Sizes #00, #000, and #0000 are from the obsolete American National
threadform specifications.
The smallest size in the main Unified (UN) specification is #0.
The nominal diameters of these threadforms are:
| UN #0 | - | 0.060 in. |
| American National #00 | - | 0.047 in. |
| American National #000 | - | 0.034 in. |
| American National #0000 | - | 0.021 in. |
For cases in which a size smaller than #0 is required, small metric (M) threadforms are recommended.
It seems unlikely that anyone would want to use our threadform calculators for the multiple-zero sizes anyway since such a small fastener would not be used as a load-bearing device, but if you insist and if precise calculations are not crucial to your application, you can improvise as follows. The American National specification is very similar to the Unified specification, except that the most common class of thread has no allowance, so it is similar to classes 3A and 3B in the Unified system. You can get approximate calculations for American National sizes #00, #000, and #0000 from our UN calculator by using the input form for UNS which allows you to enter any diameter and number of threads per inch. Use classes of fit 3A and 3B. The usual combinations of diameter and threads per inch are:
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