How much oxygen does a human breathe per day?

If you didn’t know, when we talk about carbon dioxide emissions, the reverse side of the coin is oxygen depletion. This idea sparked my exploration — I probably should say obsession — of where oxygen comes from and goes, and how long it will last. In previous articles, I calculated how much oxygen trees produce and how much oxygen cars consume. Naturally, this led to another question: how much oxygen does a human need each day? 

Previously, the only number I could find was from NASA. According to NASA, the average person doing an average amount of work needs 0.78 kilograms to 0.84 kgs of oxygen per day (24 hours) to survive. This is the amount of oxygen allotted to the astronauts on the International Space Station to use each day. To double-check this, I’m going to do the math myself, but if you want to know the answer, see the highlighted text. I’ll also put these numbers into perspective with some interesting examples. 

Here’s your first example. Surprisingly, humans and cars do much the same thing. They both combine fuel with oxygen to release energy through a process called oxidation. This process also creates the byproducts of carbon dioxide, water and heat, plus other pollutants in impure reactions.  

The chemistry is related, too. For example, glucose metabolism is often simplified as:
C₆​H₁₂​O₆​ + 6O₂​ → 6CO₂​ + 6H_2​O + energy

Fuel (sugar) + Oxygen → Carbon dioxide + Water + Energy.

Gasoline combustion looks similar:
2C₈​H₁₈ ​+ 25O₂ ​→ 16CO₂​ + 18H₂​O + energy

Fuel (petroleum) + Oxygen → Carbon dioxide + Water + Energy.

Interestingly, both humans and combustion engines convert only about 20–25% of their fuel energy into external mechanical work, with much of the remaining energy eventually dissipating as heat.

So the average person burns about 2,000 kilocalories of fuel per day. (What we call calories on food labels are actually kilocalories.) So to calculate how much oxygen we consume, we need to use the energy equivalent of oxygen. This is the amount of energy released for every liter of oxygen consumed during metabolism. While this varies slightly depending on whether you are burning carbohydrates or fats, scientists use a standard average for a mixed diet. This estimate is based on the Weir equation, the standard formula used by physiologists to calculate energy expenditure based on how much oxygen is consumed versus the carbon dioxide exhaled. (In case you are wondering, VO2 max measures the maximum amount of oxygen your body can utilize during intense exercise, but not the energy expenditure.)

So the energy equivalent of oxygen or the average energy yield ≈ 4.825 kcal per liter of O₂. 

Let’s plug into this formula our calories per day. This gives us: 
2,000 kcal / 4.82 kcal/L ≈ 414.5 Liters of pure oxygen

In other words, your body needs to consume approximately 415 liters of pure oxygen per day to survive. 415 liters of pure oxygen weighs 0.593 kg or roughly 1.3 lbs. (I’ve shown how to calculate the mass in my other articles.) So, slightly less than NASA’s number. The difference likely includes a safety margin, and I assume the astronauts are highly active. They need to exercise every day just to prevent bone loss. 

However, though human lungs are biologically impressive, they only absorb a fraction of the oxygen. When you breathe, you inhale air that contains 21% oxygen and exhale air that still contains about 15-16% oxygen. This means your body only consumes about 5% of the total volume of air you breathe in. So, to consume 415 liters of O₂, you must cycle through approximately 8,300 liters of air. In other words, you would need to breathe the volume of over 600 party balloons, or 4–5 scuba tanks, to burn 2,000 calories.

This is such a huge volume of air that I think it’s worth mentioning that your lungs are also absorbing the pollutants in the air. So, a small amount of particulates or poisonous gases adds up very quickly.

Roughly speaking, every day a human consumes about 0.5 kgs of oxygen, eats 1–2 kgs of food and drinks 2–3 kgs of water per day. But the surprising part is that to get that 0.5 kgs of oxygen, you need to process about 10 kgs of regular air!

Some final perspective taking. In a previous article, I calculated that for every 25 miles a car travels (1 gallon of gas), it requires 19 mature pine trees to produce oxygen for an entire day. Whereas to survive for one hour, a human would need only 0.07 mature pine trees. 

• See the calculation for how much oxygen a person breathes compared to a car.
• See the calculation for how many trees you need to survive.

Every breath connects us to a vast planetary exchange between plants, animals, microbes, oceans, sunlight, and even fire, especially the fire inside an engine. This was just one of my articles about the global oxygen cycle. To see all the articles, please visit: Global Oxygen Depletion​. 

What happens if the world runs out of oxygen?

The article you are reading is based on years of research. The facts and science are real. However, in my new debut sci-fi novel, I extrapolate these ideas into the worst-case scenario: A firestorm devastates the Earth’s atmosphere, and a handful of survivors question whether it is worth saving themselves.

Will humanity survive the oxygen apocalypse? Find out now.