Converting inches to millimeters is simple once you know the conversion factor. One inch equals exactly 25.4 millimeters, so you can convert any measurement by multiplying the number of inches by 25.4. This works for whole numbers, decimals, and fractional-inch measurements. NIST
Quick answer: To convert inch to mm, multiply the measurement in inches by 25.4. For example, 2 inches equals 50.8 mm, 5 inches equals 127 mm, and 10 inches equals 254 mm. The 25.4 conversion factor is exact, rather than an approximate measurement. NIST
How to Convert inch to mm
The basic conversion formula is:
Millimeters = Inches × 25.4
Or:
mm = in × 25.4
The process only requires three steps:
- Take your measurement in inches.
- Multiply it by 25.4.
- Express the result in millimeters (mm).
For example, suppose a component is 6 inches long:
6 × 25.4 = 152.4 mm
Therefore:
6 inches = 152.4 mm
The same rule works regardless of whether the original measurement is a whole number, decimal, or fraction.
Why Is 1 Inch Equal to 25.4 mm?
The relationship is not an estimate. The international inch is defined as exactly 25.4 millimeters. NIST explains that English-speaking nations agreed to unify their length standards in 1958, with the new conversion factors announced in 1959. The standard inch became exactly equivalent to 25.4 mm. NIST
This exact definition matters in fields where measurements need to move reliably between customary and metric systems, including engineering, manufacturing, hardware specifications, CAD drawings, and machining.
inch to mm Conversion Chart
Here is a quick reference table for common inch measurements. The values follow the exact 25.4 conversion factor. Metric Conversions
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.01 in | 0.254 mm |
| 0.1 in | 2.54 mm |
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 30 in | 762 mm |
| 36 in | 914.4 mm |
| 40 in | 1,016 mm |
| 50 in | 1,270 mm |
| 60 in | 1,524 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
A few conversions are especially useful to remember:
1 inch = 25.4 mm
2 inches = 50.8 mm
5 inches = 127 mm
10 inches = 254 mm
12 inches = 304.8 mm
Once you remember 25.4, you can calculate virtually any inch measurement without needing a conversion chart.
Converting Decimal Inches to Millimeters
Decimal inches are converted in exactly the same way as whole inches.
Multiply the decimal value by 25.4.
Example: 2.5 Inches to mm
2.5 × 25.4 = 63.5
So:
2.5 inches = 63.5 mm
Example: 3.75 Inches to mm
3.75 × 25.4 = 95.25
Therefore:
3.75 inches = 95.25 mm
Example: 0.1 Inch to mm
0.1 × 25.4 = 2.54
Therefore:
0.1 inch = 2.54 mm
This method is particularly useful when working with product dimensions, digital calipers, technical drawings, or equipment specifications expressed as decimal inches.
How to Convert Fractional inch to mm
Fractional inches appear frequently on rulers, tape measures, woodworking plans, fasteners, and older technical drawings.
The easiest method is to convert the fraction to a decimal first and then multiply by 25.4. MetricCalc
For example:
1/4 inch = 0.25 inch
Then:
0.25 × 25.4 = 6.35 mm
So:
1/4 inch = 6.35 mm
Common Inch Fractions in Millimeters
| Fraction | Decimal Inch | Millimeters |
|---|---|---|
| 1/16 in | 0.0625 in | 1.5875 mm |
| 1/8 in | 0.125 in | 3.175 mm |
| 3/16 in | 0.1875 in | 4.7625 mm |
| 1/4 in | 0.25 in | 6.35 mm |
| 5/16 in | 0.3125 in | 7.9375 mm |
| 3/8 in | 0.375 in | 9.525 mm |
| 1/2 in | 0.5 in | 12.7 mm |
| 5/8 in | 0.625 in | 15.875 mm |
| 3/4 in | 0.75 in | 19.05 mm |
| 7/8 in | 0.875 in | 22.225 mm |
| 1 in | 1.0 in | 25.4 mm |
Knowing a few of these values can save time when switching between imperial and metric tools.
How Many Millimeters Are in an Inch?
There are exactly 25.4 millimeters in one inch. NIST
That gives the basic relationship:
1 in = 25.4 mm
Because this value is exact, calculations such as:
2 × 25.4 = 50.8 mm
and
10 × 25.4 = 254 mm
do not contain conversion-factor uncertainty. Any practical rounding usually comes from how many decimal places you choose to display or from uncertainty in the original physical measurement.
What Are Inches and Millimeters?
Understanding the units themselves makes the conversion easier to remember.
Inch
The inch, abbreviated in, is a unit of length. There are 12 inches in one foot, and the international inch is defined as exactly 25.4 millimeters. ThousandTools
Inches commonly appear in measurements such as:
- Screen sizes
- Furniture dimensions
- Hardware
- Pipes
- Lumber
- Tools
- Product dimensions
- Construction measurements
Millimeter
The millimeter, abbreviated mm, belongs to the metric system. One millimeter is one-thousandth of a meter. Inch Calculator
In other words:
1 mm = 0.001 meter
Millimeters are useful for relatively small measurements and applications requiring finer dimensional detail, including machine parts, fasteners, technical drawings, electronics, tools, and engineering components.
Worked inch to mm Examples
A few examples make the calculation easy to understand.
What Is 1 Inch in mm?
Apply the formula:
1 × 25.4 = 25.4
Therefore:
1 inch = 25.4 mm
What Is 2 Inches in mm?
