A person sitting at a desk exhales about 18 liters of carbon dioxide every hour. Put ten people in a closed meeting room and that adds up fast. A CO₂ breathing emission calculator turns those numbers into a prediction of how high indoor CO₂ will climb and how much fresh air you need to hold it down.

For US readers, this matters in bedrooms, classrooms, offices, and gyms. Indoor CO₂ is a handy stand-in for stale air, and a CO₂ breathing emission calculator lets you check a room before you buy a sensor or change a ventilation setting. This guide shows you how to use it, what the formula means, and how to read your result with a clear head.

What Is a CO₂ Breathing Emission Calculator and Why Does It Matter?

A CO₂ breathing emission calculator estimates how much carbon dioxide occupants exhale and how high the indoor CO₂ concentration rises in a room. It follows one plain-English rule: indoor CO₂ equals outdoor CO₂ plus exhaled CO₂ divided by the airflow that dilutes it. Facility managers, HVAC engineers, teachers, and homeowners use it to size ventilation. Get it wrong and a classroom stays stuffy, people feel drowsy, or a system wastes energy heating and cooling far more outdoor air than the room needs.

What You Need Before You Start

Gather these details first so your CO₂ breathing emission calculator result reflects a real room.

The number of occupants in the space during its busiest hour.
The activity level of those people, such as sleeping, seated work, teaching, or cycling.
The room volume in cubic feet or cubic meters, found by multiplying length, width, and height.
The outdoor air supply in cubic feet per minute (cfm) or liters per second, or the air changes per hour.
The outdoor CO₂ level, which is about 420 ppm in most US locations.
The approximate age and size of occupants, since children exhale less than adults.

How to Use This CO₂ Breathing Emission Calculator — Step by Step

Follow these steps in order for a dependable estimate.

Enter the number of people who will be in the room at once.
Pick the activity that best matches what they're doing.
Choose adult or child body size, or type in body surface area if you know it.
Enter the room volume and the outdoor airflow in cfm or liters per second.
Leave outdoor CO₂ at 420 ppm unless you have a local reading.
Press calculate and note the indoor CO₂ in ppm.
Compare the result with your target, then raise the airflow input until you reach it.

The CO₂ Breathing Emission Calculator Formula — Explained Simply

The calculator works in two stages. First it estimates how much CO₂ one person exhales, based on body size and how hard they're working. Then it asks how much that CO₂ is diluted by the outdoor air entering the room.

G = 0.00276 × AD × M × RQ / (0.23 × RQ + 0.77)

Cin = Cout + (N × G × 1,000,000) / Q

G is the CO₂ generation rate per person, in liters per second. A seated adult produces about 0.005, and a person cycling hard produces about 0.03.
AD is DuBois body surface area, in square meters. Children run about 0.8 to 1.3, and adults run about 1.5 to 2.2.
M is metabolic rate in met. Sleeping is about 0.7, desk work is 1.0 to 1.2, and vigorous exercise runs 4 to 8.
RQ is the respiratory quotient, which is unitless. It is about 0.83 for typical adults and ranges from 0.7 to 1.0.
Cin is the indoor CO₂ concentration at steady state, in parts per million (ppm). Typical rooms land between 500 and 3,000.
Cout is the outdoor CO₂ concentration in ppm. It runs about 420 in clean air and 450 or more in busy cities.
N is the number of occupants, from 1 to several hundred.
Q is the outdoor airflow supplied to the room, in liters per second. One cfm equals 0.4719 liters per second.

This generation equation appears in Appendix C of ASHRAE Standard 62.1, and the dilution step is a standard mass balance described in ASTM D6245. It works because exhaled CO₂ accumulates until the air leaving the room carries it away as fast as people produce it. Double the airflow and the buildup over outdoor levels is cut in half.

CO₂ Breathing Emission Calculator — Worked Examples

Beginner Example: Two Adults Sleeping in a Closed Bedroom

Two adults sleep in a 12 ft × 12 ft × 8 ft bedroom with the door and windows closed. Leakage supplies 0.3 air changes per hour. Each person has 1.8 m² of body surface area, sleeps at 0.7 met, and has an RQ of 0.83. Outdoor CO₂ is 420 ppm.

G per person = 0.00276 × 1.8 × 0.7 × 0.83 / 0.9609 = 0.0030 L/s.
Two people: 2 × 0.003004 = 0.00601 L/s.
Room volume: 12 × 12 × 8 = 1,152 ft³ = 32,620 L.
Airflow: 0.3 × 32,620 / 3,600 = 2.72 L/s.
Rise above outdoors: 0.00601 × 1,000,000 / 2.72 = 2,210 ppm.
Indoor CO₂: 420 + 2,210 = 2,630 ppm.

