MM to Inches Converter

Why there are exactly 25.4 millimetres in an inch

The inch is defined in terms of the metre. Since the international yard and pound agreement of 1959, one inch has been exactly 25.4 millimetres — not approximately, but by definition. That means a millimetre is exactly 1/25.4 of an inch, about 0.0393701 inches, and every mm to inches conversion is a single division with no hidden rounding.

Before 1959 the United States and the United Kingdom used slightly different inches, which differed in the sixth significant figure. The agreement settled on 25.4 mm because it is a convenient, exact decimal that sits between the two old values. Engineering drawings, machine tools and digital calipers all rely on it.

How to convert millimetres to inches

Divide the number of millimetres by 25.4. If you prefer multiplying, multiply by 0.0393701 — the same thing, slightly rounded. Then decide how you want the answer: as a decimal (for engineering and specifications) or as the nearest fraction (for tape measures and drill bits).

For a slower explanation with practice questions, read how to convert mm to inches. To go the other way, multiply inches by 25.4 or use the inches to mm converter.

MM to inches as fractions

In the US and in woodworking, measurements are usually read as fractions of an inch. The table gives the exact decimal and the nearest 1/16 and 1/32 inch for common millimetre sizes:

Most metric sizes do not land exactly on an inch fraction, so these are approximations. For any value, the mm to inches fraction calculator shows the nearest 1/8, 1/16, 1/32 or 1/64 inch together with the error, so you can tell whether it is close enough.

Everyday things measured in millimetres

Millimetres are the standard unit for small, precise sizes worldwide. These examples help you picture what a millimetre measurement means in inches:

For a full printable list from 1 mm to 1,000 mm, open the mm to inches chart.

Metric and imperial tools are not interchangeable

Many metric sizes are close to an inch fraction, which causes problems. A 13 mm spanner is 0.512″ while a 1/2″ spanner is 0.500″; using the wrong one can round off a nut. A 6 mm drill bit (0.236″) makes a slightly smaller hole than a 1/4″ bit (0.250″). An 8 mm bolt (0.315″) will not fit a 5/16″ nut (0.3125″) properly because the thread pitches differ too.

When fit matters — bolts, bearings, pipe fittings, drill bits — use the tool made for the system the part was designed in, and use the exact decimal conversion rather than a rounded fraction.

Rainfall and snowfall: mm to inches

Weather services outside the US report rainfall in millimetres; American forecasts use inches. One inch of rain is exactly 25.4 mm, so a “heavy” day of 50 mm is about 2 inches.

Quick mental estimate: divide by 25

Dividing by 25 instead of 25.4 is easy to do in your head — divide by 100 and multiply by 4. The answer is about 1.6% too high, which is fine for a rough idea but not for anything that must fit:

Another handy anchor: 25 mm is just under an inch, 100 mm is just under 4 inches, and 300 mm is just under a foot (11.81″).

How many decimal places do you need?

For general use, two decimal places of an inch are plenty. Engineers and machinists usually work to three decimals (thousandths of an inch, called “thou” or “mils”); one thousandth of an inch is 0.0254 mm. Precision engineering may use four. As a rule, give the inch value to the same relative precision as the millimetre value you started with: 12 mm is “about 0.47 inches”, while 12.00 mm on a drawing calls for 0.4724″.

Reading a ruler with millimetres and inches

Many rulers and tape measures carry both scales. On the metric edge, every small line is a millimetre, the slightly longer line in the middle marks 5 mm, and the numbered lines are centimetres (10 mm each). On the imperial edge, the numbered lines are inches, and the shorter lines divide each inch into halves, quarters, eighths and sixteenths.

Laying the two edges side by side is a quick visual converter: 25.4 mm lines up with the 1″ mark, 50.8 mm with 2″, and 101.6 mm with 4″. Because one millimetre is only about 1/25 of an inch, a millimetre scale lets you read finer measurements than a tape marked in sixteenths (1/16″ is about 1.6 mm). To measure small items on your screen, try the actual-size mm ruler.

Millimetres, centimetres, inches and feet

It helps to keep the whole ladder of units in mind. Ten millimetres make a centimetre, a thousand make a metre; twelve inches make a foot and three feet a yard. The links between the two systems all come from the single exact definition of the inch.

So when a measurement is given in centimetres, multiply by 10 to get millimetres first, or use the cm to inches converter directly. For feet and inches together, the mm to inches chart lists larger sizes up to 1,000 mm in feet and inches.

Common mistakes

• Dividing by 2.54 instead of 25.4. 2.54 is the factor for centimetres; using it on millimetres gives an answer ten times too big.
• Multiplying instead of dividing. Inches are bigger than millimetres, so the inch number must be smaller.
• Rounding a fraction too coarsely. 10 mm is 0.394″; rounding to 3/8″ (0.375″) is off by nearly half a millimetre.
• Mixing up mm and cm. 1 cm = 10 mm. If a value looks ten times off, check the unit — the mm to cm converter helps.