations and Assumptions

This indoor CO₂ buildup calculation assumes that room air is perfectly mixed, so it assigns one concentration to the entire room. Real rooms can have uneven airflow near vents, windows, doors, or stagnant areas, and a monitor’s location can therefore matter. The calculator also assumes a constant generation rate per person. Activity level affects exhaled CO₂, so people exercising or doing strenuous work may not match the estimate for occupants at rest. The model fixes outdoor concentration at 400 ppm and does not provide an input for a different outdoor baseline.

The indoor CO₂ calculation also assumes that occupancy and ACH remain constant for the entire selected exposure time. People entering or leaving, a window being opened partway through a meeting, or a ventilation system cycling can change the actual concentration path. The equilibrium value is not an instant reading; it is the level approached under unchanged conditions over time. For a room with complex airflow, unusually variable use, or a need for formal assessment, use direct measurement and appropriate professional guidance in addition to this simplified estimate.

By showing how volume, occupancy, ventilation, and time interact, the Indoor CO₂ Buildup Calculator offers a focused way to reason about room-air conditions. It can help a teacher compare class sizes, a facility manager test ventilation assumptions, or a household consider how a gathering changes a room’s likely CO₂ level. Treat the output as an estimate of a well-mixed room, then use observations and measurements to confirm how the actual space performs.