Convert millimeter to inch.

About this conversion

Millimeters-to-inches: divide by 25.4 (or multiply by 0.03937). Essential whenever you're working with mechanical drawings that mix metric and imperial — fasteners, machined parts, and electronic components routinely require both.

Formula

in = mm × 0.0393701

Real-world examples

Reference values

Tips & tricks

• Common shortcut: 1 mm ≈ 0.04″ for quick mental math.
• 1/64" = 0.397 mm. So a 1 mm rounding error in inch work is more than a whole 1/64" — significant for precision machining.
• Drill bits, taps, and screw threads are routinely cross-referenced between metric and inch. Our drill bit chart shows the closest match in both systems.

Origin & history

The millimeter is one-thousandth of a meter, introduced with the metric system in 1799. The inch was redefined in 1959 as exactly 25.4 mm, making this conversion exact rather than approximate.

What is a millimeter?

One millimeter is exactly one-thousandth of a meter (1 mm = 0.001 m). It is the standard small-unit length in the SI metric system. Engineering drawings, machining tolerances, electronics dimensions, manufactured-part specifications, and rainfall measurement.

What is a inch?

The international inch is defined as exactly 25.4 millimeters since 1959, when the US, UK, and Commonwealth countries unified their definitions. US construction lumber, screen sizes (TV diagonals, phone screens), printing measurements, and pipe sizing.

Real-world examples of mm

Concrete examples often help when a unit doesn't have an intuitive feel. 300 mm is about the length of a 12-inch ruler; 0.4 mm is roughly the diameter of pencil lead; and 0.5 mm is about the thickness of a fingernail.

How to convert millimeter to inch

To convert a value from millimeter (mm) to inch (in), apply the conversion factor shown in the formula above. The calculation is the same whether you do it by hand, in a spreadsheet, or with the live converter on this page.

Steps:

1. Take your input value in mm.
2. Apply the formula see formula above.
3. The result is your value in in.

For repeated calculations, save the formula in a spreadsheet or use the live converter at the top of this page — it handles the math automatically and displays the result as you type.

Frequently asked questions

How do I convert mm to in?

Apply the formula mm → in shown above, or just type your value into the converter at the top of this page. The result updates instantly.

Which is bigger, mm or in?

This depends on the conversion ratio in the formula. If the factor multiplied by your value gives a larger number, then in is the smaller unit (so it takes more of them to express the same quantity). If the result is smaller, then in is the larger unit.

When would I use mm versus in?

Both units measure length, so the choice depends on context. Millimeter is typically used for engineering specs, construction, navigation, and everyday measurement; inch similarly. Most professional fields standardize on one or the other based on regional conventions or technical tradition.

How precise is this mm to in conversion?

The conversion factor shown is the internationally defined exact value (or the best-published approximation if the relationship is irrational, like degrees-to-radians). The live converter on this page uses double-precision floating-point math, accurate to about 15 significant digits — far beyond any practical engineering need.

Is the conversion ratio exact, or an approximation?

Most unit conversions between SI metric units, and between SI and US customary units, have been formally defined as exact values since the 1959 international yard-pound agreement and subsequent SI redefinitions. Exceptions are unit pairs that involve irrational numbers (radians, e.g.) or empirical conversions (like food calories, which depend on temperature). When in doubt, consult the formula at the top of this page.

Related conversions

See also