SAE Screw Sizes
Standard American screw sizes were originally set by the Society of Automotive Engineers (SAE), or at least everyone blames them for it, though the American Society of Mechanical Engineers (ASME) developed their own set, and now is the custodian of the “Unified” specification.
Screws with a diameter less than 1/4 inch are specified by a size number, starting with #000 for the smallest size up to #12 for the largest. Above that, the size is given in fractional inches. For machine screws, a second number is added for the pitch, indicating the number of threads per inch. There are two standard sets of threads: UNF (Unified Fine Thread) and UNC (Unified Coarse Thread). A fine thread has more threads per inch.
Thread pitch is not specified for wood screws or sheet metal screws, only machine screws that go into a threaded nut or other fitting.
Screw length is given in inches, often following the word “by”. So, for example, a “6-32 by 1/2 inch” screw would be a number 6 size (3.5mm diameter) with 32 threads per inch, and 0.5 inches (12.7mm) long.
At least this table should make it easier to identify a reasonable metric equivalent for American sizes you may encounter.
|1 inch||12 (or 14)||8||1.000||25.400|