Inches to MM

1 inch equals exactly 25.4 millimeters. This is a defined constant — not an approximation — established by international agreement in 1959 and exact to infinite decimal places worldwide.

Key multiples: 2 in = 50.8 mm  ·  6 in = 152.4 mm  ·  12 in = 304.8 mm (exactly 1 foot)  ·  36 in = 914.4 mm (exactly 1 yard). To convert mm back to inches, divide by 25.4. Enter any inch value in the calculator above for the instant full breakdown.

The exact formula is: millimeters = inches × 25.4. To reverse: inches = millimeters ÷ 25.4. Both directions are mathematically exact — the factor 25.4 is a defined constant, not a rounded approximation.

Quick mental anchors: 1″ = 25.4 mm  ·  ½” = 12.7 mm  ·  ¼” = 6.35 mm  ·  ¾” = 19.05 mm  ·  12″ = 304.8 mm (1 ft). Every whole and fractional inch value produces a millimeter result that is an exact decimal multiple of these anchors — no rounding is ever introduced by the conversion factor itself.

The exact 25.4 mm definition was established by the International Yard and Pound Agreement of 1959. Six nations agreed to define the yard as exactly 0.9144 meters, making the inch exactly 0.0254 meters — equivalently, exactly 25.4 mm to infinite precision.

Before 1959, the U.S. survey inch was approximately 25.40005 mm, causing small but real discrepancies in precision engineering and land surveying. The 1959 agreement unified the standard worldwide. NIST retired the last legacy of the old U.S. definition — the “U.S. survey foot” — on January 1, 2023. For all modern measurement, one universal value applies: 1 inch = 25.4 mm, everywhere.

¼ inch (0.25 inches) equals exactly 6.35 mm. The calculation is 0.25 × 25.4 = 6.35 mm — a terminating decimal, exact to infinite places. This is one of the most frequently referenced fractional inch conversions in woodworking (ⅼ⁄4″ plywood, ¼” router bits) and hardware (¼”-20 bolts, ¼” pipe fittings).

The complete set of quarter-inch multiples: ¼” = 6.35 mm  ·  ½” = 12.7 mm  ·  ¾” = 19.05 mm  ·  1″ = 25.4 mm. All are exact, all terminate, and all are exact multiples of 6.35 mm.

½ inch (0.5 inches) equals exactly 12.7 mm. The calculation is 0.5 × 25.4 = 12.7 mm — exact to infinite places with no rounding. ½” is one of the most important fractional inch sizes in North American trade: ½” drywall, ½” plywood, ½” pipe, ½” conduit, and ½”-13 bolts are all standard dimensions that frequently need to be expressed in millimeters for metric hardware sourcing.

Note that 12.7 mm has no clean equivalent in ISO metric hardware standards — the nearest standard metric dimensions are 12 mm and 13 mm. When sourcing metric fasteners or pipe to replace nominal ½” imperial components, always check the actual fitting dimension against the metric catalog specification rather than assuming 12 or 13 mm will fit.

¾ inch (0.75 inches) equals exactly 19.05 mm. This is the most commonly referenced fractional inch conversion in North American woodworking and construction, because ¾” nominal plywood is the standard panel thickness for cabinets, furniture, and shelving.

When ordering European or Asian sheet goods to replace ¾” nominal plywood, note that the nearest metric standard panel thickness is 18 mm or 19 mm — neither equals 19.05 mm exactly. An 18 mm panel will sit 1.05 mm below flush in a ¾” dado; a 19 mm panel will be 0.05 mm undersized (negligible). For critical joinery, specify 19.05 mm explicitly or verify the actual thickness with calipers before cutting dados.

To convert fractional inches to mm: Step 1 — convert the fraction to a decimal by dividing numerator ÷ denominator. Step 2 — add any whole inches. Step 3 — multiply the total by 25.4.

Example: 3⅝ inches. Step 1: 5 ÷ 8 = 0.625. Step 2: 3 + 0.625 = 3.625 inches. Step 3: 3.625 × 25.4 = 92.075 mm exactly. Enter 3.625 in the calculator above for the full unit breakdown including centimeters, meters, feet, and yards. The fractional reference table on this page covers all standard 16th-inch fractions for quick look-up.

12 inches equals exactly 304.8 mm — also exactly 1 foot and exactly 30.48 cm. The calculation is 12 × 25.4 = 304.8 mm. This is one of the most fundamental anchor conversions in metric-to-imperial dimensional work.

304.8 mm appears throughout construction: metric tile module layouts often reference 300 mm as “close to 1 foot,” but the exact foot is 304.8 mm — a 4.8 mm difference that adds up across multiple courses. When calculating exact room dimensions for metric flooring or tiling in a U.S.-measured space, always use 304.8 mm per foot rather than rounding to 300 mm.

Inches are an imperial unit used as the standard in the United States for construction, hardware, screen sizes, and everyday measurement. Millimeters are a metric unit — exactly 1/1000 of a meter — and are the global standard for engineering drawings, precision manufacturing, and product specifications in most countries.

One inch equals 25.4 mm exactly. Millimeters offer finer natural resolution for small measurements: expressing 25.4 mm instead of 1.0000 inch makes the four-decimal precision obvious. In professional technical contexts, mm is typically preferred over cm for dimensions below 1 meter because it avoids decimals in most precision work — e.g. 304.8 mm is cleaner than 30.48 cm for technical documentation.

The calculator uses the exact factor 1 inch = 25.4 mm with double-precision floating-point arithmetic, accurate to approximately 15 significant digits. The conversion is mathematically exact — the only rounding in your result comes from the number of decimal places you select in Display Options.

For engineering and CNC use, set precision to 6 or higher. For woodworking and construction, 3–4 decimal places provide more than sufficient accuracy. For quick estimates and purchasing, 2 is fine. The downloadable PDF records your chosen precision, the full result set, and the conversion standard, creating a complete auditable record of how each dimension was derived.