# What unit is the % of O2 in air measured in? **URL:** \ **Category:** Factual Questions\ **Created:** [October 8, 2002, 1:55pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354 "2002-10-08T13:55:50Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![alice\_in\_wonderland](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alice_in_wonderland/32/176_2.png) [@alice\_in\_wonderland](https://boards.straightdope.com/u/alice_in_wonderland)\ **Post date:** [October 8, 2002, 1:55pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/1 "2002-10-08T13:55:50Z")
I know that air is 20.9% O2, but is that Mm, or L or what? Thanks. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [October 8, 2002, 2:11pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/2 "2002-10-08T14:11:47Z")
The units that are used probably depend upon the purpose you have in mind. The amount of oxygen can be stated as partial pressure, gms/cc, moles/km[sup]3[/sup] or whatever suits your purpose. ---
**Author:** ![bordelond](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bordelond/32/150_2.png) [@bordelond](https://boards.straightdope.com/u/bordelond)\ **Post date:** [October 8, 2002, 2:14pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/3 "2002-10-08T14:14:32Z")
Alice, percentages are simply a measure of proportion. Neither cubic mm nor liters would make a difference. Consider: we’ve got a canister filled with 100 cubic mm of air. Since 20.9% of that air is oxygen, there are 20.9 cubic mm of oxygen in our canister. Now, we pull out another canister filled with 1 liter of air. This canister contais .209 liters of oxygen. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [October 8, 2002, 2:15pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/4 "2002-10-08T14:15:57Z")
I am almost positive that the 20.2% figure is a measure of volume (ususally expressed in liters). Assuming ideal gas\*, then PV=nRT would also give you the same 20.2% figure if you expressed it in terms of particle amount (moles) instead of volume (liters), so either could be considered valid. However, due to the difference in molecular weights, a percentage derived from mass (expressed in grams) would be different. \*the assumption should be valid here. I can’t imagine that the constants for O[sub]2[/sub] and N[sub]2[/sub] would be different enough to cause the percentage to be off by much. ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [October 8, 2002, 2:15pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/5 "2002-10-08T14:15:57Z")
Percent is a unitless measure. However, it could be percentage by mass or by volume (or by number of molecules) Brian ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [October 8, 2002, 4:43pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/6 "2002-10-08T16:43:58Z")
It is by volume (20.9%). e.g see [http://www.earth.monash.edu.au/firstyear/esc101102/0232ESC1.htm](http://www.earth.monash.edu.au/firstyear/esc101102/0232ESC1.htm) and lots of others. By mass it is higher, as O2 weighs 32 and N2 = 28. Approximately 23% by weight ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [October 8, 2002, 5:17pm UTC](https://boards.straightdope.com/t/what-unit-is-the-of-o2-in-air-measured-in/131354/7 "2002-10-08T17:17:59Z")
Also note that many sensor readings are on a “wet” or a “dry” basis, and, depending on the situation and the accuracy you need, you may need to take this into account.