# CFMs- 3 blade box fan vs 5 blade

**URL:** <https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531>\
**Category:** Factual Questions\
**Created:** [February 19, 2025, 7:31pm UTC](https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531 "2025-02-19T19:31:37Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![a6ka97](https://avatars.discourse-cdn.com/v4/letter/a/c2a13f/32.png) [@a6ka97](https://boards.straightdope.com/u/a6ka97)\
**Post date:** [February 19, 2025, 7:31pm UTC](https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531/1 "2025-02-19T19:31:37Z")

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Recently “inherited” an older 20"(?) box fan. It has 3 larger (surface area) metal blades. My newer 20" box fan has 5 smaller (surface area) plastic blades. Got me thinking… if the motors are running at the same RPMs is there/would there be any difference in the amount or speed of air being moved between the 2 fans because of the difference in the size or number of blades.

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**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\
**Post date:** [February 19, 2025, 8:05pm UTC](https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531/2 "2025-02-19T20:05:22Z")

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Depends on the blades and how much power the motor produces. The speed of rotation, shape and angle of the blades makes a difference for a particular motor power. They probably maximized the volume and speed of air movement for the motor and blade type used but the volume and speed of the air flow can be quite different between the two fans still.

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**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\
**Post date:** [February 21, 2025, 1:23pm UTC](https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531/3 "2025-02-21T13:23:55Z")

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From what I understand, it’s the total surface area of the blades that determines the volume of air moved, no matter how many blades it’s spread across.

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**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\
**Post date:** [February 23, 2025, 3:00am UTC](https://boards.straightdope.com/t/cfms-3-blade-box-fan-vs-5-blade/1014531/4 "2025-02-23T03:00:44Z")

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A 5-blade fan typically moves more air than a 3-blade fan at the same RPM.