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Inches to Millimetres
To convert inches to millimetres, multiply by 25.4, because an inch is exactly 25.4 mm. 3/8 inch is 0.375 × 25.4 = 9.525 mm, exactly. The nearest whole metric size is 10 mm, 0.475 mm bigger, and the nearest half millimetre is 9.5 mm, 0.025 mm smaller.
Type inches or a fraction.
| Shown as | Value |
|---|---|
| Millimetres | 9.525 mm |
| Centimetres | 0.953 cm |
| Inches, as a decimal | 0.375 in |
| Thousandths of an inch (thou) | 375 thou |
| Nearest whole millimetre | 10 mm (0.475 mm larger) |
| Nearest half millimetre | 9.5 mm (0.025 mm smaller) |
The figures above are an example: put in your own. Type a fraction (3/8), a mixed number (5 3/8 or 5-3/8) or a decimal (0.375). Worked out in this browser and kept there; nothing is sent anywhere.
Type an inch size the way it is written, as 3/8, 5 3/8, 5-3/8 or 0.375, and read it in millimetres, exactly, with thousandths of an inch and the nearest whole and half millimetre for choosing a metric part.
Inches to millimetres is a multiplication by 25.4. Since the 1959 international agreement the inch has been exactly 25.4 mm, so fractions of an inch convert without rounding.
- No sign-up
- Nothing uploaded
- Exact factors
- Works offline
- Says when it rounds
Key facts
| Detail | Answer |
|---|---|
| Formula | mm = inches × 25.4 |
| Example | 3/8 in = 9.525 mm exactly |
| 1 inch | 25.4 mm exactly |
| 1/64 inch | 0.396875 mm exactly |
| Nearest metric to 3/8 in | 10 mm (0.475 mm larger), 9.5 mm (0.025 mm smaller) |
| Where your numbers are kept | This browser only |
How to convert a fraction of an inch to millimetres
Turn the fraction into a decimal, then multiply.
Write the fraction as a decimal
Divide top by bottom: 5/16 = 0.3125.
Add any whole inches
2 5/16 inches is 2.3125.
Multiply by 25.4
2.3125 × 25.4 = 58.7375 mm.
Round only if you need to
To a tenth: 58.7 mm.
Pick a metric size
The converter shows the nearest whole and half millimetre.
Who converts inches to millimetres
People with an inch size and a metric part, tool or drawing.
Mechanics
Imperial bolts and metric sockets: which will fit.
Woodworkers
An American plan cut with a metric rule.
Engineers and machinists
Drawings in inches built in a metric shop.
Plumbers
Inch pipe and hose sizes against metric fittings.
Typesetters
Points, picas and inch margins in millimetres.
Hobbyists
Model and 3D-print parts sized in fractions.
Fractional inches to mm: an exact chart to 1/64
Every fraction of an inch with 2, 4, 8, 16, 32 or 64 underneath has an exact millimetre value, because the inch is exactly 25.4 mm and 25.4 divides by two as far as you like without the decimal going on forever. So a fraction chart can be exact, and this one is: nothing below is rounded except the last column, which gives the nearest whole millimetre for choosing a metric part.
| Inches | Decimal inch | Millimetres | Nearest whole mm |
|---|---|---|---|
| 1/64 in | 0.015625 | 0.396875 | 0 mm |
| 1/32 in | 0.03125 | 0.79375 | 1 mm |
| 3/64 in | 0.046875 | 1.190625 | 1 mm |
| 1/16 in | 0.0625 | 1.5875 | 2 mm |
| 3/32 in | 0.09375 | 2.38125 | 2 mm |
| 1/8 in | 0.125 | 3.175 | 3 mm |
| 5/32 in | 0.15625 | 3.96875 | 4 mm |
| 3/16 in | 0.1875 | 4.7625 | 5 mm |
| 7/32 in | 0.21875 | 5.55625 | 6 mm |
| 1/4 in | 0.25 | 6.35 | 6 mm |
| 5/16 in | 0.3125 | 7.9375 | 8 mm |
| 3/8 in | 0.375 | 9.525 | 10 mm |
| 7/16 in | 0.4375 | 11.1125 | 11 mm |
| 1/2 in | 0.5 | 12.7 | 13 mm |
| 9/16 in | 0.5625 | 14.2875 | 14 mm |
| 5/8 in | 0.625 | 15.875 | 16 mm |
| 11/16 in | 0.6875 | 17.4625 | 17 mm |
| 3/4 in | 0.75 | 19.05 | 19 mm |
| 13/16 in | 0.8125 | 20.6375 | 21 mm |
| 7/8 in | 0.875 | 22.225 | 22 mm |
| 15/16 in | 0.9375 | 23.8125 | 24 mm |
| 1 in | 1 | 25.4 | 25 mm |
Charts elsewhere usually round to two decimals (3/8 inch as 9.53 mm) without saying so. Two decimals is fine for a saw; for a bearing or a press fit, the third decimal is real, and 9.525 is the number to work to.
