Inch

The small length unit of the US customary system.

Length

How it's defined

Since the 1959 international agreement, the inch is defined as exactly 25.4 millimetres — customary units are officially built on metric ones underneath.

1 in = 25.4 mm — exact since 195912 in = 1 ft — exact

Convert — honestly

in equals

0.0254 m

Metre · factor exact — the base unit itself

0.0833333333 ft

Foot · factor exact — exact by definition

0.0000157828283 mi

Mile · factor exact — exact by definition

Conversions run through the metre using the stated factors — no rounding beyond display precision, and every factor says whether it's exact or measured.

Where you meet it

Screen sizes everywhere — a "15.6-inch" laptop is this unit
US construction and paper

Other ways to measure length

m Metre ft Foot mi Mile

Source

This page is an authored summary of the authoritative definition — checkable one click away.

NIST Special Publication 811 — Guide for the Use of the SI