That reading is common in tightly sealed bedrooms and explains why some people wake up groggy. Opening a window to reach 1.5 air changes per hour (13.6 L/s) drops the result to about 862 ppm. Crack a window or run a bedroom ventilation fan.

Professional Example: Sizing Outdoor Air for a 12-Person Conference Room

An HVAC engineer designs a conference room for 12 seated adults at 1.2 met, with 1.8 m² body surface area and RQ 0.83. She targets 1,120 ppm, which is 700 ppm above 420 ppm outdoor air. That is a long-used rule of thumb, not a mandatory limit.

G = 0.00276 × 1.8 × 1.2 × 0.83 / 0.9609 = 0.00515 L/s.
Total: 12 × 0.00515 = 0.0618 L/s.
Allowed rise: 1,120 − 420 = 700 ppm.
Q = 0.0618 × 1,000,000 / 700 = 88.3 L/s.
Convert: 88.3 / 0.4719 = 187 cfm.
Per person: 187 / 12 = 15.6 cfm.

The room needs about 190 cfm of outdoor air to hold that target. That is roughly three times the 5 cfm per person people-component in ASHRAE 62.1 for offices, which shows why a code minimum doesn't guarantee a low CO₂ reading. She should specify at least 190 cfm and add a CO₂ sensor for demand control.

Edge Case: A 20-Person Spin Class in a Small Studio

A fitness instructor runs a 45-minute class for 20 riders at 6.0 met. Riders have 1.9 m² body surface area and an RQ of 0.95 because hard exercise raises it. The 280 m³ studio gets 400 cfm of outdoor air.

G = 0.00276 × 1.9 × 6.0 × 0.95 / 0.9885 = 0.0302 L/s.
Total: 20 × 0.0302 = 0.605 L/s.
Airflow: 400 × 0.4719 = 188.8 L/s.
Steady-state rise: 0.605 × 1,000,000 / 188.8 = 3,204 ppm.
Time constant: 280,000 L / 188.8 L/s = 1,483 s, or 24.7 minutes.
After 45 minutes: 3,204 × (1 − e^(−45/24.7)) = 3,204 × 0.838 = 2,685 ppm, so indoor CO₂ is about 3,105 ppm.

The class never reaches steady state, so the full 3,624 ppm overstates the peak. Still, 3,105 ppm is triple the common 1,000 ppm comfort mark. To hold 1,500 ppm, the studio needs about 1,190 cfm of outdoor air.

Different Units: An Elementary Classroom Measured in CFM

A school facilities manager checks a classroom with 25 ten-year-olds and one teacher. Students have 1.1 m² of body surface area, the teacher has 1.8 m², everyone is at 1.2 met, and RQ is 0.83. The supply provides 300 cfm of outdoor air.

Child G = 0.00276 × 1.1 × 1.2 × 0.83 / 0.9609 = 0.00315 L/s.
Students: 25 × 0.00315 = 0.0787 L/s.
Teacher: 0.00515 L/s.
Total: 0.0838 L/s.
Convert airflow: 300 × 0.4719 = 141.6 L/s.
Rise: 0.0838 × 1,000,000 / 141.6 = 592 ppm, so indoor CO₂ is 1,012 ppm.

The room sits right at 1,000 ppm, with about 11.5 cfm per person. Raising outdoor air to 350 cfm (165.2 L/s) lowers the reading to about 927 ppm. He should check that the supply actually delivers 300 cfm, because many classroom units fall short.

CO₂ Breathing Emission Calculator Results — What the Numbers Mean

Read your result against these ranges. Outdoor air sits near 420 ppm. A well-ventilated occupied room usually reads 600 to 1,000 ppm, and that range means the fresh air supply is keeping up. Between 1,000 and 1,500 ppm, ventilation is likely marginal, and many people start to feel the air is stale. From 1,500 to 2,500 ppm, rooms are poorly ventilated, and occupants commonly report drowsiness or headaches. Above 5,000 ppm, you exceed the OSHA 8-hour exposure limit for workplaces, which means you should act right away.

A low result isn't a guarantee of clean air. CO₂ tracks only the exhaled breath of occupants. Smoke, cooking fumes, dust, and chemical vapors can be high while CO₂ stays low.

The most common misunderstanding is believing CO₂ itself is the main hazard at 1,000 to 2,000 ppm. It's mostly a signal of how much fresh air each person gets. The reading helps you judge ventilation, not toxicity.

If your result lands above 1,000 ppm, raise the outdoor air supply or reduce occupancy. If it lands above 1,500 ppm, treat it as a ventilation problem and call a professional. If it lands below 800 ppm, you have room to cut airflow and save energy, though you shouldn't go lower without checking other pollutants first.

