Use the CO₂ that people exhale as an indicator to size the ventilation a room needs. Change the occupancy, room volume and target CO₂ concentration to see the required ventilation rate, per-person airflow, air changes per hour and the time constant for the concentration to settle — and design an indoor environment that does not feel stuffy.
Parameters
Occupancy n
people
Number of people in the room at the same time
Per-person CO₂ generation g
L/h
About 13 at rest, 19 for light work, 30+ during exercise
Outdoor CO₂ C_oa
ppm
About 400 in suburbs, over 450 near busy city roads
Target indoor CO₂ C_t
ppm
Building codes commonly set a 1000 ppm ceiling
Room volume V
m³
Floor area × ceiling height. Affects ACH and time constant
Results
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Required ventilation Q (m³/h)
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Per-person airflow (m³/h·person)
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Air changes ACH (1/h)
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CO₂ generation G (L/h)
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Time constant τ (min)
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Per-person airflow verdict
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Room ventilation cross-section
CO₂ (dots) released by occupants spreads through the room and is pushed out by incoming fresh air. The right-hand bar is the current indoor CO₂ concentration (green ≤ 1000 ppm / orange-red above).
Required ventilation Q vs target CO₂ concentration
Transient indoor CO₂ concentration C(t) for a well-mixed room, and the time constant τ. V: room volume, Q: ventilation rate. As t → ∞, C(t) approaches the target C_t.
Air changes per hour ACH and per-person airflow q_person. Building codes and ASHRAE 62.1 use a guideline of roughly 30 m³/h per person.
What is CO₂-Based Required Ventilation?
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Why use CO₂ concentration to decide the ventilation rate? CO₂ isn't actually harmful, is it?
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Good question. CO₂ itself at around 1000 ppm has almost no direct toxicity. But CO₂ is a very convenient marker for "how much breathing is going on" in a room. When people breathe out CO₂ they also release body odour, moisture and droplets that may carry viruses. A high CO₂ level is a sign that all of those are building up too. So the idea is: "if CO₂ stays below 1000 ppm, the overall air quality is probably acceptable" — and you size the ventilation from that.
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I see — CO₂ is a "stuffiness meter" for the air. So how do you calculate the ventilation you need?
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Roughly speaking, you look for the point where the CO₂ coming in balances the CO₂ going out. Inside the room, people keep producing CO₂. Ventilation removes CO₂-laden indoor air and brings in outdoor air instead. When that in-and-out is balanced — the steady state — you get a simple formula: Q = G/(C_t − C_oa). G is the total CO₂ people produce, C_t is the target concentration, C_oa is the outdoor concentration. Raise the occupancy on the left slider and you will see the required ventilation Q shoot up in proportion.
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The denominator (C_t − C_oa) bothers me. If I lower the target from 1000 to 800 ppm, what happens to the required ventilation?
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That is the crux of it. If outdoor air is 420 ppm, a 1000 ppm target gives an allowable rise of 580 ppm. Drop the target to 800 ppm and the allowable rise shrinks to 380 ppm. The denominator becomes about two-thirds, so the required ventilation Q rises by about 1.5×. Look at the "Required ventilation vs target CO₂" chart and you will see the curve climb steeply as the target gets stricter. Better air quality means a steep jump in ventilation cost — fan power and heating/cooling loss. That is the trade-off.
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There are also cards for "ACH" and "time constant τ". What are those telling me?
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ACH is how many times the room's whole air volume is replaced per hour. τ = V/Q is how slowly the concentration settles after people enter. The "Transient indoor CO₂" chart shows exactly that — it rises fast at first, then after one τ closes 63% of the gap to the target, and is essentially steady by 5τ. What matters in practice is that a large room, like a gymnasium, has a long τ. CO₂ might only reach the ceiling 30 minutes after a class starts. So you look not just at the steady value but also at when it gets there.
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Finally, how do I connect the calculated ventilation rate to actual equipment?
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The Q [m³/h] you get is the guideline outdoor-air intake the room needs. You select exhaust fans or the outdoor-air fan of an AHU to meet it, and place supply and exhaust grilles. One caveat: this calculation assumes "perfect mixing" — uniform concentration everywhere. In reality only the area near the supply grille is fresh and the corners stagnate. So on site you place CO₂ sensors in the breathing zone (seated head height), measure, and compare with the calculated value.
Frequently Asked Questions
From a steady-state mass balance, the required ventilation rate is Q = G / (C_target − C_outdoor), where G is the indoor CO₂ generation, C_target is the target indoor CO₂ concentration and C_outdoor is the outdoor CO₂ concentration, with concentrations treated as dimensionless volume fractions. Generation G is the product of the number of occupants n and the per-person generation g. Since ppm is the usual unit in practice, it is convenient to write Q = G[m³/h] / ((C_t − C_oa)[ppm] / 10⁶).
Many building codes set an indoor CO₂ ceiling of 1000 ppm (0.1%), which is the standard target. Outdoor air is roughly 400-420 ppm, so the allowable rise is about 600 ppm. For meeting rooms or learning spaces where concentration matters, a stricter target of 800 ppm or below is sometimes used. The stricter the target, the smaller the allowable rise and the sharply higher the required ventilation rate.
Building codes typically require roughly 20-30 m³/h of fresh air per person for occupied rooms, and ASHRAE 62.1 gives about 25-30 m³/h per person of outdoor air for offices. This tool rates per-person ventilation of 30 m³/h or more as "Good", 20-30 m³/h as "Minimum", and below 20 m³/h as "Insufficient". For a fixed CO₂ generation, the per-person airflow depends only on the difference between target and outdoor concentration.
