Conversions
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Fractional Inches to MM Converter.
Convert fractional inches to millimetres and back. Type 3 1/4, 3-1/4, 1/4 or 3.25 and get an exact, reduced fraction.
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How to Use
One inch is defined as exactly 25.4 mm, so every conversion here is exact up to the rounding you choose.
Accepted input formats: "3 1/4", "3-1/4", "1/4", improper fractions like "5/4", and plain decimals like "3.25".
Results are always reduced — you get 1/2, never 8/16, and a value that rounds up to a whole inch reads as 1, never 16/16.
Pick 1/16 for framing and general carpentry, 1/32 for cabinetry and joinery, and 1/64 for machining or fine woodwork.
FAQs
How many millimetres are in an inch?
Exactly 25.4 mm. The international inch has been defined as precisely 25.4 millimetres since 1959, so inch-to-millimetre conversion is exact — the only approximation comes from rounding the fraction you read off a tape measure.
What input formats does this accept?
Mixed numbers with a space or hyphen ("3 1/4" or "3-1/4"), bare fractions ("1/4"), improper fractions ("5/4" = 1.25), and plain decimals ("3.25"). An inch mark or the word "in" on the end is fine too. Anything else returns a clear error rather than a silent wrong answer.
Why does it show 1/2 instead of 8/16?
Fractions are always reduced to lowest terms with a greatest-common-divisor step, so you get the form you would actually say out loud. For the same reason, a measurement that rounds up to a whole inch displays as "1", never "16/16".
Which precision should I choose — 1/16, 1/32 or 1/64?
1/16 in (1.59 mm) is the standard for framing, general carpentry and most tape measures. 1/32 in (0.79 mm) suits cabinetry, joinery and trim work. 1/64 in (0.40 mm) is for machining, metalwork and fine woodworking where a hair of slop matters.
How do I convert a decimal like 0.375 into a fraction?
Type it straight into the inches field. 0.375 in becomes 3/8 in (9.525 mm). The calculator rounds to your chosen denominator and then reduces, so decimals that land exactly on a fraction come out clean.