Fractional Inches to Millimetres: Workshop Conversion Chart

Convert fractional inches to decimal inches and millimetres in 1/64-inch steps, from 1/64 to 1 inch. All table values are exact. A separate thou table covers selected thousandths of an inch, including half a thou.

Download the inch conversion sheet (PDF, A4, 2 pages)

Print at actual size / 100% on A4. Page 1 covers 1/64 to 1/2 inch and 0.5 to 5 thou; page 2 covers 33/64 to 1 inch and 10 to 100 thou. Download size: 499 KiB. This reference is not a printed measuring scale.

How to read the conversion chart

One international inch is exactly 25.4 millimetres. Divide the numerator of a fraction by its denominator to get decimal inches, then multiply by 25.4 for millimetres. For example, 3/8 inch = 0.375 inch = 9.525 mm.

The chart reduces each fraction to its simplest form. Every displayed decimal terminates, so no rounding is needed in these tables. Trailing zeros are omitted: 0.5 and 0.500 represent the same number. The number of displayed decimal places does not establish a product tolerance or the accuracy of a measurement.

1/64 to 1/2 inch: exact fraction, decimal and mm chart

33/64 to 1 inch: exact fraction, decimal and mm chart

Thou to millimetres

A thou is one thousandth of an inch: 1 thou = 0.001 inch = 0.0254 mm exactly. Half a thou means 0.0005 inch, which is 0.0127 mm. Keep the unit explicit when reading drawings or notes.

Whole inches, reverse conversion and rounding

Whole inches plus a fraction: 1 3/8 inches means 1 + 3/8, not 1 multiplied by 3/8. It is 1.375 inches. Multiply the whole value by 25.4: 1.375 × 25.4 = 34.925 mm exactly.

Millimetres back to inches: divide by 25.4. For example, 12.7 mm ÷ 25.4 = 0.5 inch = 1/2 inch exactly. Other inputs may produce recurring decimals; do not assume every metric dimension maps to a neat fraction or a standard imperial size.

If your task needs a rounded display: keep the exact value until the final calculation, then apply the rounding and precision required by the drawing or specification. With half-up rounding, 3/8 inch = 9.525 mm displays as 9.53 mm to two decimal places. This is a rounded display, not the exact value shown in the chart. No general manufacturing tolerance is implied.

Exact conversion versus stock size and fit

A nominal 1/2-inch dimension converts to exactly 12.7 mm; a nominal 13 mm dimension is 0.3 mm larger. Nearby marketed metric and imperial sizes are not made identical by rounding. Actual supplied size, tolerance, condition and suitability must come from the relevant product specification and drawing.

This chart does not recommend substitute drills, spanners or stock sizes. For tool markings and nominal hexagon dimensions, see the Whitworth, BSF and AF spanner size chart. For thread diameter and pitch, see the imperial thread size chart. A unit conversion alone does not establish tool fit or thread compatibility.

Source and verification

Reviewed 17 October 2026. Revision 1.0. The conversion factor comes from NIST SI Units – Length, in its inch redefinition FAQ. NIST identifies the international inch as exactly 25.4 mm.

Workshop Standard calculated all 64 fraction rows and eight thou rows from that definition, using exact decimal and rational arithmetic, with a separate integer check. The original HTML chart and two-page PDF use the same dataset. Table values are exact, with trailing zeros omitted. These arithmetic checks do not certify the dimensions or accuracy of a physical product or measuring instrument.