Inches to Mm Converter

inches to millimeters — quick reference

The formula, worked line by line

The inch has been a metric unit by definition for longer than most workshops realise. The chain is short: the 1959 agreement fixed the yard, an inch is one thirty-sixth of a yard, and a millimeter is a thousandth of a meter. Run those together and the factor comes out exact.

Divide 0.9144 by 36 and you get 0.0254 meters, which is 25.4 millimeters with nothing after the four. No physical prototype is involved and no measurement uncertainty enters, so this is one of the few unit conversions where the word exactly is doing real work rather than being a figure of speech.

millimeters = inches × 25.4
            = inches × 0.9144 × 1000 ÷ 36
from a fraction: inches = top ÷ bottom, then × 25.4
centimeters = inches × 2.54
12 in → 12 × 25.4 = 304.8 mm

The estimate, and why it is worse here

Multiplying by 25 instead of 25.4 lands 1.6 percent low, which is a bigger relative error than most people expect from a shortcut. Twelve inches becomes 300 mm against a true 304.8. In a workshop that is nearly five millimeters over a foot, which is enough to matter on almost any joint.

Areas and volumes take a different factor

Square inches use the square of the factor, 645.16 square millimeters, and cubic inches use the cube, 16,387.064 cubic millimeters. Those come up on gasket faces, bore volumes and engine displacement, where a five litre engine is 305.1 cubic inches. Using the plain length factor on an area is out by a factor of 25.4.

For anything larger than a hand span the millimeter stops being convenient, and the centimeter or the meter takes over. The relationships are all exact multiples of the same constant, so an inch is 2.54 cm and 0.0254 m, and a foot is 30.48 cm and 0.3048 m. Pick the unit the reader will use, not the smallest one available.

How to use the inches to mm converter

This is the direction you need when the number in your head is imperial and the thing you are ordering, drawing or programming is metric. Fasteners, CNC toolpaths, CAD files, extrusion profiles and almost every technical drawing produced outside the United States expect millimeters, and they expect them without a fraction anywhere in sight.

Convert the fraction to a decimal, type it in, and the millimeters appear as you type. The tool multiplies by 25.4, an exact international definition, so the answer carries no rounding of its own. Press Swap when the traffic runs the other way and a metric size needs reading back as a fraction.

The fractions are what make this feel harder than it is. An imperial size is written as a fraction, and a fraction has to become a decimal before it can become a metric figure. Three eighths is 0.375, which is 9.525 mm. Nine sixteenths is 0.5625, which is 14.2875 mm.

Lumber is the trap worth naming, because the imperial number on the label is not the imperial size of the wood. American nominal 2 by 4 framing actually measures 1.5 by 3.5 inches once it is dried and surfaced, so the metric figure is 38.1 by 88.9 mm and not the 50.8 by 101.6 the name suggests.

US paper and photo sizes in millimeters

Paper is where most people first hit this conversion, usually while trying to print something. US Letter is 8.5 by 11 inches, which is 215.9 by 279.4 mm. A4, defined by ISO 216, is 210 by 297 mm. Letter is 5.9 mm wider and 17.6 mm shorter, so neither fits inside the other.

That mismatch is why a Letter document printed to A4 leaves a strip at the bottom and clips at the sides, and why scaling to fit is the only clean answer. The shapes differ too: A4 has the ISO ratio of the square root of two, while Letter is 1.294 to 1, so no uniform scale maps one onto the other exactly.

Photo frames carry the same problem in a smaller package. A 4 by 6 print is 101.6 by 152.4 mm while the nearest common metric size, 10 by 15 cm, is 100 by 150 mm. The frame will hold the print, with a millimeter and a half of overlap on one side and nearly two and a half on the other.

Why every inch fraction lands on an exact millimeter

There is a small piece of arithmetic here that makes imperial to metric work cleanly, and almost nobody points it out. Imperial fractions are always halves of halves: 1/2, 1/4, 1/8, 1/16, 1/32, 1/64. Dividing 25.4 by any power of two always terminates, because 25.4 is 254 tenths and halving a decimal never produces a repeat.

So 1/2 inch is 12.7 mm exactly. A quarter is 6.35, an eighth is 3.175, a sixteenth is 1.5875, a thirty-second is 0.79375 and a sixty-fourth is 0.396875. Six exact figures with no tail, which is why a fraction can be typed into a metric CAD system without anyone having to decide where to truncate.

The direction that does not behave is the reverse one. A whole millimeter almost never lands on a fraction, because dividing by 25.4 introduces a repeating decimal immediately. That asymmetry is why imperial parts drop into metric drawings cleanly and metric parts arrive in imperial drawings as awkward four-place decimals.

Every inches value, worked out

31 common inches figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask