Convert mm to in millimetres ↔ inches Free Online Secure
Type a value in millimetres and read it in inches at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert mm to in (millimetres ↔ inches)
| mm | in |
|---|---|
| 1 | 0.0393700787402 |
| 2 | 0.0787401574803 |
| 3 | 0.11811023622 |
| 5 | 0.196850393701 |
| 10 | 0.393700787402 |
| 20 | 0.787401574803 |
| 50 | 1.96850393701 |
| 100 | 3.93700787402 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
Why do engineering drawings use millimetres?
Because a millimetre is small enough that nearly every dimension of a manufactured part is a whole number or a short decimal, which leaves no room for the arithmetic mistakes that fractions invite. Drawings in millimetres state the unit once and then drop it, and tolerances are written the same way, in hundredths or thousandths of a millimetre.
Why is 1 inch exactly 25.4 mm?
Because in 1959 the standards bodies of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed to make it so. Before that the American inch and the British inch differed in the seventh decimal place, which was enough to matter when parts made on one side of the Atlantic had to fit parts made on the other. The agreed value is a definition, not a measurement, so it will not be revised.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
When the socket set speaks inches: how to convert mm to in
Open a toolbox that came from another continent and you may find that your 13 mm bolt needs a 1/2 inch socket, or that the 10 mm spanner is a hair too small for a 3/8 inch nut. Millimetres and inches live in separate worlds, but they meet at hardware stores, in car repairs, in product specifications and in weather reports. Whenever a metric dimension must be read against an imperial one, you have to convert mm to in. This page does it as you type, giving the answer as a decimal and, below it, as the nearest fraction of an inch.
The conversion is a division by 25.4, because the inch is defined as exactly 25.4 millimetres. It is exact, but it is not easy to do in the head, which is why a few approximations are worth learning. The sections below set out the rule, give worked examples, tell you a little about each unit, compare them, tour the places where the conversion matters and list the usual mistakes.
How to convert mm to in: divide by 25.4
The exact rule
One inch is exactly 25.4 mm. To convert mm to in, divide the number of millimetres by 25.4, or multiply by 0.0393701. 25.4 mm is 1 in, 10 mm is 0.3937 in, 50 mm is 1.9685 in, 100 mm is 3.937 in and 1,000 mm is 39.37 in. In the other direction, multiply the inches by 25.4.
Examples
A bolt of 13 mm is 0.512 in, a hair over half an inch. A phone 7.8 mm thick is 0.307 in. A 300 mm ruler is 11.81 in. A pipe of 22 mm is 0.866 in, between 3/4 and 7/8. A sensor of 36 mm is 1.417 in.
Mental shortcut
For a mental convert mm to in, divide by 25 and subtract 1.6 per cent. For 100 mm: 4 minus 0.064 is 3.936, against the exact 3.937. For 50 mm: 2 minus 0.032 is 1.968. For a very rough check, remember that 25 mm is about an inch, so 100 mm is about 4 in and 250 mm about 10 in.
To a fraction of an inch
Tools are sized in fractions, so turn the decimal into the nearest sixteenth: multiply by 16 and round. 13 mm is 0.512 in times 16, which is 8.19, so 8/16 or 1/2 in. 9 mm is 0.354 in, 5.67 sixteenths, so 11/32 is the closest common size.
The millimetre
The workshop unit
The millimetre is one thousandth of a metre and the standard for mechanical drawings worldwide. Hardware, screws, tubes and electronic parts are specified in millimetres. A tolerance of 0.1 mm is common in manufacturing.
Strengths
Millimetres give whole numbers for most workshop sizes and decimals only for the finest ones. They integrate with the rest of the metric system by powers of ten.
The inch
A twelfth of a foot
The inch is one twelfth of a foot and exactly 25.4 millimetres. Its name comes from the Latin uncia, a twelfth. For centuries it was defined by the width of a thumb or by three barleycorns. Since 1959 it has been fixed in metric terms.
Where it lives
In the United States the inch is used for tools, lumber, screens, tyres and clothing. In other countries it survives in pipes, screens and some tools. Its fractions, halves, quarters, eighths, sixteenths and thirty-seconds, suit hand work.
Millimetre and inch compared
An inch is 25.4 times a millimetre. A small dimension in inches is a long one in millimetres: a 1/8 in gap is 3.175 mm. A tolerance of a thousandth of an inch is 0.0254 mm, about 25 micrometres, so inch drawings use decimals in thousandths where metric drawings use hundredths of a millimetre.
When both units appear in one document, state which one governs. A drawing in millimetres with inches in brackets treats the brackets as references and the millimetres as master dimensions. Do not mix them in calculations.
Where you will need it
Tools and fasteners
Sockets are sold in metric and inch sets. A 10 mm socket is 0.394 in, between 3/8 and 7/16 in. Using a 3/8 in socket (9.525 mm) on a 10 mm bolt will round the head. A 13 mm and a 1/2 in spanner (12.7 mm) differ by 0.3 mm, enough to cause slop but not to be interchangeable.