How Your Inputs Change the Result

Occupancy has a straight-line effect. Each added person adds a fixed slice of CO₂. In a room with 100 L/s of outdoor air and seated adults at 0.00515 L/s each, the result climbs steadily with headcount.

Occupants	Rise above outdoors (ppm)	Indoor CO₂ (ppm)
5	257	677
10	515	935
20	1,030	1,450
40	2,060	2,480

Activity level has a bigger effect than most users expect. A person cycling hard produces six times the CO₂ of someone sitting still. For an adult with 1.8 m² of body surface area and an RQ of 0.83, output scales directly with met.

Activity	Met	CO₂ per person (L/h)
Sleeping	0.7	10.8
Seated, resting	1.0	15.4
Office work	1.2	18.5
Brisk walking	2.0	30.9
Aerobic class	4.0	61.8
Hard cycling	6.0	92.7

Ventilation has a diminishing return. Take ten seated adults, and watch what each doubling of airflow buys you.

Outdoor air (L/s)	Outdoor air (cfm)	Indoor CO₂ (ppm)
25	53	2,480
50	106	1,450
100	212	935
200	424	677

Going from 25 to 50 L/s cuts the reading by 1,030 ppm. Going from 100 to 200 L/s cuts only about 258 ppm. Fix badly ventilated rooms first, because that's where each extra cfm pays off most.

The remaining inputs shift things more gently. A city block with 450 ppm outdoors adds 30 ppm to every reading. Switching RQ from 0.7 to 1.0 changes a seated adult's output from 0.00448 to 0.00596 L/s, a swing of about a third. Neither moves the answer as much as headcount, activity, or airflow.

CO₂ Breathing Emission Calculator — Common Mistakes to Avoid

Mixing airflow units. Entering cfm where the calculator expects liters per second throws the result off by a factor of about 2. Remember that 1 cfm equals 0.4719 L/s.
Using supply air instead of outdoor air. Recirculated air has CO₂ in it, so only the fresh outdoor portion dilutes the room. Check your HVAC balance report for the outdoor air figure.
Assuming steady state in short events. A 30-minute meeting in a large room may never reach the steady value. Use a transient calculation for short sessions.
Guessing leakage. A sealed modern home might have 0.2 air changes per hour, while an older leaky house might have 1.0. Test with a CO₂ sensor if the answer matters.
Ignoring other CO₂ sources. Gas stoves, unvented heaters, candles, and fireplaces add CO₂ that this calculator doesn't count. High readings with few people point to combustion, which needs inspection.
Applying US thresholds abroad. Europe uses EN 16798-1 categories, and other countries set their own limits. Check which standard governs your building before comparing numbers.
Treating the calculator as a safety device. It doesn't detect carbon monoxide or other hazards. Install working CO alarms and don't rely on estimates in emergencies.

Key Terms Explained

CO₂ Generation Rate

The CO₂ generation rate is the volume of carbon dioxide one person exhales per unit of time, usually in liters per second.

Metabolic Rate (Met)

One met is the heat output of a seated, resting adult, about 58 watts per square meter, and activity levels are expressed as multiples of it.

Respiratory Quotient (RQ)

The respiratory quotient is the ratio of CO₂ exhaled to oxygen consumed, which varies with diet and exercise intensity.

Body Surface Area

Body surface area is the skin area of a person in square meters, estimated here with the DuBois formula from height and weight.

Air Changes per Hour (ACH)

Air changes per hour is how many times the room's full air volume is replaced by outdoor air in one hour.

Steady-State Concentration

Steady-state concentration is the CO₂ level a room settles at when exhaled CO₂ entering the air equals the CO₂ leaving with ventilation.

Parts Per Million (ppm)

Parts per million expresses CO₂ as the number of CO₂ molecules per million air molecules, where 10,000 ppm equals 1 percent.

How This Calculator Works — Methodology

This CO₂ breathing emission calculator uses the CO₂ generation equation from Appendix C of ASHRAE Standard 62.1 (2022 edition) and a single-zone steady-state mass balance consistent with ASTM D6245. It assumes the room air is well mixed, outdoor CO₂ is constant at 420 ppm unless you change it, and airflow stays steady. It excludes combustion sources, plants, pets, and CO₂ removal by scrubbers, because each needs inputs outside this tool's scope. Short events use a time-constant correction when you enter a duration. Generation rates round to four significant digits, concentrations round to the nearest whole ppm, and airflow rounds to the nearest whole cfm. Results are estimates, and a calibrated sensor always beats a calculated number.

