Millimeters to Inches Converter

To convert mm to inches, divide by 25.4. One inch is exactly 25.4 millimeters, so 10 mm is 0.3937 in, 25 mm is 0.9843 in and 100 mm is 3.937 in. The converter above works in both directions. It gives the decimal result, the nearest 1/16, 1/32 and 1/64 inch with the rounding error in millimeters, and a ruler view so you can see where the length falls. For inches to mm, you can type fractions such as 5/8 or 1 1/8.

Below the converter you will find a full mm to inches chart for 1 to 100 mm, a chart of common larger sizes up to 1,000 mm, a table of small decimal sizes, and a reverse chart that turns every 64th of an inch into millimeters.

How to convert millimeters to inches

The formula has one step: inches = millimeters ÷ 25.4. For the reverse, millimeters = inches × 25.4. NIST Special Publication 811 lists the inch as 2.54 × 10⁻² meter and prints the factor in bold, which marks it as exact. NIST Handbook 44 states it the same way: 1 inch = 2.54 centimeters, exactly. Because the factor is exact, any small difference between two converters comes only from how many decimals they show, not from the conversion itself.

1. Take the length in millimeters, for example 74 mm.
2. Divide by 25.4: 74 ÷ 25.4 = 2.91339 in.
3. Round only at the end. For a drawing, two or three decimals are usual: 2.913 in.
4. For a tape measure, turn the decimal part into sixteenths: 0.913 × 16 = 14.6, so round to 15/16 and the result is 2 15/16″.

If you start from centimeters, multiply by 10 first. Also, 1 mm is 0.1 cm, so 55 mm is 5.5 cm and 2.1654 in.

Why 25.4 mm is exact

The inch is not measured against a physical bar any more; it is defined from the meter. A Federal Register notice from the National Bureau of Standards (now NIST), effective July 1, 1959, set 1 yard = 0.9144 meter exactly and noted that the inch derived from it is exactly 25.4 millimeters. Dividing 0.9144 m by 36 gives 0.0254 m. The notice also records that 25.4 mm had already been approved in 1933 for industrial inch–millimeter conversion. Before 1959 the US yard was 3600/3937 meter, which made the old inch about 2 parts per million longer. The difference is far too small to see on a ruler, but it is why the notice kept a separate US survey foot for existing geodetic survey data.

Reading the result as an inch fraction

Imperial tapes and rules are divided in halves of halves: 1/2, 1/4, 1/8, 1/16, and on finer rules 1/32 and 1/64. A millimeter value rarely lands exactly on one of these marks, so the converter rounds to the nearest mark and reports how far off that mark is. A positive error means the fraction is longer than your part; a negative one means it is shorter.

• 1/16 inch (about 1.6 mm per step): woodworking, framing, flooring and general DIY. The worst rounding error is half a step, 0.79 mm.
• 1/32 inch (about 0.8 mm per step): cabinetry, trim and sheet goods, where a 1 mm gap is visible. Worst error 0.40 mm.
• 1/64 inch (about 0.4 mm per step): machining, drill bits and hardware. Worst error 0.20 mm. For tighter work, use the decimal inch value and calipers.

Example: 190 mm is 7.4803 in. Rounded to 1/16 it becomes 7 1/2″, which is 0.50 mm longer than the real part. Rounded to 1/64 it is 7 31/64″, off by only 0.10 mm. Choose the precision your tool can actually read; reporting 1/64 from a tape marked in sixteenths adds false precision.

Reverse: inches to mm and fractions to mm

To convert an inch fraction to millimeters, turn the fraction into a decimal and multiply by 25.4. For 5/8 inch: 5 ÷ 8 = 0.625, and 0.625 × 25.4 = 15.875 mm. For a mixed number such as 1 1/8, add the whole inches first: 1.125 × 25.4 = 28.575 mm. Every 64th of an inch is a finite decimal, so the millimeter values in the chart below are exact rather than rounded; the 1/64 step itself is 0.396875 mm.

A useful pattern: common fractions rarely equal a whole number of millimeters. 1/4 in is 6.35 mm, 3/8 in is 9.525 mm and 1/2 in is 12.7 mm. That is why metric and inch wrenches, bits and sockets are not interchangeable even when they look close; a 13 mm socket on a 1/2 inch nut has 0.3 mm of play.

Quick mental math (approximate)

For a rough check without a calculator, use 1 inch ≈ 25 mm or 1 mm ≈ 0.04 in. Both are approximations. Dividing by 25 instead of 25.4 makes the answer 1.6% too large: 250 mm comes out as 10 in instead of 9.843 in. That is fine for guessing whether a box fits in a drawer, but not for ordering parts. Another trick is to remember that 100 mm is almost exactly 4 inches (3.937 in), so 300 mm is just under 12 inches, or about a foot.

Choosing between a metric and an inch measurement

If a product is specified in millimeters, the millimeter number is the real dimension and the inch figure is a conversion. Keep the metric value on your notes and treat the fraction as a reading aid. A part listed as 67 mm, for instance, is 67 mm; calling it 2 5/8 in (66.675 mm) is close, but a third of a millimeter short, which can matter for a thread or a press fit. The reverse holds for inch-designed hardware: a 5/8 inch bolt is 15.875 mm, and a 16 mm hole may be loose. When you have to compare two parts made to different systems, convert both to millimeters with at least two decimals and subtract. That difference, not the rounded fractions, tells you whether they fit. For a quick visual check, the ruler view in the converter lines up the inch sixteenths against the millimeter scale for the inch you are in, so you can see how many millimeter lines fall between each fraction mark.

Common mistakes

• Rounding before converting. Convert the full number, then round. Rounding 25.4 to 25 or the result to one decimal can add a millimeter of error on longer parts.
• Mixing up mm and cm. 35 cm is 350 mm, not 35 mm. When a label reads 350 mm, the length is 13.78 in.
• Reading a fraction as exact. 7 1/2 in is 190.5 mm, not 190 mm. Check the rounding error before cutting.
• Confusing mm with mM. Lowercase mm is millimeter, a length. Capital M in mM means millimolar, a concentration; see the molarity unit converter.
• Dropping the feet. From 304.8 mm (12 in) up, the converter also shows feet and inches, which is how US tape measures and plans are read.

mm to inches chart: 1 to 100 mm

Each row gives the exact decimal inches rounded to four places, then the nearest 1/16 and 1/64 inch marks.

Common sizes from 105 to 1,000 mm in inches

Steps of 5 mm to 300 mm, then 25 mm to 1,000 mm, plus frequently searched sizes such as 135 mm, 228 mm, 255 mm, 350 mm and 800 mm.

Small and decimal mm sizes in inches

Sheet thickness, wire, nozzle and gap sizes often use tenths of a millimeter, such as 0.2 mm, 1.3 mm, 1.5 mm or 5.5 mm.

Inch fractions to mm chart (every 1/64 inch)

Exact millimeter values for every 64th of an inch up to 1 inch. Add 25.4 mm for each whole inch.

Common inch sizes in mm

For other everyday conversions, see mL to ounces, Celsius to Fahrenheit and the mixed fraction calculator for adding inch fractions.

Frequently asked questions

Sources & method

• NIST SP 811, Appendix B.8 — inch to meter factor 2.54 E-02 (bold = exact)
• NIST Handbook 44 (2026), Appendix C — 1 inch = 2.54 centimeters (exactly)
• Federal Register Notice 59-5442 (June 30, 1959) — Refinement of values for the yard and the pound; inch = 25.4 mm exactly
• NIST — SI Units: Length (meter and metric prefixes)

Results are estimates for general information. Found an error? It helps everyone — see our methodology.