Millimetres to Inches

What is a millimetre?

The millimetre (symbol mm, US spelling millimeter) is the SI unit of length equal to one thousandth of a metre — 1 mm = 0.001 m. It is the everyday metric unit for small precise measurement: a credit card is about 0.76 mm thick, a 2B pencil lead is 2 mm, an AA battery is 14.5 mm in diameter, a sheet of A4 paper is 297 mm tall. Across almost every country, the millimetre is the working unit of mechanical engineering drawings, machined-part tolerances, jewellery sizes, watch movements, smartphone dimensions and electronic component spacings. Fastener metric sizes — M3, M4, M5, M6, M8, M10 — are nominal thread diameters in millimetres. Centimetres (10 mm) and metres (1000 mm) are its main related units.

What is an inch?

The inch (symbol in or ″) is the smallest customary unit of length in the US Customary system and UK Imperial system. Its modern definition is exact: 1 inch = exactly 25.4 millimetres = 2.54 centimetres, agreed internationally in 1959 to standardise across the US, UK, Canada, Australia and other Anglosphere countries that had previously used slightly different inch lengths. Twelve inches make one foot, 36 inches one yard. Inches survive in engineering domains where US standards dominate — US thread sizes (¼-20, ⅜-16, ½-13), US construction lumber, US plumbing pipe, US tool drawer slides — and in consumer domains worldwide: TV and monitor diagonals, smartphone screen sizes, vinyl-record diameters, firearm calibres.

The conversion formula

inches = millimetres ÷ 25.4. The reverse is multiplication by 25.4. A worked example: a 50 mm watch case converts to 50 ÷ 25.4 = 1.9685 in. A 1080p phone screen height of 154.8 mm becomes 154.8 ÷ 25.4 = 6.0945 in. A bolt diameter of 6 mm is 6 ÷ 25.4 = 0.2362 in, just under ¼ in. Because the inch is defined by an exact 25.4 mm equivalence, the conversion is mathematically precise; any rounding you see is purely cosmetic truncation at the display step.

Common reference values

A handful of conversions worth memorising:

• 1 mm ≈ 0.0394 in — the building block.
• 25.4 mm = 1 in — the literal definition.
• 3 mm ≈ 0.118 in — about ⅛ in (0.125 in).
• 6 mm ≈ 0.236 in — about ¼ in (0.250 in).
• 10 mm ≈ 0.394 in — close to ⅜ in.
• 12.7 mm = 0.5 in — exactly half an inch.
• 25.4 mm = 1 in — exactly one inch.
• 100 mm ≈ 3.937 in — about 4 in.
• 152.4 mm = 6 in — exactly six inches.
• 304.8 mm = 12 in (1 ft) — exactly one foot.

The reference table on the page widens this list and updates live as you change the input value.

The arithmetic anchors on the exact 1959 definition: 25.4mm = 1 inch. From there, a handful of common queries fall out directly — 10mm = 0.394 inches (close to ⅜ in, useful for jewellery and watch-strap widths); 100mm = 3.937 inches (just under 4 in, a common reference for engineering and screen specs); 12.7 mm = 0.500 in (exactly half an inch); 152.4 mm = 6 in. The same 25.4 ratio drives every “convert mm to inches” calculation, whether the input is a mechanical drawing dimension, a jewellery measurement or a phone chassis size.

Why people convert mm to inches

Engineering and drafting. Mechanical drawings outside the US are in millimetres; US drawings are in inches (often as fractions or decimals to three places — “0.250 in” or “¼ in”). When a metric drawing arrives at a US shop, or vice versa, every dimension needs converting for tools, gauges and stock. CAD systems can dual-dimension, but machinists frequently want a quick value to enter on the controller.

Jewellery and watches. Ring sizes, watch case diameters, lug widths and strap widths are listed in millimetres almost universally (an 18 mm strap, a 42 mm case) — even on US websites. Customers familiar with imperial measurements often want the inch equivalent to visualise wrist or finger fit; 22 mm is just under ⅞ in.

Phones, tablets and screens. Screen diagonals are listed in inches (a 6.1” iPhone, a 13.3” laptop) but device dimensions are typically in millimetres (a 146.7 × 71.5 × 7.85 mm chassis). Comparing case sizes or wall-mount specs needs the dimensions converted.

Fasteners and hardware. Metric bolts (M3, M6, M8, M10) name thread diameter in millimetres; imperial bolts (#6, #10, ¼”, ⅜”) name the same in fractional inches. When stocking a workshop that mixes metric and US-spec equipment, mm ↔ in conversion is constant.

3D printing and CNC. Slicer and CAM software accepts either unit but printer firmware and stock-material sizes often arrive in one system or the other. A 0.4 mm nozzle is 0.0157 in; a 1.75 mm filament is 0.0689 in.

How to use this Millimetres to Inches converter

1. Type your value in millimetres into the input box. Decimals work directly. The result in inches updates as you type — no Convert button.
2. The reference table below shows several common millimetre values and their inch equivalents, rendered to the same precision as the live result.
3. To go the other way (inches → millimetres) tap Swap or open the dedicated Inches to Millimetres landing.
4. Tap the copy icon next to the result to put the inches value on your clipboard, ready to paste into a CAD model, drawing annotation or shop order.
5. For fractional-inch output, multiply the decimal inch result by 16 and round to express in sixteenths (e.g. 0.625 × 16 = 10/16 = ⅝).

Privacy

The conversion is a single division — it runs locally on your device with no network call. There are no analytics on the numbers you type, no server-side logging, and the page works the same way offline once it has loaded. Confirm in your browser’s Network panel if you want; the only requests you’ll see are the one-time page load.

Compatibility notes

The math works in every browser ever made — it’s literally a single division. Result formatting uses the modern Intl-based number formatting that ships in every browser released since 2017, so very large and very small results display in scientific notation rather than as a wall of zeroes.