Frequently Asked Questions About CO₂ Breathing Emission Calculator

1. What does a CO2 breathing emission calculator actually calculate?

A CO₂ breathing emission calculator estimates how much carbon dioxide people exhale and how high indoor CO₂ climbs in a room. It uses body size, activity level, headcount, ventilation, and outdoor CO₂. The result shows indoor concentration in parts per million, plus the fresh airflow needed to hold a target such as 1,000 ppm.

2. How accurate is a CO2 breathing emission calculator?

Expect accuracy within roughly 10 to 25 percent for a well-mixed room with known inputs. The biggest uncertainty is real ventilation, because leakage, open doors, and weather change daily. Occupancy also varies. Check the estimate against a calibrated NDIR CO₂ sensor, which reads actual concentration and shows whether your assumptions hold.

3. What is the CO2 emission formula in simple terms?

Each person's output equals 0.00276 × body surface area × activity level × respiratory quotient, adjusted by a small correction factor, giving liters per second. Indoor CO₂ then equals outdoor CO₂ plus total exhaled CO₂ divided by the outdoor airflow. A seated adult exhales about 0.005 liters per second, so a roomful adds up quickly.

4. Is CO2 measured in ppm or percent, and does it differ by country?

Both are used, and 10,000 ppm equals 1 percent. Indoor air quality work uses ppm. Airflow units differ by region: the US uses cubic feet per minute, while most other countries use liters per second, and one cfm equals 0.4719 L/s. Europe also applies EN 16798-1 categories instead of ASHRAE guidance.

5. When does a CO2 breathing emission calculator not apply?

It doesn't apply when CO₂ comes from sources other than breathing, such as gas stoves, unvented heaters, fireplaces, or CO₂ tanks. It also fails in poorly mixed spaces and sealed environments like submarines that need dynamic models. A high reading with few people usually signals a combustion source and calls for prompt inspection.

6. How can I get a more precise indoor CO2 estimate?

Measure instead of guess. Place a calibrated NDIR sensor at breathing height, away from windows and direct exhaled breath, and log readings through a full occupied period. Use real headcounts, a realistic activity level, and the actual outdoor air supply from your HVAC balance report. Then adjust the ventilation input until the calculator matches your readings.

7. What is a normal indoor CO2 level?

Outdoor air holds about 420 ppm, and well-ventilated indoor spaces usually read 600 to 1,000 ppm. Offices and classrooms often reach 800 to 1,500 ppm, while sleeping bedrooms with closed windows can exceed 2,000 ppm. Readings below 1,000 ppm are a widely used target for good ventilation in occupied rooms.

8. Should I see a professional about high CO2 indoors?

Yes, if readings stay above 1,500 ppm, people report headaches or drowsiness, or you suspect a combustion source. A licensed HVAC engineer or industrial hygienist can measure ventilation, balance airflow, and check equipment. Seek medical help for symptoms like dizziness or confusion, and leave the space immediately if a carbon monoxide alarm sounds.

Related Calculators Worth Bookmarking

Air Changes per Hour Calculator: Use it to convert your room volume and airflow into ACH, which feeds directly into this calculator's ventilation input.
CFM Calculator: Use it to work out the airflow a fan or duct delivers before you enter outdoor air into this tool.
Room Volume Calculator: Use it to get the cubic feet or cubic meters of an irregular room that this calculator needs.
Ventilation Rate Calculator: Use it to find required outdoor air per person and per square foot for a specific space type.
Metabolic Rate Calculator: Use it to estimate calories burned and met values for unusual activities not listed here.
Maximum Occupancy Calculator: Use it to set a headcount limit so your CO₂ target stays achievable.
Air Purifier CADR Calculator: Use it to size filtration for particles and smoke, which this calculator does not address.
Carbon Footprint Calculator: Use it to compare breathing emissions with the larger CO₂ footprint of heating, cooling, and travel.

References and Further Reading

ASHRAE. Standard 62.1-2022: Ventilation and Acceptable Indoor Air Quality. ASHRAE, 2022.
ASTM International. ASTM D6245-18: Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation. ASTM International, 2018.
Persily, A., and de Jonge, L. Carbon Dioxide Generation Rates for Building Occupants. Indoor Air, Wiley, 2017.
Satish, U., et al. Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance. Environmental Health Perspectives, 2012.
National Institute for Occupational Safety and Health. NIOSH Pocket Guide to Chemical Hazards. U.S. Department of Health and Human Services, 2007.
NOAA Global Monitoring Laboratory. Trends in Atmospheric Carbon Dioxide. National Oceanic and Atmospheric Administration, 2024.

Important Disclaimer

This calculator is for informational and educational purposes only. Results are estimates based on the inputs you provide and simplified assumptions about airflow and mixing. This tool does not constitute medical or safety advice. Always consult a qualified HVAC engineer, industrial hygienist, or physician before making real decisions.