Inch to MM Calculator

Inch to mm: the essential conversion for engineering, DIY and fixings

Millimetres are the standard unit for precision measurement on UK engineering drawings, tools and most modern components — but a huge amount of hardware still uses imperial (inch and fractional-inch) sizing, especially fasteners, plumbing fittings, older machinery, US-manufactured parts and specific trades like woodworking. That mix is exactly why "inch to mm" is one of the most consistently searched conversions among tradespeople, engineers and DIY enthusiasts, not just casual users.

One inch equals exactly 25.4mm — a fixed, internationally agreed figure with zero approximation, so an inch-to-mm conversion is always precise arithmetic. This calculator handles both directions: inches into millimetres, for reading an imperial spec (from a drill bit, bolt, or US component) in the metric figure needed for a UK tool or drawing, and millimetres back into inches for the reverse.

Quick conversions

The most commonly looked-up fractional-inch conversions, calculated exactly:

Fraction to mm conversion table

The full set of sixteenths-of-an-inch — the standard fractional gradations used on imperial rulers, drill bits, sockets and spanners:

Whole inches to mm and cm

For larger measurements — timber, pipes, screen sizes and room dimensions:

Conversion formula and method

The inch-to-mm relationship is a single fixed multiplication:

Millimetres = Inches × 25.4
Inches = Millimetres ÷ 25.4

Worked example: inches to mm

Take 5/8 inch, a common socket size. Convert the fraction to a decimal first: 5 ÷ 8 = 0.625. Multiply by 25.4: 0.625 × 25.4 = 15.875mm. So 5/8" converts to 15.875mm, usually rounded to 15.88mm.

Worked example: mm to inches

Take 10mm, a common metric bolt size. Divide by 25.4: 10 ÷ 25.4 = 0.3937in. This is the calculation the "MM → Inches" mode above runs automatically, and the calculator also rounds to the nearest 1/16" for a practical ruler reading.

Converting a mixed fraction

For a measurement like 2 3/8 inches, convert the fraction first (3 ÷ 8 = 0.375), add it to the whole number (2.375in), then multiply by 25.4: 2.375 × 25.4 = 60.325mm.

Where this conversion actually gets used

• Fasteners and fixings: UK hardware stores sell both metric (M6, M8, M10...) and imperial (1/4", 5/16", 3/8"...) bolts, nuts and drill bits — telling them apart and cross-referencing sizes requires this exact conversion.
• Plumbing: UK plumbing fittings are a well-known mix of metric pipe and older imperial fitting sizes (1/2", 3/4", 1" BSP threads are still standard), making inch-to-mm conversion a routine trade task.
• Woodworking and US-sourced plans: American woodworking plans and lumber dimensions are specified in inches, which UK woodworkers using metric tools and timber need to convert.
• Engineering and manufacturing: imported components, especially from the US, are frequently specified in inches and fractional inches, requiring conversion to match metric drawings and tolerances.
• Screen and display sizing: screen sizes are conventionally quoted in inches worldwide, but bezel, mounting and cable specifications are often in millimetres.

Metric vs imperial fastener quick-reference

Because UK hardware stores stock both standards, it's useful to know which metric bolt sizes sit closest to common imperial fractions — though "closest" doesn't mean interchangeable, since the pitch and tolerances differ:

These pairings are close enough to cause confusion at a hardware counter, but not close enough to be safely substituted — an M8 bolt (8mm) will not thread properly into a 5/16" (7.94mm) tapped hole, even though the two figures are within a fraction of a millimetre of each other. Always match fastener standards exactly rather than relying on an "approximately equal" size.

Reading a tape measure: understanding the fractional markings

Standard UK and US tape measures mark inches in a repeating binary sequence — halves, then quarters, then eighths, then sixteenths — rather than decimals, which is why the fraction-to-mm table above is arranged in sixteenths rather than a decimal step. The longer lines on a tape measure mark whole inches and halves; progressively shorter lines mark quarters, eighths and finally sixteenths. Recognising this pattern makes it much faster to read off a fractional measurement directly, without needing to count every tick mark from zero.

Historical context: two systems bridged by a single fixed number

The inch has ancient origins, historically estimated from body proportions such as the width of a thumb, and varied slightly between countries and eras before formal standardisation. The millimetre is a purely modern metric unit — one-thousandth of a metre — introduced when the metric system was formalised in France in the 1790s and later adopted internationally through the Metre Convention of 1875. For most of their shared history, the two units had no single agreed exact relationship, meaning conversions were approximate and varied slightly by country.

That changed in 1959 with the International Yard and Pound Agreement, signed by the UK, US, Australia, Canada, New Zealand and South Africa, which fixed the inch at exactly 25.4mm. This single, deliberately chosen bridge value is why every inch-to-mm conversion today, in every English-speaking country, uses precisely the same factor — a level of international consistency that didn't exist before 1959, when UK and US inches could differ by tiny but real fractions of a millimetre.

Common mistakes when converting inches to mm

• Misreading a fraction's denominator. 3/16" and 3/8" look superficially similar written down but convert to very different millimetre values (4.76mm vs 9.53mm) — always double-check the denominator before converting or ordering a part.
• Rounding a metric-imperial near-match too casually. 10mm and 3/8" (9.525mm) are close but not identical — using one bolt or socket size in place of the other can strip threads or fail to fit properly, even though the numbers look close.
• Forgetting that "mm to nearest sixteenth" is an approximation. When a calculator rounds a millimetre figure to the nearest 1/16" for ruler-reading convenience, that's a practical rounding for marking a cut — not the exact converted value, which should be used for anything precision-critical.
• Confusing metric (M) bolt sizes with millimetre diameter directly. An M8 bolt has an 8mm nominal shaft diameter, but thread pitch and head size vary by standard — the "M" number isn't a simple, complete substitute for a full spec when ordering imperial-metric equivalents.

Accuracy and precision note

Because 1 inch = 25.4mm is a fixed, internationally agreed definition rather than a measured approximation, every conversion on this page and in the calculator above is mathematically exact. Millimetre figures are shown to two decimal places above for the precision that engineering and fastener work typically needs — full floating-point precision is available in the calculator for anything requiring an even finer figure.

A quick note

All conversions above use the exact internationally defined relationship between the inch and the millimetre. For anything precision-critical (engineering, structural or commercial use), always confirm against the source figures rather than a rounded display value.

References

• Weights and Measures Act 1985 (legislation.gov.uk)
• UK product conformity marking (gov.uk)