Weather and rainfall
American weather reports give rain in inches. A storm of 50 mm is 1.97 in, nearly 2 inches. An annual rainfall of 600 mm, typical of Bucharest, is 23.6 in. Seattle gets 37 in, 940 mm. To compare climates, run the “convert mm to in” calculation on one side.
Screens and photography
A sensor of 36 by 24 mm is 1.42 by 0.94 in. A film of 35 mm is 1.378 in. A lens filter of 77 mm is 3.03 in. A tablet 8 mm thick is 0.315 in. Spec sheets often list one unit and the user manual the other.
Bicycles and cars
A bicycle tyre is 622 mm bead seat diameter, 24.5 in, labelled 28 in or 700C. A car wheel rim of 406 mm is 16 in. Brake pads must be replaced at 3 mm, 0.118 in, of material. Mixing the units in a parts list is a common source of the wrong order.
3D printing and electronics
Print bed and nozzle
A print bed of 220 mm is 8.66 in. A nozzle of 0.4 mm is 0.0157 in. Layers of 0.2 mm are 0.0079 in, about 8 thousandths. A model designed in inches must be scaled by 25.4 before slicing. A 4 in cube is 101.6 mm.
Boards
A printed circuit board of 1.6 mm is 0.063 in. Pin spacing of 2.54 mm is exactly 0.1 in, which is the reason that 2.54 appears in so many electronic parts. A 100 by 60 mm board is 3.94 by 2.36 in.
Language and symbols
Symbols
The symbols are mm and in, or the double prime mark. The abbreviation in is confusing in a sentence, so the quote mark is common in tables: 0.5". Put a space between the number and unit when you write mm.
Words
Milli comes from Latin mille, a thousand. Inch from Latin uncia, one twelfth, the same root as ounce. The two units come from different traditions of counting: tens and twelves.
A worked example: choosing a drill bit
The task
A bolt of 10 mm needs a clearance hole of 10.5 mm. To convert mm to in, divide by 25.4: 0.413 in. The nearest fractional bit is 13/32 in, 0.406 in, 10.32 mm, slightly small; the next is 27/64 in, 0.422 in, 10.72 mm. Choose 27/64 in, since the hole must be larger than the bolt.
Check
27/64 in times 25.4 is 10.72 mm, more than 10.5 and less than 11, so the bolt slips through with 0.72 mm to spare in diameter. That is the right amount for a clearance hole.
Final checks
Round trip
Multiply your answer in inches by 25.4. If you return to the millimetres you started with, no digit has slipped. A result about 2.54 times too large or too small means you used 25.4 where 10 was needed, or the reverse.
Notes
Write both figures in your notes. If an order or plan is checked later, both numbers are there for comparison.
Paper, printing and envelopes
Letter and A4
American Letter paper is 8.5 by 11 in, which is 215.9 by 279.4 mm, wider and shorter than A4 at 210 by 297 mm, which is 8.27 by 11.69 in. A document set up for A4 and printed on Letter loses 18 mm, 0.7 in, at the bottom unless it is scaled. When you convert mm to in for a form, compare the margins too: a 25 mm margin is 0.98 in, nearly an inch.
Envelopes and labels
A DL envelope is 110 by 220 mm, 4.33 by 8.66 in; the American number 10 envelope is 4.125 by 9.5 in, 104.8 by 241.3 mm. Labels in sheets of 38.1 by 99.1 mm are 1.5 by 3.9 in, a size chosen in inches even on A4 sheets.
Bolts, taps and thread pitch
Metric against imperial threads
A metric bolt M6 has a diameter of 6 mm, 0.236 in, close to a 1/4 in bolt, 6.35 mm, but the threads differ: M6 has a pitch of 1 mm, 1/4-20 has 20 threads per inch, a pitch of 1.27 mm. The two look alike and will start into each other for a turn, then jam. Always check pitch as well as diameter.
Common pairs
M8 is 0.315 in against 5/16 in, 7.94 mm; M10 is 0.394 in against 3/8 in, 9.53 mm; M12 is 0.472 in against 1/2 in, 12.7 mm. A table of these pairs, kept in the shed, prevents the commonest hardware error.
Camera filters and lens threads
Filter sizes
Filter threads are given in millimetres: 49, 52, 58, 67, 72, 77 mm, which are 1.93, 2.05, 2.28, 2.64, 2.83 and 3.03 in. A step-up ring adapts a 58 mm lens to a 67 mm filter. Old American lenses have series filters measured in inches, such as Series VI, 1.6 in. A photographer buying second-hand gear must convert mm to in to check that the ring will fit.
Tripod screws
Camera tripod screws are 1/4 in, 6.35 mm, or 3/8 in, 9.53 mm. A European accessory with an M6 thread is 6 mm, so it will not fit a 1/4 in socket without an adapter. Keep a small bag of adapters with the tripod.
Wood, board and sheet
Nominal against actual
American lumber is sold by nominal size: a 2 by 4 is really 38 by 89 mm, 1.5 by 3.5 in. Plywood of 3/4 in is 18 mm. Drywall of 1/2 in is 12.7 mm. Metric boards of 18 mm, 12 mm and 9 mm correspond to 23/32, 15/32 and 3/8 in.
