Bolt Size Chart
Standard SAE bolt sizes run from numbered sizes like #4 through fractional sizes like 3/4", each with a fixed decimal diameter and thread pitch. A 1/4" bolt measures exactly 0.250" (6.35 mm) across its major diameter, with 20 threads per inch in the coarse (UNC) series.
Numbered vs Fractional Bolts
SAE bolts smaller than 1/4" use a numbered designation, while 1/4" and larger use the fraction directly as the size — and only one of those two conventions tells you the actual diameter just by reading it.
A #10 bolt doesn't measure "10" of anything — it's 0.190" in diameter, following a numbering scale that predates easy decimal measurement. A 1/4" bolt, by contrast, genuinely measures 1/4" (0.250") across its major diameter — the fractional designation and the physical size are the same number. Every bolt also carries a threads-per-inch (TPI) rating, which is separate from its diameter and determines how finely or coarsely it's threaded.
Bolt Size to Millimetres
Once you know a bolt's decimal diameter, converting to millimetres uses the same exact constant as everywhere else.
MM equals the bolt's decimal major diameter multiplied by 25.4.
- INThe bolt's decimal major diameter, looked up from the table below.
- MMThe millimetre equivalent: IN × 25.4.
Worked calculation — 3/8" bolt
- 3/8" = 0.375"Decimal diameter
- 0.375 × 25.4 = 9.525Convert to millimetres
3/8" bolt = 0.375" = 9.525 mm major diameter
Standard SAE Bolt Sizes
| Size | Decimal diameter | UNC threads/in | MM diameter |
|---|---|---|---|
| #4 | 0.112" | 40 | 2.845 mm |
| #6 | 0.138" | 32 | 3.505 mm |
| #8 | 0.164" | 32 | 4.166 mm |
| #10 | 0.19" | 24 | 4.826 mm |
| 1/4" | 0.25" | 20 | 6.35 mm |
| 5/16" | 0.3125" | 18 | 7.938 mm |
| 3/8" | 0.375" | 16 | 9.525 mm |
| 7/16" | 0.4375" | 14 | 11.112 mm |
| 1/2" | 0.5" | 13 | 12.7 mm |
| 9/16" | 0.5625" | 12 | 14.287 mm |
| 5/8" | 0.625" | 11 | 15.875 mm |
| 3/4" | 0.75" | 10 | 19.05 mm |
For the matching pilot hole size for each of these threads, see the tap drill chart.
Frequently Asked Questions
What is the decimal diameter of a 1/4 inch bolt?
0.250" exactly — a 1/4" bolt's nominal fractional size is its actual major diameter.
What does UNC mean on a bolt size chart?
UNC stands for Unified National Coarse, one of the two standard US thread series (the other is UNF, Unified National Fine). A UNC thread has fewer threads per inch than a UNF thread at the same diameter.
What is the metric diameter equivalent of a 3/8 inch bolt?
3/8" equals 9.525 mm, found by multiplying 0.375 by 25.4.
Why do some bolts have two thread counts?
A bolt size available in both UNC (coarse) and UNF (fine) versions serves different needs: UNC for general-purpose fastening and quicker assembly, UNF for higher clamping force and use in thinner materials where more threads engage.
What is the difference between a bolt's nominal size and its actual diameter?
For a numbered size like #10, the designation doesn't literally state the diameter — #10 measures 0.190", not 10 of anything measurable. For a fractional size like 1/4", the nominal size is the actual major diameter: 1/4" really does measure 0.250" across the threads.
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