To convert millimeters to inches, divide by 25.4. To convert inches to millimeters, multiply by 25.4. One inch equals exactly 25.4 millimeters, so the same factor works in both directions. For example, 10 mm ÷ 25.4 is about 0.394 inches, while 2 inches × 25.4 equals 50.8 mm.
This is useful when a craft pattern lists millimeters but your ruler shows inches, or when a product description uses a different unit from your measuring tool. Keep the original dimension beside the converted result, especially when comparing small objects or deciding whether something will fit.
Why is the conversion factor 25.4?
The factor comes from the definition of the inch, rather than an approximate comparison between two rulers. NIST’s length reference explains that the standard inch is exactly equivalent to 25.4 mm. You can use 25.4 directly without adding extra digits.
The abbreviation mm means millimeters, and in means inches. Because an inch is larger than a millimeter, the number gets smaller when you convert mm to inches. A result of 254 inches for 10 mm is therefore a warning that you multiplied when you needed to divide.
Millimeters to decimal inches and nearby fractions
The decimal column below is rounded to four decimal places. The final column gives the nearest 1/16 inch, reduced to a simpler fraction where possible. These fractions are approximations for reading a ruler, not interchangeable part specifications. Even 25 mm is only near one inch; one inch is 25.4 mm.
| Millimeters | Decimal inches (rounded) | Nearest 1/16 inch (approx.) |
|---|---|---|
| 1 | 0.0394 | 1/16 |
| 2 | 0.0787 | 1/16 |
| 3 | 0.1181 | 1/8 |
| 4 | 0.1575 | 3/16 |
| 5 | 0.1969 | 3/16 |
| 6 | 0.2362 | 1/4 |
| 8 | 0.3150 | 5/16 |
| 10 | 0.3937 | 3/8 |
| 12 | 0.4724 | 1/2 |
| 15 | 0.5906 | 9/16 |
| 20 | 0.7874 | 13/16 |
| 25 | 0.9843 | 1 |
| 25.4 | 1.0000 | 1 |
| 30 | 1.1811 | 1 3/16 |
| 40 | 1.5748 | 1 9/16 |
| 50 | 1.9685 | 1 15/16 |
| 100 | 3.9370 | 3 15/16 |
For example, 6 mm converts to approximately 0.2362 inches. Its nearest sixteenth is 1/4 inch, but 1/4 inch equals 6.35 mm. That leaves a 0.35 mm difference. Whether a difference matters depends on the project’s requirements; the chart alone cannot establish a suitable fit.
How do you calculate a fraction yourself?
First divide the millimeter measurement by 25.4. Then multiply the decimal inches by the denominator your ruler uses, such as 16. Round that result to the nearest whole number and place it over 16. Reduce the fraction and separate any whole inches.
For 20 mm, the calculation is 20 ÷ 25.4 = 0.787401… inches. Multiply by 16 to get 12.5984…, which rounds to 13. The nearest sixteenth is therefore 13/16 inch. For a ruler marked in thirty-seconds, use 32 instead of 16. Label the result approximate rather than replacing your original measurement.
How do you convert inches back to millimeters?
Multiply the inch value by 25.4. A decimal value of 0.75 inches gives 0.75 × 25.4 = 19.05 mm. For a fraction, divide the numerator by the denominator first: 3/8 inch is 0.375 inches, so 3/8 inch equals 9.525 mm.
With a mixed number, add the whole inches to the fractional part before multiplying. For 1 1/2 inches, calculate 1 + 1 ÷ 2 = 1.5, then 1.5 × 25.4 = 38.1 mm. Do not read 1 1/2 as 1.2 inches or multiply only the fractional part.
Everyday examples: ribbon, beads and small openings
Suppose a craft listing describes ribbon as 12 mm wide. Dividing by 25.4 gives approximately 0.4724 inches, which is a little less than half an inch. A ribbon labeled 1/2 inch is 12.7 mm wide. For a decorative bow that comparison may be enough; for a narrow channel, check the actual opening and material.
A bead listed as 8 mm across is approximately 0.3150 inches across. That dimension says nothing about the hole through its center. If you need to thread cord through it, find the hole diameter separately and compare that with the cord diameter, allowing for the seller’s specifications and the material.
For a small box opening measured as 50 mm, the conversion is approximately 1.9685 inches. Calling it “about two inches” may be useful for describing its size, but an exactly two-inch-wide item measures 50.8 mm. Rounding the opening upward does not create that extra clearance.
How much should you round?
Keep the full calculator result while working, then round the final number to the precision your task needs. Four decimal places in a chart make comparisons easier, but they do not mean your ruler measured that accurately. A rough reading remains rough after conversion.
Rounding to the nearest 1/16 inch can introduce up to 1/32 inch of rounding difference, equivalent to 0.79375 mm. That is substantial for some small projects. A finer fraction reduces the rounding step, but it cannot correct an inaccurate starting measurement or a tool that is unsuitable for the job.
Check the measurement before checking the arithmetic
Start at the ruler’s zero mark, which may not be at its physical edge. Keep the object aligned with the scale and read straight down rather than at an angle. For a diameter, measure across the center; for thickness, measure the intended surfaces without squeezing a soft material.
With calipers, follow the tool’s instructions for cleaning the contact faces, checking zero and positioning the jaws. A display with several digits is not proof of accuracy. For fitted components, follow the specified dimensions and tolerances rather than selecting a nearby imperial fraction. This conversion does not establish compatibility between screws, threads, bearings or other precision parts.
What if the starting measurement is in centimeters?
One centimeter equals 10 millimeters. Multiply centimeters by 10 to get millimeters, or divide millimeters by 10 to get centimeters. Thus 3 cm is 30 mm, and 30 ÷ 25.4 is approximately 1.1811 inches. Keep cm and mm separate: confusing them changes the answer by a factor of ten.
For optional measuring supplies, browse metric and imperial rulers on Amazon. Check the smallest marked divisions and measuring range before choosing one. This is a general browse link, not a tested product recommendation.
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