CO2 Breathing Emission Calculator
Every breath you exhale contains about 4 percent CO2. This calculator works out your personal CO2 breathing emission in grams per hour, kilograms per day, and kilograms per year using your body size and activity level. The math is based on the Persily and de Jonge (2017) formula adopted by ASHRAE ventilation standards, with body surface area computed from the DuBois equation. Switch between metric and imperial units, pick your typical activity, and set how many hours per day you spend at that intensity.
Formula
Worked example
An adult who weighs 70 kg and stands 175 cm has a DuBois area of 0.203 x 1.75^0.725 x 70^0.425 = 1.853 m^2. Sitting (1.3 MET): VCO2 = 8.747 x 1.853 x 1.3 = 21.1 L/h = 41.4 g/h. Over 8 h sitting and 16 h resting (1.0 MET), daily output is 41.4 x 8 + 31.8 x 16 = 331.2 + 508.8 = 840 g, or about 0.84 kg.
Why do you exhale CO2?
Every cell in your body burns glucose (and fat) with oxygen in a process called cellular respiration, producing energy, water, and carbon dioxide. The CO2 diffuses into the bloodstream and is transported to the lungs, where it crosses into the alveolar air and is expelled with each breath. Exhaled air is roughly 4 percent CO2, compared with 0.04 percent in the outdoor atmosphere, a 100-fold enrichment. The amount you exhale depends on how fast your cells are working, which scales with physical activity: running produces about ten times more CO2 per hour than sleeping.
The Persily-de Jonge formula and DuBois surface area
This calculator uses the Persily and de Jonge (2017) model, the method cited in ASHRAE Standard 62.1 for estimating ventilation requirements in buildings. The formula is VCO2 = 0.00276 x AD x M x RQ / (0.23 x RQ + 0.77), where AD is the DuBois body surface area in m^2, M is the metabolic equivalent (MET), and RQ is the respiratory quotient (0.85 for a typical mixed diet). The DuBois surface area captures body size in a single number: AD = 0.203 x H^0.725 x W^0.425, where H is height in metres and W is weight in kilograms. A larger body has more cells burning fuel and therefore exhales more CO2 at every activity level.
Is human breathing a significant source of CO2 emissions?
Biologically, no. The carbon you exhale came from food you ate, which came from plants that took the same carbon from the atmosphere via photosynthesis. The cycle is closed: exhaled CO2 is re-absorbed by plants, so breathing adds no net CO2 to the atmosphere. It is fundamentally different from burning fossil fuels, which releases carbon that has been locked underground for millions of years. A 70 kg adult exhales roughly 280 to 350 kg of CO2 per year from breathing, but their total carbon footprint from energy, transport, and food is typically 4-15 tonnes per year in a high-income country, between 10 and 50 times larger.
How activity level affects your output
MET (metabolic equivalent of task) is the ratio of your metabolic rate during an activity to your resting metabolic rate. Sitting quietly is about 1.3 MET, walking at a normal pace is about 3.5 MET, and hard running can reach 11 MET or more. Because the Persily formula scales linearly with MET, your CO2 output during hard running is roughly 8 times higher than at rest. The table below shows computed rates for a 70 kg, 175 cm adult across common activities. Values for other body sizes can be estimated by scaling proportionally with your DuBois surface area relative to the 1.866 m^2 reference.
Typical CO2 emission rates by activity level
| Activity | MET | CO2 (L/h) | CO2 (g/h) | kg per 8-hour period |
|---|---|---|---|---|
| Sleeping | 0.95 | 15.4 | 30.2 | 0.24 |
| Resting / lying awake | 1.0 | 16.2 | 31.8 | 0.25 |
| Sitting quietly | 1.3 | 21.1 | 41.4 | 0.33 |
| Standing / light desk work | 1.8 | 29.2 | 57.3 | 0.46 |
| Light housework / slow walk | 2.5 | 40.5 | 79.6 | 0.64 |
| Walking at normal pace | 3.5 | 56.7 | 111.4 | 0.89 |
| Cycling / moderate exercise | 6.0 | 97.3 | 191.0 | 1.53 |
| Jogging | 8.0 | 129.7 | 254.7 | 2.04 |
| Running hard | 11.0 | 178.3 | 350.2 | 2.80 |
Estimated CO2 emission rates for a reference adult (70 kg, 175 cm) computed with the Persily-de Jonge formula. Higher body mass or height increases all figures proportionally.
Frequently asked questions
How much CO2 does an average person exhale per day?
A typical adult weighing around 70 kg exhales roughly 700-900 grams of CO2 per day, depending on their mix of activity levels. Published estimates range from about 200 g at very low activity (mostly sleeping) to over 1 kg on a highly active day. The widely cited figure of "about 1 kg per day" assumes a moderately active adult spending most of the day at desk work and sleeping the rest.
Does breathing contribute to climate change?
No. Human breathing is part of the biogenic carbon cycle. The carbon in your exhaled CO2 came from food grown by plants, which captured it from the atmosphere by photosynthesis. When you exhale, the carbon returns to the atmosphere and can be re-absorbed. This is a closed loop with no net addition to atmospheric CO2, unlike burning fossil fuels, which releases carbon that was stored underground and adds new carbon to the active cycle.
Why does body size affect CO2 output?
Larger bodies have more cells, which together consume more oxygen and produce more CO2 even at rest. The Persily formula uses DuBois surface area, which captures total body size in a single number, as the main scaling factor. A person with a 10 percent larger body surface area exhales about 10 percent more CO2 at the same activity level. Height and weight both contribute, but weight has a larger influence because of the exponent in the DuBois formula (0.425 for weight vs 0.725 for height).
What is MET and how does it relate to CO2 exhaled?
MET stands for metabolic equivalent of task. One MET is the energy your body uses when resting completely, about 1 kcal per kg per hour. Running hard at 11 MET burns about 11 times more energy per unit of body mass than resting. Because CO2 is a direct byproduct of energy metabolism, your CO2 exhale rate scales almost linearly with MET. This calculator uses published MET values from the Compendium of Physical Activities.
How is the calculation different from room CO2 concentration calculators?
Room CO2 concentration calculators estimate how quickly exhaled CO2 builds up in an enclosed space, which depends on the room volume, ventilation rate, and number of occupants. This calculator is simpler: it tells you how many grams or kilograms of CO2 you personally exhale per hour or day, without considering where that CO2 goes afterwards. The per-person emission rate used in this calculator feeds into room-level models as an input.
How accurate is the Persily-de Jonge formula?
For a healthy adult at moderate activity levels, the formula is accurate to within about 10-15 percent compared with direct measurements. Accuracy decreases at very high exercise intensities, where the respiratory quotient rises above 0.85 as the body shifts to burning more carbohydrate. Individual variation in basal metabolic rate also introduces scatter. The formula is best used as a practical estimate rather than a precise physiological measurement.