Air changes per hour ACH = Q / V is how many times the room's air is replaced per hour. The time constant τ = V / Q is its inverse and measures how fast the concentration approaches its steady value. After one τ about 63% of the gap to the target is closed, after 3τ about 95%, and after 5τ it is essentially steady. A large room has a long τ, so it takes longer for the concentration to settle after people enter.
Real-World Applications
Office and meeting-room HVAC design: In office-building HVAC design, the required outdoor-air rate is derived from occupancy — often estimated as a people density per floor area. Meeting rooms are a classic case: many people gather in a short time, the people density is high, and CO₂ rises quickly. Raise the occupancy in this tool to meeting-room levels and you can feel how the per-person airflow swings to "Minimum" or "Insufficient". Demand-controlled ventilation (DCV) — throttling the outdoor-air rate to measured CO₂ — is now mainstream for balancing energy and air quality.
School and classroom ventilation planning: Classrooms have a high pupil density per floor area and are spaces where CO₂ readily becomes a problem; school environmental-health standards also use indoor CO₂ as an indicator. Because CO₂ keeps rising during a lesson, a common practice is to reset it by opening windows during breaks and to combine that with mechanical ventilation. A large gymnasium has a long time constant τ, so concentration rises slowly even with a crowd at an event — but once raised, it is correspondingly slow to come back down.
Infection control and post-pandemic ventilation upgrades: The risk of airborne aerosol infection is known to correlate with indoor CO₂ concentration. Treating CO₂ as a proxy for "how much of other people's exhaled air you are breathing", many operators adopted stricter targets of 800 ppm or below in crowded spaces. Lowering the target in this tool makes the required ventilation jump, giving a quantitative sense of how much extra cost a ventilation upgrade for infection control entails.
Residential continuous ventilation and sick-building measures: Many countries require dwellings to have continuous ventilation of at least roughly 0.5 ACH. That is mainly aimed at VOCs such as formaldehyde, but CO₂-based assessment of occupant density is also a useful livability indicator. Enter a small room volume and few occupants for a residential case and you can examine the relationship between required ventilation and air changes per hour at house scale.
Common Misconceptions and Pitfalls
The biggest misconception is applying the perfect-mixing assumption directly to a real space. The C(t) and steady concentration in this tool assume CO₂ is uniform throughout the room. A real room is fresh near the supply grille and stagnant in the far corner or behind desks, so concentrations can differ by hundreds of ppm within the same room. If the ventilation effectiveness — how well supply air actually displaces polluted indoor air — is poor, the CO₂ where people sit can exceed the target even when the calculated rate is adequate. Always check the layout of supply and exhaust grilles and whether there is short-circuiting (supply air going straight to exhaust).
Next, looking only at the steady state and ignoring the transient. Q = G/(C_t − C_oa) is the value after enough time has passed. But in meeting rooms or classrooms where people come and go a lot, the occupancy often changes before steady state is reached. In rooms with a long time constant τ — a large volume or a small ventilation rate — the concentration may still be rising 30 minutes to an hour after people enter. A "steady value within the limit" does not guarantee "within the limit at the peak", so evaluate the transient response curve and the time constant together.
Finally, "more ventilation is always better" is not true. More ventilation does lower CO₂, but bringing in outdoor air increases the heating and cooling load and the fan power. In peak summer and winter, ventilation accounts for a large share of energy use. That is exactly why demand-controlled ventilation (DCV) — throttling the outdoor-air rate when the space is empty or lightly occupied — matters. And in regions where the outdoor air itself is polluted with PM2.5 or pollen, indiscriminately bringing in outdoor air creates a different air-quality problem, so you must judge holistically, including outdoor-air condition and filter performance.
How to Use
Enter occupancy count (nNum) and select range: typical office 5-50 persons, classroom 20-100 persons
Input room volume (m³) via nRange slider; set outdoor CO₂ concentration (coaNum, typically 400-420 ppm)
Define target indoor CO₂ setpoint (ctNum, e.g., 800 ppm for comfort, 1200 ppm for economy) and run simulation to calculate required ventilation Q in m³/h
Worked Example
Conference room: 8 occupants, 120 m³ volume, outdoor CO₂ 410 ppm, target 900 ppm indoor. Each person generates ~16.5 L/h CO₂ (total G = 132 L/h). Simulator calculates required Q = 485 m³/h ventilation rate (≈60.6 m³/h per person), producing 6.8 air changes/hour (ACH). Time constant τ = 17.6 minutes indicates CO₂ response speed. If per-person airflow falls below 7 m³/h·person ASHRAE minimum, verdict flags inadequacy.
Practical Notes
CO₂ generation rate assumes ~0.2 L/min per adult metabolic equivalent; adjust gNum/gRange for children (lower) or intense activity (higher, up to 0.3 L/min)
Outdoor CO₂ coaNum varies seasonally: urban areas 450+ ppm, rural 380-400 ppm; use measured data from local monitoring stations
Demand-controlled ventilation systems use this simulator's CO₂ setpoint (ctNum) logic to modulate fan speed, reducing energy 20-40% versus fixed-schedule operation
ACH verdict: below 2/h indicates poor flushing; above 8/h typical for healthcare/lab standards; classroom comfort zone 4-6 ACH at 900-1000 ppm target