Convert in to mm inches ↔ millimetres Free Online Secure
Type a value in inches and read it in millimetres 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 in to mm (inches ↔ millimetres)
| in | mm |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 50 | 1270 |
| 100 | 2540 |
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 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.
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.
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.
Precision in two languages: why you may need to convert in to mm
When a drawing from an American supplier lands on a metric workshop floor, the first question is always the same: what is that in millimetres? A shaft is specified as 1.25 in, a plate as 0.375 in thick, a hole as 0.201 in. The machine, the caliper and the people who must convert in to mm think in millimetres, so the figures must be brought across. That step is to convert in to mm, and this page performs it as you type, with the exact result and a rounded one.
The conversion factor is as simple as they come: since 1959 the inch has been exactly 25.4 millimetres. What makes the task slippery is not the multiplication but the habits around it: fractions of an inch, tolerances in thousandths, the temptation to round too early. The sections below set out the rule, give worked examples, introduce both units, compare them, tour the situations in which the conversion matters and list the mistakes that cost the most.
How to convert in to mm: multiply by 25.4
The exact rule
One inch is exactly 25.4 mm. To convert in to mm, multiply the number of inches by 25.4. Two inches are 50.8 mm, five are 127 mm and ten are 254 mm. A decimal behaves the same way: 0.5 in is 12.7 mm, 0.375 in is 9.525 mm and 0.001 in is 0.0254 mm. The result is exact, which is rare among conversions, because the inch is defined in millimetres.
Worked examples
A plate 3/4 in thick is 19.05 mm. A bolt 2 1/2 in long is 63.5 mm. A pipe with an outside diameter of 1.315 in is 33.4 mm. A part toleranced at plus or minus 0.005 in is plus or minus 0.127 mm. In each case the working is a plain multiplication, and the only decision left is how many decimals to keep.
Mental shortcut
For a mental convert in to mm, multiply by 25 and add 1.6 per cent. For 4 in: 100 plus 1.6 gives 101.6 mm, exactly right. Or count that four inches are about 100 mm and every extra quarter inch is 6.35 mm. The 25 times table is easier to do in your head than 25.4.
Fractions of an inch
Convert fractions to decimals before multiplying: 5/16 is 0.3125, so 7.9375 mm. A table of the common ones is worth keeping: 1/16 is 1.5875, 1/8 is 3.175, 1/4 is 6.35, 3/8 is 9.525, 1/2 is 12.7, 5/8 is 15.875, 3/4 is 19.05, 7/8 is 22.225.
The inch
Origin and definition
The inch comes from the Latin uncia, a twelfth, and was long defined by the width of a thumb or three barleycorns. In 1959 the United States and the Commonwealth countries agreed on exactly 25.4 mm. Before that, the American and British inches differed by about two parts per million, a tiny gap that mattered in precision work.
Modern use
In engineering, inch drawings use decimals and thousandths, called thou or mils. A part drawn as 1.2500 in plus or minus 0.0005 is a typical machinist specification. Elsewhere inches appear on tape measures, in screen sizes and in pipe diameters.
The millimetre
A thousandth of a metre
The millimetre is a thousandth of a metre and the unit in which most of the world draws mechanical parts. A hair is about 0.07 mm thick, a credit card 0.76 mm, a coin 2 mm. A tolerance of 0.01 mm is common in precision machining.
Why drawings use it
Whole millimetres describe most dimensions without decimals, and the metric system's tens make addition and scaling easy. International standards for screws, bearings and tools all use millimetres.
Inch and millimetre compared
An inch is 25.4 times longer than a millimetre. Numbers in millimetres are therefore larger and, for the same precision, need fewer decimals. A tolerance of 0.001 in is 0.0254 mm, about 25 micrometres, so inch drawings use three decimals where metric drawings use two.
When you convert, remember to convert the tolerance as well. Plus or minus 0.002 in becomes plus or minus 0.0508 mm, often rounded to 0.05. Writing the converted size with too many digits suggests a precision the original never had.
Where you will need it
Machining and fabrication
An American drawing calls for a 1.5 in diameter shaft with a plus 0.000, minus 0.001 tolerance. In millimetres that is 38.1 mm with plus 0, minus 0.0254. The machinist chooses a standard 38 mm bar and turns it down, or buys 1.5 in stock. The conversion is the first step in deciding.
Fasteners
Bolts in the UNC series are named for the diameter in inches: 1/4 in is 6.35 mm, 3/8 in 9.525 mm, 1/2 in 12.7 mm. The nearest metric sizes are M6, M10 and M12, which are close but do not interchange. Threads differ too: 1/4-20 UNC has 20 threads per inch, a pitch of 1.27 mm.
