In To MM Conversion

Engineering drawings, drill-bit sets, fastener specifications and 3D-printing files are drawn in millimetres, while the products they describe are still sold in inches across North America. This converter turns inches into millimetres exactly, so a 10-inch part becomes 254 mm and a quarter-inch bolt becomes 6.35 mm — the figures a metric workshop actually works to.

Quick Answer

Multiply the inch value by 25.4: mm = inches × 25.4, and 25.4 is exact by definition. For example, 10 in × 25.4 = 254 mm.

The Most Exact Conversion in the Workshop

Most unit conversions involve measured quantities and inherited approximations. This one does not. Since 1959 the inch has been defined internationally as exactly 25.4 millimetres — a matter of treaty-level agreement, described in the history of the inch itself — so the factor carries no uncertainty at all, at any scale, for any purpose from watchmaking to shipbuilding. When a machinist converts 3 inches and gets 76.2 mm, that is not a close estimate; it is the same length wearing metric clothes.

The pairing is unavoidable because manufacturing globalised around both standards at once. Machine tools, bearings, pipe threads, camera filter threads, bicycle parts and electronic enclosures are specified in millimetres by their designers and sold into inch markets, or designed in inches and manufactured abroad. Fasteners show the split at its starkest: a bolt bin may hold M6 bolts (6 mm thread diameter) beside 1/4-inch bolts (6.35 mm) that look interchangeable and are not — close enough to start threading, different enough to strip. Conversions at this precision are not pedantry; they are how parts come to fit.

Note the direction of travel in precision culture: drawings in millimetres usually tolerate no fractions, while inch drawings speak fluently in 1/16, 1/32 and 1/64. Converting fractional inches into decimal millimetres is therefore half the job of this page, and the worked example takes it head-on.

How to Use the In to MM Converter

1. Read the dimension in inches from your part, drawing, product page or tape measure. If it is written as a fraction, convert it to a decimal first: divide the top by the bottom (3/8 = 0.375).
2. Enter the decimal inch value into the input field.
3. Read the millimetre result, calculated with the exact 25.4 factor.
4. Check the size before acting on it: millimetre answers are always about twenty-five times the inch input, so a result near the input's size means a slip.
5. Round to the precision your task honestly needs — one decimal for carpentry and printing, two or three for machining — rather than copying every digit the converter offers.
6. Convert each dimension of a part separately, and if the part must mate with a metric component, compare the converted size against the metric part's nominal size, not its measured sample.

Fractional Inches From a Real Tape Measure

A fitter needs the metric size of three common fractional dimensions. A quarter inch: 0.25 × 25.4 = 6.35 mm. Three-eighths: 0.375 × 25.4 = 9.525 mm. Three-quarters: 0.75 × 25.4 = 19.05 mm. And the whole-number anchor: a 10-inch tablet screen diagonal is 10 × 25.4 = 254 mm.

The fractions machinists reach for, converted:

• 1/16 in = 1.5875 mm
• 1/8 in = 3.175 mm
• 1/4 in = 6.35 mm
• 3/8 in = 9.525 mm
• 1/2 in = 12.7 mm
• 5/8 in = 15.875 mm
• 3/4 in = 19.05 mm
• 1 in = 25.4 mm

Keep the near-misses from this table in mind: 1/2 inch (12.7 mm) is not 13 mm, and 3/4 inch (19.05 mm) is not 19 mm. In clearance holes the difference is invisible; in threads and press fits it is the whole story.

Near Enough Is Not the Same Size

The dangerous assumption in this conversion's neighbourhood is that adjacent metric and inch sizes are interchangeable. They are not. A 6.35 mm (1/4 in) shaft in a 6 mm bearing is a press fit that was never designed; a 19 mm spanner on a 3/4-inch (19.05 mm) nut rounds its corners. Where a conversion produces a value like 9.525 mm, treat it as information about the original design — the part was designed in inches — and source the inch part or re-engineer the fit deliberately, rather than grabbing the closest metric stock.

Paper and photography offer a gentler version of the same lesson: a 4 × 6 inch photo print is 101.6 × 152.4 mm, and metric frames sold as 10 × 15 cm (100 × 150 mm) are nominally for the same print with slightly different cropping. And in electronics, "2.54 mm pitch" pin headers are simply tenth-of-an-inch spacing in metric dress — 0.1 in × 25.4 = 2.54 mm exactly — so circuit boards straddle both systems by design. For the reverse conversion, when the drawing is metric and the supplier is American, the MM to Inch converter divides by the same factor, and larger-scale dimensions belong on the CM to Inches converter.

Pipe sizes are the conversion's most notorious ambush. A fitting sold as 1/2-inch pipe thread does not measure half an inch across anything: the designation is nominal, inherited from the pipe's approximate bore in an older standard, and the thread's actual outside diameter is about 0.84 inches — 21.34 mm. BSP and NPT threads add national differences on top, so a converted measurement can be perfectly accurate and the fitting still wrong. Measure the real diameter, convert it with this page, and then match against the thread standard's table rather than the product's headline size. The arithmetic is exact; the naming never was.

Pocket change offers a pocket check: a US quarter is specified at 0.955 inches in diameter, which is 0.955 × 25.4 = 24.26 mm — and indeed a ruler shows just over 24 mm, confirming both the coin and the conversion at once.

Precision Work Starts With This Number

25.4 is the hardest-working constant in any bilingual workshop, and this page applies it without rounding surprises. Keep it open next to the parts catalogue, the drawing or the printer settings, and the two measurement worlds stop colliding. The remaining length conversions are gathered on the converters category page.