Converting a project
A bookshelf design in inches calls for boards 3/4 in thick, 11.25 in wide, 36 in long. With metric boards of 18 by 300 by 900 mm, the thickness is 0.709 in, the width 11.8 in, the length 35.4 in. The differences are small but cumulative.
Bullets, hoses and pipes
Calibres and bores
A 9 mm calibre is 0.354 in; a 5.56 mm round is 0.219 in; a 12.7 mm is 0.5 in. Pipe sizes for hydraulics are given in both units, a 12 mm hose being 0.472 in, close to 1/2 in. Compression fittings sold abroad may differ by half a millimetre, which matters for sealing.
Checking fit
When fitting a hose to a barb, measure the outside diameter of the barb. If it is 12.5 mm, 0.492 in, a hose with 1/2 in inner diameter, 12.7 mm, will slide on with a clamp.
Sewing and crafts
Notions in two units
Buttons are 11 mm, 15 mm or 20 mm: 0.433, 0.591 and 0.787 in, or 7/16, 19/32 and 25/32 in. Zippers are 20 cm, 7.87 in, but the tape width is 25 mm, 1 in. Knitting needles of 4 mm are 0.157 in, sold in the US as size 6.
Seam allowances
A European pattern has seams of 10 mm, 0.394 in, while an American pattern uses 5/8 in, 15.9 mm. Mixing the two leads to garments that are a few millimetres too big or small on each seam.
Medical and dental
Small sizes in two units
A tumour of 20 mm is 0.79 in, nearly 3/4 in. A needle of 0.5 mm, gauge 25, is 0.0197 in. A surgical wound of 30 mm is 1.18 in. American doctors often describe sizes in centimetres in reports, so the millimetre figure and inch figure both appear in a case discussion.
Why the unit changes
Dental instruments are sized in millimetres, but a patient's mouth opening may be given in inches in some textbooks. A normal mouth opening is 35 to 50 mm, 1.4 to 2 in.
Spreadsheet formulas
Formula
To convert mm to in for a column, put millimetres in column A and enter =A2/25.4 in column B. Format with three decimals. For the nearest sixteenth, enter =ROUND(B2*16,0)/16. To reverse, multiply by 25.4.
Check values
Enter 25.4: result 1. Enter 12.7: 0.5. Enter 100: 3.937. If you see 0.0394 for 1, you are multiplying by 0.0394 and dividing again.
Rounding
Matching precision
A measurement of 52 mm to the nearest millimetre is 2.047 in, but a sensible report gives 2.05 in. If you need the nearest sixteenth, 2 3/64 is meaningless; use 2 1/16, 2.0625.
Fits and clearances
For a hole that must be large enough for a pin, round the size up to the next standard. For a pin that must fit in a hole, round down. A 13 mm hole is 0.512 in; a drill of 33/64 in is 13.1 mm, a better choice than 1/2 in, which is 12.7 mm.
Teaching the idea
A double ruler
To teach the “convert mm to in” calculation, print a ruler in inches and one in millimetres side by side. Students mark 25.4 mm on the metric ruler and find it lines up with 1 in. They then measure objects and record both units, discovering the 25.4 ratio.
Estimating
Ask students to guess lengths in inches, convert by multiplying by 25 and add 1.6 per cent, and check with a ruler. The error should be well below 2 per cent.
History of two measures
Barleycorns and metres
A statute of 1324 in England defined the inch as three barleycorns, round and dry, laid end to end. The millimetre arrived with the metric system in the 1790s as a thousandth of a metre. For two centuries they were incompatible until the 1959 agreement defined the inch as 25.4 mm.
The effect
The agreement was a boon to engineering: the same drawing could be read in either system without error. The conversion factor never changes.
More values for convert mm to in
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 mm = 0.00393701 in
- 0.5 mm = 0.019685 in
- 3 mm = 0.11811 in
- 4 mm = 0.15748 in
- 7 mm = 0.275591 in
- 15 mm = 0.590551 in
- 20 mm = 0.787402 in
- 75 mm = 2.95276 in
- 200 mm = 7.87402 in
- 500 mm = 19.685 in
- 2,000 mm = 78.7402 in
- 10,000 mm = 393.701 in
Mistakes when you convert mm to in
Multiplying instead of dividing
The inch is longer, so the number of inches is smaller. If 100 mm becomes 2,540, you multiplied.
Using 2.54 for millimetres
2.54 is centimetres per inch. For millimetres, use 25.4.
Rounding too early
Convert, then round to a fraction. Rounding the decimal first adds error.
Quick answers
How many millimetres in an inch?
Exactly 25.4.
How do I convert mm to in?
Divide by 25.4.
What is 100 mm in inches?
3.937 in.
Is the conversion exact?
Yes, the inch is defined as exactly 25.4 mm.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with lengths, these conversions are close at hand: Convert mm to cm, Convert mm to dm, Convert dm to in, Convert ft to in, Convert in to ft, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert in to mm.
In short, to convert mm to in divide by 25.4, round to the nearest sixteenth when you work with tools, and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.