Millimeters to Inches Converter

Background.

A DIYer replacing a stripped bolt pulls it from the workbench, measures the head across the flats with a caliper, and gets 10 millimeters — but the hardware store's imperial socket set only shows fractions of an inch. Metric and imperial fasteners sit side by side often enough, on cars, appliances, and furniture, that converting a millimeter measurement into the nearest usable inch fraction is one of the most frequent unit conversions in a home workshop. This calculator divides the millimeter value by the exact factor of 25.4 — the number of millimeters in one inch, fixed since 1 July 1959 by the International Yard and Pound Agreement and confirmed by NIST as the current definition, not a rounded approximation. Because US drill bits, sockets, and wrenches are conventionally sized in 64ths of an inch, the calculator also returns the nearest 1/64-inch fraction alongside the exact decimal value, so the result maps directly onto a tool you can actually buy or already own. A 6 mm bolt, for instance, is closest to a 15/64-inch socket, not the 1/4-inch (16/64) size many people reach for first. Enter any millimeter length to get both the precise decimal inch equivalent and the nearest standard imperial fraction.

What is millimeters to inches calculator?

The millimeter is a metric unit of length equal to one-thousandth of a meter, while the inch is a US customary/imperial unit. Since 1 July 1959, the inch has been defined as exactly 25.4 millimeters by international agreement, so converting millimeters to inches means dividing by 25.4 rather than applying an approximate ratio. This calculator performs that division in exact decimal arithmetic and also reports the nearest 1/64-inch fraction, the standard increment used for US drill bits, socket wrenches, and many fasteners, so the result can be matched directly to a physical tool.

How to use this calculator.

1. 01Enter the length you want to convert, in millimeters.
2. 02The calculator returns the precise equivalent in inches.
3. 03Check the nearest 1/64-inch output to find the closest standard imperial drill bit, socket, or wrench size.
4. 04Remember that the nearest fraction is a practical rounding, not an exact match — verify fit for close-tolerance fasteners.

The formula.

The conversion divides the input value in millimeters by 25.4, the exact number of millimeters in one inch, fixed by the 1959 International Yard and Pound Agreement. In the worked example, a 6-millimeter bolt-head dimension divides by 25.4 to give 0.2362204724 inches. Because imperial sockets and drill bits are stepped in 64ths of an inch, the calculator multiplies that decimal by 64, rounds to the nearest whole number using standard half-up rounding, and divides by 64 again, returning 15/64 inch (0.234375). That is closer to the true metric dimension than the more commonly assumed 1/4-inch (16/64) socket, which is roughly 0.65 mm too large — enough to slip on a fastener head that needs a snug grip. The calculator performs every step in arbitrary-precision decimal arithmetic, not native floating-point, so the fraction is derived from the exact millimeter-to-inch relationship rather than an approximated one.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST. SI Units — Length.
2. [2]NIST (2008). Special Publication 811: Guide for the Use of the International System of Units (SI), Appendix B — Conversion Factors.