Sheet metal and plate
Sheet metal is described by gauge, but the thickness in inches converts directly: 16 gauge steel is 0.0598 in, 1.52 mm; 11 gauge is 0.1196 in, 3.04 mm. Plate comes in fractions: 1/4 in is 6.35 mm, 3/8 in 9.525 mm.
3D printing and CNC
Slicers and CAM software use millimetres by default. A model downloaded with dimensions in inches must be scaled by 25.4. A part 4 by 3 by 2 in is 101.6 by 76.2 by 50.8 mm and fits a printer bed of 220 mm.
Engines and pistons
Bore and stroke
American engines are specified in inches: a bore of 4.000 in, 101.6 mm, and a stroke of 3.48 in, 88.4 mm. The displacement of a cylinder is the bore squared times pi over four times stroke: 0.7854 times 16 times 3.48 is 43.7 cubic inches, which is 716 cc. Eight of them give 5.7 litres, a well-known engine size.
Clearances
Piston-to-wall clearance of 0.002 in is 0.0508 mm. Valve lash is often 0.012 in, 0.305 mm. These small figures are the ones where a conversion mistake destroys a part.
Why exactly 25.4?
A compromise
Before 1959 the US inch was 25.4000508 mm and the British inch 25.399977 mm. By choosing 25.4 as the common value, the countries accepted a difference of about two parts per million from their old standards, invisible in everyday work.
Survey feet
In the US, land surveys continued with the older definition of the foot until 2022. This only matters for very long distances: a mile differs by about 3 mm between the two feet.
Teaching the conversion
A caliper in the classroom
Give students a caliper that reads both units, and have them measure coins, pencils and books. Writing the same reading in inches and millimetres shows the 25.4 ratio directly.
Estimation games
Have students guess a length in inches, convert it by multiplying by 25, and check with a ruler. The error will usually be less than 2 per cent, and the guess trains the eye.
Reading a mixed drawing
Dual dimensions
Some drawings show both units, with the inch value first and the millimetre in brackets: 2.000 (50.8). Always check that the two numbers agree, because a mismatch signals a typo in one of them.
Which to trust
The unit stated in the general notes is the master. If the note says dimensions are in inches, the bracketed numbers are references and may be rounded.
Small numbers: thousandths
The thou
One thousandth of an inch is called a thou in Britain and a mil in the United States, and it is 0.0254 mm, or 25.4 micrometres. A sheet of paper is about 4 thou, 0.1 mm. A human hair is about 3 thou, 0.075 mm.
Tenths
Machinists speak of tenths, meaning a tenth of a thousandth: 0.0001 in, or 2.54 micrometres. To convert in to mm for a part toleranced at 3 tenths is to say 0.00762 mm, about 7.6 micrometres.
A worked job: converting a bracket drawing
From inch drawing to metric shop
Say a bracket is drawn 3.000 in wide, 2.250 in tall and 0.250 in thick, with two holes of 0.375 in on 1.500 in centres. Run each figure through the “convert in to mm” calculation step: 76.2 mm wide, 57.15 mm tall, 6.35 mm thick, with holes of 9.525 mm on 38.1 mm centres. A metric shop would drill 9.5 mm or 9.6 mm, so choose the one that suits the bolt and note the change on the drawing.
Documenting the change
Write the original inch value in brackets on the shop copy so inspectors can compare with the customer's drawing. Rounded values should be marked as such, with the allowed deviation in the notes.
Online orders and metric catalogues
Finding the closest standard
Parts bought online come in standard metric sizes, and the nearest to an inch figure is not always the right one. A 1/2 in rod, 12.7 mm, is closer to 13 mm than 12 mm, but 13 mm is 0.3 mm too thick to enter a bore reamed to inches, while 12 mm leaves a loose 0.7 mm gap. When you convert in to mm for a purchase, compare the clearance both ways before choosing.
Ask for the drawing
If a supplier lists only the nominal size, ask for a dimensioned drawing. A two-line email saves a return shipment and the delay that goes with it.
Cameras, lenses and film
Sensor and film sizes
Photographic film of 35 mm is 1.378 in wide. A medium format film of 120 is 2.44 in, 62 mm. Sensor sizes are quoted in odd fractions of an inch, such as 1/2.3 in, which refers to the tube diameter of old video cameras and is about 7.7 mm along the diagonal, not 11.
