# Air pressure inside a balloon

**URL:** <https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109>\
**Category:** Factual Questions\
**Created:** [February 18, 2008, 4:13pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109 "2008-02-18T16:13:22Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\
**Post date:** [February 18, 2008, 4:13pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109/1 "2008-02-18T16:13:22Z")

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Okay, please settle a dispute I had 20 years ago with my high school chemistry teacher. He said that the air pressure inside an inflated balloon is the same as the outside air pressure. I said the pressure would be higher inside than outside, because the latex is stretched and the contractive force (for lack of a better phrase) of the latex exerts additional pressure on the air inside the balloon. Who is right?

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**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\
**Post date:** [February 18, 2008, 4:21pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109/2 "2008-02-18T16:21:31Z")

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If there is tension in the surface of the balloon, then the pressure inside is higher

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**Author:** ![Mycroft\_H](https://avatars.discourse-cdn.com/v4/letter/m/898d66/32.png) [@Mycroft\_H](https://boards.straightdope.com/u/Mycroft_H)\
**Post date:** [February 18, 2008, 4:28pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109/3 "2008-02-18T16:28:30Z")

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To add a bit more to **Giles’** correct and succinct answer, a proof of this is when you let go of the balloon (assuming the opening is not tied off.) The fact that air rushes out and propels the balloon means that the pressure inside is greater than it is outside. Otherwise it would be more like a paper bag.

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**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\
**Post date:** [February 18, 2008, 5:39pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109/4 "2008-02-18T17:39:47Z")

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You’re right.

FWIW it’s pretty difficult to create a pressure of 3 psi by blowing into something, maybe 4 psi, depending on whether you’re using your lungs or closing your throat and using your cheeks and tongue. I’d guess typical balloons take 1/2 or 1 psi to inflate them.

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**Author:** ![SiouxChief](https://avatars.discourse-cdn.com/v4/letter/s/d6d6ee/32.png) [@SiouxChief](https://boards.straightdope.com/u/SiouxChief)\
**Post date:** [February 18, 2008, 5:57pm UTC](https://boards.straightdope.com/t/air-pressure-inside-a-balloon/438109/5 "2008-02-18T17:57:54Z")

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The point the professor was probably trying to make was that the internal forces (pushing outward) are equal to the external forces (pushing in). That statement is correct, otherwise the balloon would either expand or contract. But the internal _air pressure_ must be greater, as the OP stated, to offset the contractive force of the balloon.