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Millimetres to Inches
To convert millimetres to inches, divide by 25.4, because an inch is exactly 25.4 mm. 18 mm ÷ 25.4 = 0.709 inches. On a tape that is 23/32 inch (0.26 mm over) or 11/16 inch (0.54 mm under), so 18 mm plywood is not 3/4 inch, which is 19.05 mm.
Type millimetres.
| Shown as | Value |
|---|---|
| Inches, as a decimal | 0.709 in |
| Nearest 1/16 inch | 11/16 in (0.54 mm under) |
| Nearest 1/32 inch | 23/32 in (0.26 mm over) |
| Nearest 1/64 inch | 45/64 in (0.14 mm under) |
| Thousandths of an inch (thou) | 708.7 thou |
| Centimetres | 1.8 cm |
The figures above are an example: put in your own. A decimal comma (0,5) works too. Worked out in this browser and kept there; nothing is sent anywhere.
Type millimetres and read inches the way a tape, rule or drawing gives them: as a decimal, as the nearest 1/16, 1/32 and 1/64 inch with the gap in millimetres, and in thousandths of an inch.
Millimetres to inches is a division by 25.4. The millimetre is a thousandth of a metre and the inch is exactly 25.4 of them, so a millimetre is 5/127 of an inch.
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- Says when it rounds
Key facts
| Detail | Answer |
|---|---|
| Formula | inches = mm ÷ 25.4 |
| Example | 18 mm = 0.709 in ≈ 23/32 in (0.26 mm over) |
| 1 mm | 0.03937 in, 39.37 thou |
| 1/16 inch | 1.5875 mm exactly |
| 1/32 inch | 0.79375 mm exactly |
| Where your numbers are kept | This browser only |
How to convert millimetres to a fraction of an inch
Divide, then find the nearest mark on the scale you are using.
Divide by 25.4
18 mm ÷ 25.4 = 0.7087 in.
Choose your scale
A tape reads 1/16; a steel rule often 1/32.
Multiply by the scale
0.7087 × 32 = 22.68.
Round to a whole number
23, so 23/32 inch.
Check the gap
23/32 in is 18.26 mm: 0.26 mm over. Decide if that is close enough.
Who converts millimetres to inches
Anyone with a metric part and an inch tape, or the other way round.
Carpenters and joiners
Metric sheet goods against inch plans and router bits.
Mechanics
Metric fasteners and inch spanners: which ones swap.
Machinists
Drawings in mm, a micrometer in thou.
3D printing
Filament and nozzle sizes against inch parts.
Jewellers and crafters
Bead and wire sizes in mm, patterns in inches.
Car owners
Tyre widths in mm on inch wheels.
Millimetres on an inch tape: the nearest 1/16 and 1/32
A decimal inch figure is what a calculator gives and nothing a tradesperson can use. A tape measure in the US is marked to 1/16 inch, a steel rule often to 1/32, and machinists’ scales to 1/64. So the useful answer to “what is 18 mm in inches” is not 0.709 but the mark to cut to, and how far that mark is from 18 mm. The converter gives the nearest 1/16, 1/32 and 1/64, each with the gap in millimetres, so you can see when the rounding matters.