Lens threads
Filters are given in millimetres, but older lenses use inches or fractions: a 1 1/8 in thread is 28.6 mm. Adapters connect the two worlds.
Coins and small parts
Coin sizes
A US quarter has a diameter of 0.955 in, 24.26 mm. A penny is 0.750 in, 19.05 mm. A dime is 0.705 in, 17.91 mm, and a nickel 0.835 in, 21.21 mm. A vending machine slot designed for inches rejects a coin that is 0.1 mm out.
Washers, shims and spacers
Shim stock comes in thicknesses such as 0.002, 0.005 and 0.010 in, which are 0.05, 0.13 and 0.25 mm. Stacking them is how mechanics set clearances. Keep a table to avoid mistakes.
Glass, plastics and plywood
Standard thicknesses
Window glass comes in 1/8 in, 3.18 mm, and 3/16 in, 4.76 mm. Acrylic sheets are 1/8, 3/16, 1/4 in: 3.18, 4.76 and 6.35 mm. Plywood sold as 3/4 in is actually 23/32 in, 18.26 mm, close to a metric 18 mm board.
Nominal versus actual
Many American products are named for a size they do not quite have. When you convert in to mm for fit, measure the real item with a caliper and use the measured value, not the label.
Pipes and tubes
Outside diameters
Copper tube of 1/2 in nominal has an outside diameter of 5/8 in, 15.875 mm, and 3/4 in tube has 7/8 in, 22.225 mm. These match the metric 15 and 22 mm fittings only approximately, so push-fit adapters are sold for the transition.
Wall thickness
A wall of 0.045 in is 1.14 mm. Pressure rating depends on it, and a conversion error could mean choosing a thinner tube than needed.
Jewellery and watchmaking
Small distances
Watch case thickness is quoted in millimetres, while old American catalogues gave it in inches: 0.3 in is 7.62 mm. A ring band of 1/8 in is 3.18 mm. For jewellery, the conversion is often rounded to the nearest 0.1 mm.
Bead and wire sizes
Beads of 1/4 in are 6.35 mm. Wire of 0.025 in is 0.635 mm, close to 22 gauge. When you buy from abroad, check the conversion before ordering large quantities.
Measuring tools
Calipers and micrometers
Digital calipers switch between inches and millimetres at the press of a button, but their resolution differs: 0.0005 in is 0.0127 mm, whereas the metric display reads to 0.01 mm. A micrometer with a thimble marked in 0.001 in reads in steps of 0.0254 mm.
Why a calculator still helps
When you must convert in to mm to record a reading in a report that uses the other unit, the conversion should be done once, in a spreadsheet, not by hand each time.
Rounding and tolerances
How many decimals
Keep one more decimal in the converted number than the precision of the source. If the drawing gives 0.250 in, you may write 6.350 mm. For 0.25 in written with two decimals, 6.35 mm is enough.
Avoid cumulative error
Do not convert and round each dimension of a chain and then add them; add them in inches, then convert the total. Rounding each of ten dimensions to the nearest 0.1 mm can leave a total error of 0.5 mm.
Spreadsheet formulas
A column of inches
To convert in to mm for a whole column, put the inches in column A and enter =A2*25.4 in column B. Format with two decimals. For fractions, type =5/16*25.4. For a reverse check, divide by 25.4.
Using named constants
Name a cell INCH_MM and give it 25.4. Formulas such as =A2*INCH_MM read clearly and are easy to audit.
More values for convert in to mm
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 in = 2.54 mm
- 0.5 in = 12.7 mm
- 3 in = 76.2 mm
- 4 in = 101.6 mm
- 7 in = 177.8 mm
- 15 in = 381 mm
- 20 in = 508 mm
- 75 in = 1,905 mm
- 200 in = 5,080 mm
- 500 in = 12,700 mm
- 2,000 in = 50,800 mm
- 10,000 in = 254,000 mm
Mistakes when you convert in to mm
Multiplying by 2.54
2.54 gives centimetres. For millimetres, multiply by 25.4.
Rounding too early
Keep full precision until the final step.
Forgetting the tolerance
Convert tolerances along with sizes, or the part may not fit.
Quick answers
How many millimetres in an inch?
Exactly 25.4.
How do I convert in to mm?
Multiply the inches by 25.4.
What is 1/4 in in millimetres?
6.35 mm.
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 in to dm, Convert in to ft, Convert µm to mm, Convert cm to mm, Convert mm to m, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert mm to in.
In short, to convert in to mm multiply by 25.4, convert tolerances with the sizes, and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.