CO₂ From Breathing Emission Calculator
Enter body weight, activity level, and duration — the calculator returns exhaled CO₂ mass and volume from human respiration using published metabolic-equivalent (MET) values.
CO₂ exhaled over the period
588.8 g (0.589 kg)
- MET value
- 1
- Oxygen uptake VO₂ (mL/min)
- 245
- CO₂ production VCO₂ (mL/min)
- 208.25
- Volume of CO₂ (L)
- 299.88
- Rate (g CO₂ per hour)
- 24.53
Oxygen uptake is estimated as VO₂ = MET × 3.5 × body-weight-kg (mL/min). CO₂ output equals VO₂ × RER, with respiratory exchange ratio RER = 0.85 for a mixed adult diet. Volume is converted to mass using the STP density of CO₂ (≈ 1.964 g/L, from molar mass 44.01 g/mol ÷ molar volume 22.414 L/mol).
How to use this calculator
Enter your body weight in kilograms, choose an activity level (from sleeping through sprint effort), and set the duration in hours — use 24 for a whole day. The calculator returns the mass of CO₂ exhaled in grams and kilograms, the exhaled volume in litres, and the per-hour rate. Weight in pounds? Multiply by 0.4536 first.
How the calculation works
The metabolic equivalent (MET) rates the intensity of an activity relative to sitting quietly. One MET is defined as 3.5 mL of oxygen consumed per kilogram of body weight per minute, so oxygen uptake VO₂ (mL/min) = MET × 3.5 × weight-kg. Carbon-dioxide production VCO₂ is VO₂ multiplied by the respiratory exchange ratio (RER), which for a mixed adult diet averages 0.85. Converting the exhaled gas volume to mass uses the standard density of CO₂ at 0 °C and 1 atm: molar mass 44.01 g/mol divided by molar volume 22.414 L/mol, giving about 1.964 g per litre.
Worked example
A 70-kg adult resting for a full 24-hour day. Oxygen uptake VO₂ = 1 × 3.5 × 70 = 245 mL/min. CO₂ production VCO₂ = 245 × 0.85 ≈ 208.25 mL/min. Over 1,440 minutes the volume exhaled is 208.25 × 1,440 / 1,000 ≈ 299.9 L. Multiplying by 1.964 g/L gives about 589 g, or 0.59 kg, of CO₂ per day at complete rest. A sedentary day averaging ~1.4 MET lifts the figure closer to 0.8 kg, which matches the commonly quoted "about 1 kg CO₂ per person per day" for an active adult.
Frequently asked questions
How much CO₂ does a person breathe out per day?
A resting 70-kg adult exhales roughly 0.6 kg of CO₂ per day; a typical sedentary adult exhales about 0.8–1 kg; an active adult 1.2–1.5 kg. The figure scales linearly with body weight and with the time-averaged MET value of the day.
What is a MET value?
A metabolic equivalent, or MET, is the ratio of a task’s metabolic rate to the resting metabolic rate. One MET equals 3.5 mL of oxygen consumed per kilogram of body weight per minute, set by the American College of Sports Medicine. The Ainsworth Compendium lists MET values for hundreds of activities.
What is the respiratory exchange ratio (RER)?
RER is the ratio of CO₂ produced to O₂ consumed at the mouth. Burning pure carbohydrate gives RER 1.0, pure fat gives about 0.7. A mixed adult diet averages 0.85, which is the value this calculator uses. During short intense exercise RER can briefly exceed 1.0, but that averages out over longer durations.
Does the CO₂ I exhale add to climate change?
No. Human-exhaled CO₂ is part of the short carbon cycle — the carbon comes from food, which comes from plants that pulled that carbon out of the atmosphere weeks or months earlier. Fossil-fuel emissions add new carbon from long-buried stores, which is what shifts the atmospheric balance. Breathing calculators are useful for indoor-air quality and ventilation sizing, not for climate footprints.
Why is this useful for indoor ventilation?
ASHRAE Standard 62.1 sizes ventilation partly by expected CO₂ output per occupant. Knowing how much CO₂ occupants produce lets you size fresh-air flow so indoor CO₂ stays below the ~1,000 ppm level associated with drowsiness and impaired concentration. This calculator gives the per-person source term used in that sizing.
How accurate is a MET-based estimate?
MET-based CO₂ estimates are typically within 10–20% of measured values for adults of average build. Accuracy drops for very lean or very obese subjects (RER shifts), for children (higher metabolic rate per kg), and for very short intense bursts (RER > 1). For engineering sizing that safety margin is more than adequate.