Convert mm to inches

Convert millimetres to inches. 1 inch = 25.4 mm by definition.

inch

0.3937inch

History

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Formula

Formula: inch = mm ÷ 25.4

Design notes

Converting between inches and millimetres comes up in mechanical design and whenever you check a component against its standard. Imperial-standard hardware is usually dimensioned in inches, and 1 inch = 25.4 mm is an exact definition fixed by ISO.

When you need this

For mechanical part selection and for fitting overseas components into metric mechanical designs — screws, connectors, rack mounting and chassis work, where parts are standardised in inches.

1 inch = 25.4 mm exactly

The 1959 international yard and pound agreement defined the inch as exactly 25.4 mm. It is a definition, not a measured approximation, so there is no residual error.

Conversion is therefore exact, but rounding still needs care. 0.1 inch = 2.54mm is clean, but fractions such as one third of an inch do not terminate. State significant figures on drawings.

Dimensions that come up

Inches	mm	Where
0.05"	1.27 mm	SOP package pitch (50mil)
0.1"	2.54 mm	DIP and pin header pitch
1/4"	6.35 mm	Phone jack, tripod thread
1/2"	12.7 mm	Rack-related base dimension
1"	25.4 mm	Reference
1.75"	44.45 mm	1U rack height
19"	482.6 mm	Standard rack width

Cautions

Inch and metric threads are not interchangeable. Pitch and thread angle differ even when they look alike, and forcing them strips the thread. Keep UNC / UNF distinct from M.
Get comfortable with fractions. American drawings routinely use 3/8" or 5/16". Convert to decimal first.
Convert tolerances too. ±0.005" is ±0.127mm. When converting a drawing, do not leave the tolerances behind.

Frequently asked questions

Why exactly 25.4mm?

The 1959 international agreement defined the yard as 0.9144m. With 36 inches to the yard, one inch is 0.9144 / 36 = 0.0254m exactly. Before that, slightly different values were in use in different countries.

What is 1U in millimetres?

1.75 inches = 44.45mm, used with the 19-inch (482.6mm) rack width. It looks arbitrary in metric but is a round 1.75" in inches.

How many digits should I keep when converting a drawing?

Match the significant figures of the original. Writing four decimal places for a part toleranced at ±0.1mm serves no purpose. Conversely, on precision parts, machining to a rounded value can fall outside tolerance.

Standards and references

ISO 2768 — General tolerances for dimensions without individual indication.
ASME Y14.5 — Geometric dimensioning and tolerancing (inch-based drawing standard).

Last updated: 2026-08-29