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The measurement in inches.
Enter fractions as decimals — 3/8 is 0.375, 5/16 is 0.3125. A sixteenth is 0.0625, which is a useful number to remember when reading a tape.
Length
304.8 mm
1 inch = 25.4 mm exactly, by international agreement since 1959. Converting a dimension does not make imperial and metric fixings interchangeable — match those by specification.
- Conversion factor applied
- 25.4 mm per in
They open the calculator with your figures already in it
Inches to Millimetres Calculator: 305 mm — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- International yard and pound agreement (1959): 1 inch = 25.4 mm exactly
Inputs used
- Length (in)
- 12
Intermediate steps
- Conversion factor applied
- 25.4 mm per in
Confidence note: 1 inch = 25.4 mm exactly, by international agreement since 1959. Converting a dimension does not make imperial and metric fixings interchangeable — match those by specification.
Add the equipment this sizes
This result is a specification — 304.8 mm — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-26 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations1
- International yard and pound agreement (1959): 1 inch = 25.4 mm exactly
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Anyone working across markets meets this constantly: an American drawing against European sheet goods, or a metric fixing schedule against imperial timber. The factor is exact, so the arithmetic never introduces error. What does cause trouble is assuming that converting a number makes components compatible. An M10 bolt is not a three-eighths bolt, plasterboard sold as twelve and a half millimetres is not the same sheet as half-inch board, and a converted hole diameter will not accept a fastener from the other system without being redrilled. Convert dimensions freely for setting out and cutting; specify fixings, threads and sheet products by their actual designation.