What is a Inch?

The inch is the global standard for screen sizes, pipe diameters, and shoe sizing in Anglo-Saxon countries.

Overview

The inch equals exactly 25.4 millimeters under the 1959 international yard and pound agreement. The word derives from the Old English 'ynce,' from Latin 'uncia' meaning one twelfth (the inch is one twelfth of a foot). Originally based on the width of a thumb, the inch has been standardized for centuries. It is the global standard for screen sizes (a 15-inch laptop or 65-inch TV), pipe and tubing diameters (1-inch plumbing, 2-inch exhaust), and shoe sizes in Anglo-Saxon countries. Construction lumber, photographic prints, paper sizes (US Letter is 8.5 × 11 in), and rainfall in the US all use inches. The inch relates to the millimeter (25.4 mm = 1 in exactly), the foot (12 in = 1 ft), and the yard (36 in = 1 yd). Subdivisions in fractions (½, ¼, ⅛) remain common in carpentry.

Symbol

Convert Inch to all units

Live result

Meter0.0254 m Kilometer0.0000254 km Centimeter2.54 cm Millimeter25.4 mm Mile0.00001578 mi Yard0.027778 yd Foot0.083333 ft Nautical Mile0.00001371 nmi Micrometer25,400 µm Nanometer25,400,000 nm Light Year2.685 × 10^-18 ly Furlong0.000126 fur Fathom0.013889 ftm Angstrom254,000,000 Å

Relationship to Other Length Units

1 in equals

Visual reference for how the inch relates to other length units. Each row links to the full converter for that pair.

1 in=0.0254 m 1 in=0.0000254 km 1 in=2.54 cm 1 in=25.4 mm 1 in=0.00001578 mi 1 in=0.027778 yd 1 in=0.083333 ft 1 in=0.00001371 nmi 1 in=25,400 µm 1 in=25,400,000 nm 1 in=2.685 × 10^-18 ly 1 in=0.000126 fur

See all 15 length units in matrix view →

When Is the Inch Used?

Screen and monitor diagonals worldwide (phone, tablet, TV)
US and UK shoe sizes
Industrial pipe and fitting diameters

Real-world examples

A 65-inch TV has a 165 cm diagonal. A smartphone screen is typically 6–7 inches. Standard copper plumbing in the UK is ½ inch or ¾ inch.

Tips for Using the Inch

1 inch = 2.54 cm exactly by international agreement — no rounding.
Screen sizes are always diagonal, never width. A 27-inch monitor is about 60 cm wide.
To convert cm to inches mentally, divide by 2.5 and subtract 1.6%.

Common Mistakes

Reading a screen size as width. A 65-inch TV is about 143 cm wide — the 165 cm figure is diagonal.
Using 2.5 instead of 2.54 on precision drawings — the 1.6% error matters in manufacturing.
Confusing inches of length with inches of mercury (inHg) — a pressure unit, not length.

Convert Inch to Other Length Units

Inch to Nautical Mile

1 in = 0.00001371 nmi

1 in = 2.685 × 10^-18 ly

Convert Other Units to Inch

Nautical Mile to Inch

1 nmi = 72,913.386 in

Micrometer to Inch

1 µm = 0.00003937 in

Nanometer to Inch

1 nm = 3.937 × 10^-8 in

Light Year to Inch

1 ly = 372,468,503,937,007,900 in

1 Å = 3.937 × 10^-9 in

FAQ About the Inch

3 questions

What does the Inch (in) measure?

The inch measures length. The inch is the global standard for screen sizes, pipe diameters, and shoe sizing in Anglo-Saxon countries.

When is the Inch used?

The inch is used in: Screen and monitor diagonals worldwide (phone, tablet, TV); US and UK shoe sizes; Industrial pipe and fitting diameters. A 65-inch TV has a 165 cm diagonal. A smartphone screen is typically 6–7 inches. Standard copper plumbing in the UK is ½ inch or ¾ inch.

How accurate are conversions involving the Inch?

All conversions on Units Converter use NIST SP 811 and BIPM reference values, accurate to 8 significant figures.