CO₂ Breathing Emission Calculator
Work out how much carbon dioxide a person exhales per hour, per day and per year from body mass, age band, sex and activity level.
🌱 What is a CO₂ Breathing Emission Calculation?
Human CO₂ breathing emission is the rate at which a person's metabolism releases carbon dioxide into the air around them. Every cell burns fuel from food using oxygen, and carbon dioxide is the waste product carried back to the lungs and breathed out. The rate is not a fixed number per person, it scales directly with how hard the body is working, so a sleeping adult and the same adult running produce carbon dioxide at rates that differ by roughly a factor of nine.
Building services engineers need this number to size ventilation. Demand-controlled ventilation systems measure indoor CO₂ and modulate fresh-air supply to keep concentration under about 1000 ppm, and the design calculation starts from the occupants' generation rate. Anaesthetists and intensive-care clinicians use the same quantity when setting minute ventilation on a ventilator. Divers and submarine engineers use it to size carbon dioxide scrubbers for a sealed atmosphere. Spacecraft and habitat designers use it for the same reason. Schools and science communicators use it to answer the perennial question of whether human breathing warms the planet.
The answer to that last question is a firm no, and it is the biggest misconception attached to this number. Breathing is carbon neutral over an annual cycle, because the carbon leaving your lungs entered the food chain when a plant pulled it out of the atmosphere weeks or months earlier. Fossil fuel combustion is different because it releases carbon that has been out of circulation for tens of millions of years. A second misconception is that body weight is the main driver. It is not. The metabolic rate equation has a large constant term, so activity level moves the result far more than body size does.
This calculator applies the standard building-science formula for metabolic carbon dioxide generation, combining Schofield basal metabolic rate coefficients with a met-unit activity multiplier and a respiratory quotient. It reports the rate in litres per second (the unit used in ventilation standards), litres and grams per hour, kilograms over the period you enter, and tonnes per year. A comparison table shows the same person's output at every activity level, and the chart shows how output varies with body mass across all five age bands.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Average Adult Over a Full Day
70 kg male aged 30 to 60, standing relaxed (1.2 met), RQ 0.85, 24 hours
Example 2 — An Eight Hour Walking Shift
60 kg female aged 18 to 30, walking at 4 km/h (2.6 met), RQ 0.85, 8 hours
Example 3 — A Teenager in a Six Hour School Day
35 kg male aged 10 to 18, seated light office work (1.4 met), RQ 0.85, 6 hours
Example 4 — One Hour of Hard Exercise
80 kg male aged 18 to 30, heavy exercise (6.0 met), RQ 0.85, 1 hour
❓ Frequently Asked Questions
🔗 Related Calculators
How much CO2 does a human exhale per day?
A 70 kg adult male aged 30 to 60, standing relaxed at 1.2 met for 24 hours, produces about 15.18 litres of CO2 per hour, which is 30.0 grams per hour or roughly 0.72 kg per day. A typical mixed day of sitting, walking and sleeping lands most adults between 0.7 and 1.1 kg of CO2 daily.
What is the formula for human CO2 generation rate?
The standard building-science formula is VCO2 = RQ x BMR x M x 0.000569 / (0.23 x RQ + 0.77), giving litres per second. RQ is the respiratory quotient (about 0.85), BMR is basal metabolic rate in megajoules per day, and M is the activity level in met units.
What is a met unit?
One met is the metabolic rate of a person sitting quietly, roughly 58 watts per square metre of body surface. Sleeping is about 0.7 met, standing relaxed 1.2 met, walking at 4 km/h about 2.6 met, and hard running 6 to 8 met. Multiplying basal metabolic rate by the met value gives total energy expenditure.
Does breathing contribute to climate change?
No, not on a net basis. The carbon in exhaled CO2 came from food, and that food's carbon was pulled from the atmosphere by plants during the same growing season. Human respiration therefore cycles carbon rather than adding it, unlike fossil fuel combustion which releases carbon locked away for millions of years.
What is the respiratory quotient and what value should I use?
The respiratory quotient is the ratio of CO2 exhaled to oxygen consumed. It is 1.0 for pure carbohydrate metabolism, about 0.7 for pure fat, and roughly 0.85 for a mixed Western diet. Use 0.85 unless you have measured data, and stay inside the 0.6 to 1.0 range.
How is basal metabolic rate estimated here?
The calculator uses the Schofield body-mass-only equations, which give BMR in megajoules per day from body mass, sex and age band. For a male aged 30 to 60 the relation is BMR = 0.048 x mass + 3.653, so a 70 kg man has a basal rate of about 7.013 MJ per day.
Why does a child produce almost as much CO2 as an adult?
Because metabolic rate per kilogram is much higher in growing children. A 35 kg boy aged 10 to 18 has a basal rate of about 5.34 MJ per day against 7.01 MJ for a 70 kg adult man, so at half the body mass he still runs at about three quarters of the adult metabolic output.
How much CO2 do people add to a closed room?
Multiply the per-person litres per hour by the number of occupants. Ten adults at 15 litres per hour add 150 litres of CO2 per hour. In a 100 cubic metre sealed room that raises the concentration by about 1500 ppm per hour, which is why unventilated meeting rooms become stuffy so quickly.
Does exercise really multiply CO2 output that much?
Yes. CO2 generation scales directly with the met value in this formula, so an 80 kg man aged 18 to 30 goes from about 14.3 litres per hour seated quietly to 85.87 litres per hour at 6 met during heavy exercise, a sixfold increase in exhaled carbon dioxide.
Are the litre values at room temperature or standard conditions?
Standard conditions, 0 degrees C and 101.325 kPa, matching the source formula. The gram figures use a CO2 density of 1.977 g/L on that same basis. At 20 degrees C the same mass of gas occupies about 7 percent more volume.
Can I use this to size ventilation for a building?
It gives the CO2 generation term you need, but full ventilation design also requires outdoor CO2 concentration, target indoor concentration, occupancy schedule and room volume. Use this figure as the source term, then apply a mass-balance or steady-state ventilation equation.
Why do the results change when I switch sex or age band?
Each combination has its own Schofield coefficients. Males generally have higher lean mass and a higher basal rate at the same body weight, and metabolic rate per kilogram declines with age, so an 18 to 30 band gives a higher figure than an over-60 band at identical mass.