Fraction to Decimal Chart

Every fraction from 1/64 through 1 inch, in simplest form, with its decimal inch equivalent and millimetre conversion.

Page last changed 2026-10-19 · based on n/a — the conversions are exact arithmetic, not tabulated reference data

Quick Answer

Fraction, Decimal and Millimetre Equivalents

Only fractions in lowest terms are listed — 2/8, 4/16 and 1/4 are all the same size. The 64ths column shows the equivalent in 64ths of an inch, which is the granularity most fractional drill and stock sizes use.

The Conversion

decimal inch = numerator ÷ denominator
millimetres = decimal inch × 25.4
64ths = decimal inch × 64

All three columns are exact arithmetic from the fraction — there is nothing to look up and no rounding other than the display precision.

Reducing Fractions to Lowest Terms

A fraction is in lowest terms when the numerator and denominator share no factor other than 1. Divide both by their greatest common divisor:

8/32 → divide both by 8 → 1/4
6/16 → divide both by 2 → 3/8
10/64 → divide both by 2 → 5/32

This matters when reading a drawing: a dimension written 12/32 and one written 3/8 are the same size, and mixing reduced and unreduced forms is a common source of measurement error. The 64ths column here lets you compare any fraction against the 1/64 in increments most stock and drill sizes are sold in.

Frequently Asked Questions

What is 1/8 as a decimal?

0.1250 in, or 3.175 mm. Divide 1 by 8 to get the decimal, then multiply by 25.4 for millimetres.

What is 3/8 as a decimal?

0.3750 in, or 9.525 mm. Divide 3 by 8 for the decimal, then multiply by 25.4 for millimetres. In 64ths it is exactly 24/64.

What is 5/16 as a decimal?

0.3125 in, or 7.938 mm. Note that 5/16 is exactly 20/64, so it sits on a 64th boundary.

How do I convert a decimal to a fraction?

Multiply by the denominator you want and round. To express 0.4375 in 64ths, multiply by 64 to get 28, giving 28/64, which reduces to 7/16. The 64ths column in the table does this conversion for every common fraction.

Why are fractions written in 64ths on drawings?

1/64 in is the practical resolution of a machinist's rule and the increment most fractional drill and stock sizes come in. Dimensions finer than 1/64 are normally given in decimals rather than fractions.

Is 2/8 the same as 1/4?

Yes — 0.2500 in either way. 2/8 reduces to 1/4 by dividing both terms by 2. Only reduced forms are listed in the table, but any equivalent fraction gives the same decimal.

Related

• Drill Size Chart
• Sheet Metal Gauge Chart
• AWG Wire Gauge Chart
• Tap Drill Size Chart

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References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

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No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

Derived values — the following values on this page are calculated, not taken directly from the standard:

Decimal values are shown to four places. Any fraction can be evaluated exactly; the display precision is the only rounding.

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