Inch fraction chart: fractions to decimals and millimeters

Reviewed on September 30, 2026 by the Skala Convert team

How do you convert a fraction of an inch to a decimal?

Divide the top number by the bottom one. 5/16 is 5 ÷ 16 = 0.3125 inch. For millimeters, multiply that decimal by 25.4: 0.3125 × 25.4 = 7.9375 mm. Because the inch is defined as exactly 25.4 mm, every value in the chart is exact rather than rounded — 1/64 inch is 0.396875 mm, with nothing left over. Fractions of an inch always have a power of two underneath (2, 4, 8, 16, 32, 64), and those always give a decimal that ends.

For whole inches plus a fraction, convert the fraction and add the inches: 2 3/4 inches is 2.75 inches, or 69.85 mm. The inches to mm converter takes the decimal directly.

How do you find the nearest fraction for a millimeter measurement?

Turn millimeters into inches, then into sixteenths. Divide by 25.4, multiply by 16, round to the nearest whole number and put it over 16. For 10 mm: 10 ÷ 25.4 = 0.3937 inch; × 16 = 6.3; rounded, 6/16, which reduces to 3/8 inch (9.525 mm). If half a millimeter matters, use 64 instead of 16: 0.3937 × 64 = 25.2, so 25/64 inch, or 9.921875 mm.

Going the other way with a decimal — say 0.6875 inch from a drawing — multiply by 16: 0.6875 × 16 = 11, so it is 11/16. The mm to inches converter gives the decimal; this chart gives the mark to look for on the tape.

Why are tape measures marked in sixteenths?

Because the inch is divided by halving. Each step cuts the previous mark in two — halves, quarters, eighths, sixteenths — and each level gets a shorter tick, which is how you read a tape without counting. At 1/16 inch the marks are about 1.6 mm apart, roughly the finest step that stays legible on a tape blade. Steel rules for machining keep going to 1/32 and 1/64.

The online ruler shows the same tick pattern on screen, and the printable ruler puts it on paper at actual size.

Which fractions do not end in a clean decimal?

Any fraction whose bottom number, once reduced, has a factor other than 2 or 5. 1/3 is 0.333…, 1/6 is 0.1666…, 1/7 is 0.142857142857… and so on forever. That is why the inch scale never produces them, and why the common-fractions table marks them with an ellipsis. In everyday use, four decimal places is more than enough: 1/3 inch rounded to 0.3333 is off by less than a thousandth of a millimeter.

Where does the 25.4 mm come from?

From the international inch of 1959. That year the national standards laboratories of the US, the UK, Canada, Australia, New Zealand and South Africa agreed on a common yard of exactly 0.9144 m — which makes the inch exactly 25.4 mm — and in the US the National Bureau of Standards (now NIST) announced it, effective 1 July 1959. NIST Special Publication 811, the US guide to the SI, lists the inch as 2.54 × 10⁻² m with no rounding, and NIST Handbook 44, the reference used by US weights-and-measures inspectors, gives the same exact value in its tables of equivalents. The inches to centimeters page explains the history in more detail.

Frequently asked questions

What is 5/16 of an inch as a decimal?

5/16 inch is 0.3125 inch, which is exactly 7.9375 mm. Divide 5 by 16 for the decimal, then multiply by 25.4 for millimeters.

How many millimeters is 3/8 of an inch?

Exactly 9.525 mm. 3/8 inch is 0.375 inch, and 0.375 × 25.4 = 9.525.

What fraction of an inch is 0.375?

0.375 inch is 3/8 inch. Multiply the decimal by 8, 16, 32 or 64 until you get a whole number: 0.375 × 8 = 3, so the fraction is 3/8.

How big is 1/16 of an inch in millimeters?

1/16 inch is 0.0625 inch, or 1.5875 mm — a little over one and a half millimeters. It is the smallest mark on most US tape measures.

Is 10 mm the same as 3/8 inch?

Close, but not the same. 3/8 inch is 9.525 mm, about half a millimeter short of 10 mm. The nearest 64th is 25/64 inch, which is 9.921875 mm.