| Millimetres | Inches | Nearest 1/16 | Nearest 1/32 |
|---|---|---|---|
| 1 mm | 0.039 | 1/16 (0.59 mm over) | 1/32 (0.21 mm under) |
| 2 mm | 0.079 | 1/16 (0.41 mm under) | 3/32 (0.38 mm over) |
| 3 mm | 0.118 | 1/8 (0.18 mm over) | 1/8 (0.18 mm over) |
| 4 mm | 0.157 | 3/16 (0.76 mm over) | 5/32 (0.03 mm under) |
| 5 mm | 0.197 | 3/16 (0.24 mm under) | 3/16 (0.24 mm under) |
| 6 mm | 0.236 | 1/4 (0.35 mm over) | 1/4 (0.35 mm over) |
| 8 mm | 0.315 | 5/16 (0.06 mm under) | 5/16 (0.06 mm under) |
| 10 mm | 0.394 | 3/8 (0.48 mm under) | 13/32 (0.32 mm over) |
| 12 mm | 0.472 | 1/2 (0.7 mm over) | 15/32 (0.09 mm under) |
| 13 mm | 0.512 | 1/2 (0.3 mm under) | 1/2 (0.3 mm under) |
| 15 mm | 0.591 | 9/16 (0.71 mm under) | 19/32 (0.08 mm over) |
| 16 mm | 0.63 | 5/8 (0.13 mm under) | 5/8 (0.13 mm under) |
| 18 mm | 0.709 | 11/16 (0.54 mm under) | 23/32 (0.26 mm over) |
| 19 mm | 0.748 | 3/4 (0.05 mm over) | 3/4 (0.05 mm over) |
| 20 mm | 0.787 | 13/16 (0.64 mm over) | 25/32 (0.16 mm under) |
| 22 mm | 0.866 | 7/8 (0.23 mm over) | 7/8 (0.23 mm over) |
| 25 mm | 0.984 | 1 (0.4 mm over) | 31/32 (0.39 mm under) |
| 32 mm | 1.26 | 1 1/4 (0.25 mm under) | 1 1/4 (0.25 mm under) |
The gap column is the point. 19 mm is within 0.05 mm of 3/4 inch, so a 19 mm and a 3/4-inch part are the same for any job done with a saw. 18 mm is 0.54 mm short of 11/16 and 1.05 mm short of 3/4, which is enough to leave a visible step in a joint. The distance between two sixteenth marks is 1.5875 mm, so the nearest 1/16 can be up to 0.79 mm out; the nearest 1/32 is never more than 0.4 mm out.
Metric sheets and spanners against inch sizes
Sheet materials are where millimetres and inch fractions most often get swapped, because a metric sheet is sold as the “equivalent” of an inch size it does not match. Plans written in one system and built from materials in the other need the real thickness.
| Metric sheet | In inches | Often sold as | That is | Difference |
|---|---|---|---|---|
| 3 mm | 0.118 | 1/8 in | 3.175 mm | 0.18 mm |
| 6 mm | 0.236 | 1/4 in | 6.35 mm | 0.35 mm |
| 9 mm | 0.354 | 3/8 in | 9.525 mm | 0.53 mm |
| 12 mm | 0.472 | 1/2 in | 12.7 mm | 0.7 mm |
| 15 mm | 0.591 | 5/8 in | 15.875 mm | 0.88 mm |
| 18 mm | 0.709 | 3/4 in | 19.05 mm | 1.05 mm |
| 25 mm | 0.984 | 1 in | 25.4 mm | 0.4 mm |
A dado or groove cut for 3/4-inch plywood leaves 18 mm sheet loose by just over a millimetre, which is why router bits are sold in “undersize” plywood widths. Spanners show the same pattern: a 13 mm spanner on a 1/2-inch head (12.7 mm) slips by 0.3 mm and rounds the corners, while 19 mm and 3/4 inch differ by only 0.05 mm and swap freely.
Tyres, filament and thou
Some sizes mix the two systems in one code, and converting the right half matters. A car tyre marked 205/55 R16 is 205 mm wide, its sidewall is 55% of that width, and it fits a 16-inch wheel. In inches the width is 8.07 in; the sidewall is 112.75 mm, 4.44 in; and the overall diameter is 16 in plus two sidewalls, 24.88 in or 631.9 mm. The 16 is a real inch figure for the rim, so only the millimetre parts need converting.
| Item | Metric size | In inches | Nearest fraction |
|---|---|---|---|
| 3D printer filament | 1.75 mm | 0.069 in | about 1/16 in (0.16 mm under) |
| Pencil lead | 0.5 mm | 0.020 in | about 20 thou |
| Plastic sheeting, 6 mil | 0.1524 mm | 0.006 in | 6 thou: a mil is a thousandth of an inch |
| Smartphone thickness | 7.8 mm | 0.307 in | 5/16 in (0.14 mm over) |
| Car tyre width, 205/55 R16 | 205 mm | 8.071 in | 8 1/16 in (0.21 mm under) |
Thousandths of an inch (thou, or mil in the US) are how machinists, plastics and electronics quote thin material, and they are exact: 1 thou is 0.0254 mm, listed by NIST as an exact factor. Do not confuse a mil with a millimetre: 6 mil sheeting is 0.15 mm, not 6 mm.
