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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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Set your values

Results update as you type.

Direction

Try 3 1/4, 3-1/4, 1/4, 5/4 or 3.25.

Fraction precision
Millimetres: 82.55 mm

Millimetres

82.550mm
Fractional inches: 3 1/4"

Fractional inches

3 1/4"

Breakdown

Decimal inches3.2500
Rounded to1/16

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.

Cite this calculator

Canonical URL: https://mathify.one/fr/conversion/fractional-inches

Cite as: Mathify. (2026). Convertisseur de Pouces Fractionnaires en MM. https://mathify.one/fr/conversion/fractional-inches

Use Cases

DIY and woodworking projects

When working with imperial measurements on tools or plans, quickly convert fractional inches to millimetres for metric rulers or hardware.

Example: Convert 5/16 inch to mm to drill a hole with a metric bit.

Engineering and design

Switch between imperial and metric dimensions in CAD or technical drawings without manual calculation errors.

Example: Convert 2 3/8 inches to mm for a mechanical part specification.

Frequently Asked Questions

How do I convert a fractional inch like 3 1/4 to millimetres?
Enter '3 1/4' (with a space) or '3-1/4' (with a hyphen) into the converter. The tool will interpret it as 3 and 1/4 inches, convert to millimetres (82.55 mm), and also show the reduced fraction.
Can I enter a decimal like 3.25 instead of a fraction?
Yes, you can type '3.25' and the converter will treat it as 3.25 inches, convert to millimetres (82.55 mm), and also display the equivalent reduced fraction (3 1/4).
What does 'reduced fraction' mean?
A reduced fraction is one where the numerator and denominator have no common factors other than 1. For example, 2/8 is reduced to 1/4. The converter automatically simplifies fractions for you.

Tips & Common Mistakes

Tips

  • Use a space or hyphen between the whole number and fraction (e.g., 3 1/4 or 3-1/4) for mixed numbers.
  • For fractions only, just type like 1/4 or 3/16.
  • Decimals are accepted too – type 3.25 and get the reduced fraction.
  • Double-check your input format to avoid misinterpretation (e.g., 3-1/4 is not 3 minus 1/4).

Common Mistakes to Avoid

  • Entering '3 1/4' without a space or hyphen (e.g., '31/4') – the tool may read it as 31/4 inches.
  • Confusing decimal points with fraction slashes – use '.' for decimals and '/' for fractions.
  • Assuming the result is rounded – the converter gives exact millimetre values based on 1 inch = 25.4 mm.

Last updated: August 13, 2026