Which metric size replaces an inch size
The question behind most inch-to-millimetre lookups is practical: I have an inch size, which metric part will do? The converter’s last two rows give the nearest whole and half millimetre and how far off each is. Whether that is close enough depends on what the part does.
| Inch size | Exact mm | Nearest metric | Off by | Interchangeable? |
|---|---|---|---|---|
| 1/4 in | 6.35 | 6 mm | 0.35 mm | For a drilled clearance hole, yes; for a socket, no |
| 5/16 in | 7.9375 | 8 mm | 0.06 mm | Usually, even for spanners |
| 3/8 in | 9.525 | 10 mm | 0.475 mm | No for spanners; a 10 mm rounds a 3/8 head |
| 1/2 in | 12.7 | 13 mm | 0.3 mm | Rarely for spanners; yes for clearance |
| 3/4 in | 19.05 | 19 mm | 0.05 mm | Yes, spanners and sheet alike |
| 1 in | 25.4 | 25 mm | 0.4 mm | For pipe clips and tube, check the fit |
As a rule of thumb, a spanner or socket wants a gap under about 0.1 mm, a clearance hole tolerates a few tenths, and anything that slides or seals should be bought in the system it was designed in.
Bolts, jacks, hoses and points
Fractional inches survive in the trades and in a few products everyone meets. These are the places the conversion to millimetres comes up, with the exact figure.
| Item | Inch size | In mm |
|---|---|---|
| Standard bolt (UNC 1/4-20) | 1/4 in diameter, 20 threads per inch | 6.35 mm; pitch 1.27 mm |
| Headphone jack | 1/4 in (6.35 mm) and the 3.5 mm mini jack | 3.5 mm is 0.138 in, about 9/64 |
| Garden hose (US) | 5/8 in bore | 15.875 mm |
| Drill bit set, fractional | 1/16 to 1/2 in by 64ths | 1.588 to 12.7 mm |
| Typographic point | 1/72 in | 0.3528 mm (0.352777…) |
| Page margin in a manuscript | 1 in | 25.4 mm |
The typographic point is the odd one out: 1/72 inch does not have an exact millimetre value with a finite decimal, because 72 has a factor of 3. To check a small length in inches on screen, the online inch ruler reads it to 1/16. For page measures in both units, see the KDP margin calculator and the standard manuscript format guide.
What this converter does not do
This converts inches to millimetres. The nearest-size rows show a difference, not a tolerance; it does not choose fasteners, drill bits or threads for you. Thread pitches, wire gauges (AWG, SWG) and sheet metal gauges are separate scales that are not inches and are not converted here. Fractions must have a non-zero number underneath; negative lengths are refused.
This converter or a decimal-only one
What this page adds to a typical one-formula converter.
| This converter | A typical converter | |
|---|---|---|
| Fractions typed as written | 3/8, 5 3/8 and 5-3/8 | Decimal box, or a fraction list |
| Exact millimetres | 9.525, not 9.53 | Often rounded silently |
| Nearest metric size | Whole and half mm, with the gap | Not given |
| Thousandths | Thou given | Not given |
| Your entries leave the browser | Never | Varies |
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Frequently asked questions
How many mm is 3/8 inch?
3/8 inch is exactly 9.525 mm. The nearest whole metric size is 10 mm, which is 0.475 mm larger, so a 10 mm spanner is loose on a 3/8-inch head and can round it. The nearest half millimetre, 9.5 mm, is only 0.025 mm smaller.
How do you convert inches to mm?
Multiply by 25.4. For a fraction, divide the top by the bottom first: 5/8 is 0.625, and 0.625 × 25.4 = 15.875 mm. The converter takes the fraction directly, including the tape style 5-3/8, and gives the exact result.
How many mm is 1/4 inch?
1/4 inch is exactly 6.35 mm. A 6 mm part is 0.35 mm smaller, close enough for a drilled clearance hole but not for a socket or a press fit. A 1/4-inch UNC bolt has 20 threads per inch, which is a pitch of 1.27 mm.
How many mm is 1/2 inch?
1/2 inch is exactly 12.7 mm. The nearest whole metric size, 13 mm, is 0.3 mm larger, which is why a 13 mm spanner on a 1/2-inch nut is loose. 12 mm plywood is sold as a substitute for 1/2-inch and is 0.7 mm thinner.
How many mm is 5/16 inch?
5/16 inch is exactly 7.9375 mm, so 8 mm is only 0.0625 mm larger, one of the closest inch and metric pairs. An 8 mm drill makes a hole that fits a 5/16-inch pin, and the two sizes are commonly swapped for spanners too.
Is every fraction of an inch an exact number of mm?
Every fraction with 2, 4, 8, 16, 32 or 64 underneath is, because 25.4 divided by any power of two ends: 1/64 inch is 0.396875 mm. A fraction with 3 underneath is not: 1/3 inch is 8.4666… mm, and the typographic point, 1/72 inch, is 0.352777… mm.
What is SAE to metric?
SAE sizes are the inch fractions used for US tools and fasteners, named after the Society of Automotive Engineers. Converting SAE to metric is inches to millimetres: multiply by 25.4. The useful extra is the nearest metric size and the difference, which this converter shows in its last two rows.
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