What this converter does not do
This converts millimetres to inches. The nearest fraction is the nearest mark, not a tolerance: whether 0.54 mm is close enough depends on the job, and the converter shows the gap so you can decide. It does not choose drill sizes or bolt sizes for you. For centimetres use the cm to inches converter; for the reverse direction, with fractions typed in, the inch to mm converter. To add or subtract inch fractions, use the fraction calculator. To measure something small in millimetres on screen, use the online ruler; its inch twin, the online inch ruler, is marked in sixteenths.
This converter or a decimal-only one
What this page adds to a typical one-formula converter.
| This converter | A typical converter | |
|---|---|---|
| Fractions | 1/16, 1/32 and 1/64 at once | One fraction, or none |
| Gap shown | In mm, over or under | Not shown |
| Thousandths | Thou given | Not given |
| Sheet sizes | 18 mm against 3/4 in, with the gap | Not covered |
| Your entries leave the browser | Never | Varies |
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Frequently asked questions
Is 18mm the same as 3/4 inch?
No. 3/4 inch is exactly 19.05 mm, so 18 mm is 1.05 mm thinner. 18 mm is 0.709 inches, nearest to 23/32 inch. Metric plywood sold as a 3/4-inch replacement fits a 3/4-inch groove loosely, which is why undersize router bits exist. 19 mm, by contrast, is within 0.05 mm of 3/4 inch.
How many inches is 10 mm?
10 mm is 0.3937 inches, which is 25/64 inch to the nearest sixty-fourth, or 13/32 to the nearest thirty-second. It is close to, but not the same as, 3/8 inch (9.525 mm): the difference is 0.475 mm, enough for a 10 mm spanner to slip on a 3/8-inch head.
How do you convert mm to inches as a fraction?
Divide the millimetres by 25.4, multiply by the fraction you want (16, 32 or 64), round to a whole number and put it over that number, then reduce. 6 mm is 0.236 inches; 0.236 × 16 = 3.78, which rounds to 4, so 4/16 or 1/4 inch, 0.35 mm over.
How many mm is 1/16 inch?
1/16 inch is exactly 1.5875 mm, so the marks on a tape measure in sixteenths are about one and a half millimetres apart. Half of it, 1/32 inch, is 0.79375 mm, and 1/64 inch is 0.396875 mm. Because an inch is exactly 25.4 mm, every binary fraction of an inch has an exact millimetre value.
How many inches is 25 mm?
25 mm is 0.984 inches, 1/64 inch short of one inch, or 0.4 mm. One inch is 25.4 mm. For most cutting the two are interchangeable; for a fit, such as a 25 mm pipe in a 1-inch clip, the 0.4 mm can matter.
What is a thou?
A thou is a thousandth of an inch, 0.0254 mm exactly, called a mil in the US. Machinists and plastics makers use it for thin dimensions. 1 mm is 39.37 thou. Do not confuse mil with millimetre: 6 mil plastic sheeting is 0.15 mm thick.
Can I convert mm to inches in my head?
Divide by 25, then take off about 2% (25.4 is 1.6% more than 25): 18 mm is 0.72, less 2% is about 0.706, close to the true 0.709. For fractions, remember that 1/16 inch is about 1.6 mm, so 6 mm is close to 4/16 (1/4 inch) and 12 mm is close to 8/16 (1/2 inch).
Why do tape measures use sixteenths?
Because the inch is divided by halving: halves, quarters, eighths, sixteenths, which are easy to mark and to read by the length of the line. A sixteenth, 1.6 mm, is about as fine as the eye can reliably read on a tape. Finer scales, to 1/32 and 1/64, are found on steel rules.
Related guides
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