Hong Life CalculatorsUnderstanding the breathing CO₂ calculator — concept and principle
This calculator estimates how much carbon dioxide a person breathes out per day and per year, and the flow of CO₂ in litres per hour. It is used for ventilation planning in rooms and classrooms, for CO₂-monitor experiments, and to answer the common question of whether human breathing adds to climate change.
How it is calculated
- Basal metabolic rate (BMR, MJ/day) from the Schofield equations used by FAO/WHO/UNU, by sex and age group. Men 30–60: BMR = 0.048 × weight + 3.653.
- CO₂ flow (L/s) = RQ × BMR × M × (T ÷ P) × 0.000211, with respiratory quotient RQ = 0.85, M the activity multiple, T in kelvin and P = 101.325 kPa.
- For a whole day M is the physical activity level (PAL): 1.55 sedentary or light, 1.85 active, 2.2 vigorous.
- Mass per day = flow at 0 °C × 86,400 s ÷ 22.414 L/mol × 44.01 g/mol.
- Hourly flow is shown at 20 °C, both at the daily PAL and at rest (M = 1).
The principle
Your body gets energy by oxidising carbohydrate, fat and protein, and the carbon in that food leaves through the lungs as CO₂. The respiratory quotient, the ratio of CO₂ produced to oxygen used, is about 0.85 for a mixed diet. So exhaled CO₂ tracks energy use: a bigger or more active person makes more. Persily and de Jonge turned this into a building-ventilation formula. Gas volume grows with temperature, so the same mass of CO₂ takes up more litres at 20 °C than at 0 °C.
Worked example
Default inputs: male, 30–60 years, 75 kg, sedentary or light activity (PAL 1.55), 1 person.
- BMR = 0.048 × 75 + 3.653 = 7.253 MJ/day = 7.253 ÷ 0.004184 = 1,734 kcal/day; daily use × 1.55 = 2,687 kcal.
- Flow at 0 °C = 0.85 × 7.253 × 1.55 × (273.15 ÷ 101.325) × 0.000211 = 0.005435 L/s.
- Per day: 0.005435 × 86,400 = 469.6 L ÷ 22.414 × 44.01 g = 0.922 kg; per year × 365.25 = 337 kg.
- At 20 °C: 21 L/h on average and 13.5 L/h at rest.
| Body weight (kg) | CO₂ breathed out per day (kg) |
|---|---|
| 18.8 | 0.579 |
| 37.5 | 0.693 |
| 56.2 | 0.808 |
| 75 | 0.922 |
| 93.8 | 1.037 |
| 112 | 1.151 |
| 150 | 1.38 |
How to read the result
About 0.9–1 kg of CO₂ a day is typical for an adult. For a room, the litres-per-hour figure tells you how fast CO₂ builds up and how much fresh air is needed to keep indoor levels near outdoor air. Breathing does not add to long-term warming the way fossil fuels do, because the carbon in food was taken from the air by plants within the last year or so.
Common mistakes
- Counting breathing in a personal carbon footprint as if it were fossil CO₂.
- Using the vigorous level for a whole day; few people sustain that.
- Comparing litres at different temperatures without converting.
- Applying adult equations to young children.
Frequently asked questions
How much CO₂ does a person exhale per day?
Roughly 0.9–1 kg for an adult at light activity, more for larger or more active people.
Does human breathing cause climate change?
Not in the way burning fuel does. The carbon came from plants that recently took it from the air, so it is part of a short cycle.
Sources: A. Persily & L. de Jonge, "Carbon dioxide generation rates for building occupants", Indoor Air 27(5):868–879 (2017) · FAO/WHO/UNU, Human Energy Requirements, FAO Food and Nutrition Technical Report Series 1 (2004)