# Question about SAE Screw Size nomenclature

**URL:** <https://boards.straightdope.com/t/question-about-sae-screw-size-nomenclature/724779>\
**Category:** Factual Questions\
**Created:** [July 10, 2015, 5:09pm UTC](https://boards.straightdope.com/t/question-about-sae-screw-size-nomenclature/724779 "2015-07-10T17:09:01Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![RearEchelon](https://avatars.discourse-cdn.com/v4/letter/r/b4bc9f/32.png) [@RearEchelon](https://boards.straightdope.com/u/RearEchelon)\
**Post date:** [July 10, 2015, 5:09pm UTC](https://boards.straightdope.com/t/question-about-sae-screw-size-nomenclature/724779/1 "2015-07-10T17:09:01Z")

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#6-32, #10-20… Why the #'s? Apparently small screw sizes are given as a # instead of the actual diameter of the screw, and a site I was perusing a few minutes ago said to get the actual diameter of an SAE screw #n, you take the number #n, multiply it by 0.013, then add 0.06. Why? Where did 0.013 and 0.060 come from? I can’t find an answer to this anywhere.

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**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\
**Post date:** [July 10, 2015, 5:29pm UTC](https://boards.straightdope.com/t/question-about-sae-screw-size-nomenclature/724779/2 "2015-07-10T17:29:59Z")

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Here’s a Wiki page that goes over the specifications - and the math might explain the constants used for the standard but my eyes glazed over.

> **[Unified Thread Standard](https://en.wikipedia.org/wiki/Unified_Thread_Standard)**
>
> The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millime...

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**Author:** ![RearEchelon](https://avatars.discourse-cdn.com/v4/letter/r/b4bc9f/32.png) [@RearEchelon](https://boards.straightdope.com/u/RearEchelon)\
**Post date:** [July 10, 2015, 6:23pm UTC](https://boards.straightdope.com/t/question-about-sae-screw-size-nomenclature/724779/3 "2015-07-10T18:23:51Z")

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Okay, so you add 0.060 because that’s the diameter of a #0 screw ((#0 x 0.013) + 0.060 = 0.060). But I still don’t get where 0.013 comes from. Thanks for the link.