2 × 25.4 = 50.8
Therefore:
2 inches = 50.8 mm
What Is 5 Inches in mm?
5 × 25.4 = 127
Therefore:
5 inches = 127 mm
What Is 10 Inches in mm?
10 × 25.4 = 254
Therefore:
10 inches = 254 mm
What Is 12 Inches in mm?
12 × 25.4 = 304.8
Therefore:
12 inches = 304.8 mm
Because 12 inches make one foot, this also means:
1 foot = 304.8 mm
What Is 25 Inches in mm?
25 × 25.4 = 635
Therefore:
25 inches = 635 mm
What Is 50 Inches in mm?
50 × 25.4 = 1,270
Therefore:
50 inches = 1,270 mm
What Is 100 Inches in mm?
100 × 25.4 = 2,540
Therefore:
100 inches = 2,540 mm
How to Convert mm Back to Inches
Sometimes you need to reverse the calculation.
Instead of multiplying by 25.4, divide the millimeter measurement by 25.4:
Inches = Millimeters ÷ 25.4
For example, convert 254 mm to inches:
254 ÷ 25.4 = 10
Therefore:
254 mm = 10 inches
One millimeter is approximately:
1 mm ≈ 0.0393701 in Inch Calculator
You can therefore use either:
in = mm ÷ 25.4
or approximately:
in = mm × 0.0393701
Dividing by 25.4 is generally the clearer method because it directly uses the exact inch-to-millimeter relationship.
When Should You Round an inch to mm Conversion?
The conversion factor itself is exact, but your final answer may need rounding.
Suppose you convert 5/16 inch:
5 ÷ 16 = 0.3125 inch
Then:
0.3125 × 25.4 = 7.9375 mm
Depending on the application, you might write this as:
7.9375 mm — higher displayed precision
7.94 mm — rounded to two decimal places
7.9 mm — rounded to one decimal place
8 mm — rounded to the nearest whole millimeter
Which one is appropriate depends on the original measurement and what you are doing with it.
For a casual household measurement, the nearest millimeter may be enough. For machining, engineering, or manufacturing, rounding too early can introduce an unacceptable dimensional difference.
A useful rule is to keep the unrounded value during calculations and round only the final result.
Exact Conversion vs. Measurement Accuracy
There is an important distinction between an exact unit conversion and an exact physical measurement.
The relationship:
1 inch = 25.4 mm
is exact.
But imagine measuring an object with a basic ruler and recording its width as 2.3 inches. Converting that value gives:
2.3 × 25.4 = 58.42 mm
The mathematical conversion is correct, but that does not mean the object was physically measured to an accuracy of 0.01 mm.
Your final precision should reflect the accuracy of the original measurement and the measuring instrument.
This distinction becomes especially important with calipers, machining tolerances, manufacturing specifications, and engineering drawings.
Where Inch-to-Millimeter Conversions Are Used
Switching between inches and millimeters is common whenever US customary or imperial-style dimensions need to work with metric specifications.
Engineering and Manufacturing
Engineering drawings, CAD models, CNC setups, and machine components may use either inches or millimeters. Converting correctly helps ensure that components match the intended dimensions. MetricCalc
Bolts and Fasteners
Hardware is frequently described using either fractional-inch or metric dimensions.
For example:
1/4 inch = 6.35 mm
However, a converted measurement does not automatically mean a metric fastener is an exact functional replacement for an imperial one. Thread pitch, nominal sizing, tolerance, and standards also matter.
Woodworking
Woodworking plans commonly use fractions such as 1/8, 1/4, 1/2, and 3/4 inch.
Converting these measurements into millimeters is useful when your rulers, saw settings, drill bits, or other tools use metric markings.
3D Printing and CAD
Many 3D modeling and slicing applications use millimeters as their default dimensional unit. A design created from dimensions listed in inches may therefore require conversion before modeling or printing.
For example, an object measuring 4 × 2 inches becomes:
101.6 × 50.8 mm
Product Dimensions
International product listings may mix metric and customary measurements. Converting dimensions can make it easier to compare furniture, electronics, components, packaging, and other products.
Screen Sizes
Televisions, monitors, laptops, and smartphone displays are commonly marketed using diagonal inches.
For example:
24 inches = 609.6 mm
Remember, however, that a 24-inch display specification normally describes its diagonal, not its width or height.
Common Mistakes When Converting Inches to Millimeters
The arithmetic is straightforward, but several mistakes occur regularly.
Dividing Instead of Multiplying
For inch to mm, you multiply:
mm = inches × 25.4
Division is used for the reverse conversion from millimeters to inches.
Using 2.54 Instead of 25.4
One inch equals:
2.54 centimeters
but:
25.4 millimeters
Mixing centimeters and millimeters creates a tenfold error.
Rounding Too Early
If several calculations are involved, keep the full value until the end. Early rounding can compound small errors.
Treating Nominal Sizes as Exact Measurements
Some products are described using nominal dimensions rather than their exact physical size. Converting the nominal number does not necessarily reveal the item’s actual measured dimensions.
Ignoring Fractions
Do not multiply only the numerator of a fractional measurement.
For 3/8 inch, first calculate:
3 ÷ 8 = 0.375
Then:
0.375 × 25.4 = 9.525 mm
So:
3/8 inch = 9.525 mm
A Simple Mental Conversion Method
For an exact answer, multiplying by 25.4 is best.
For a quick estimate without a calculator, multiply the inch measurement by 25 and then add another 0.4 times the measurement.
For 8 inches:
8 × 25 = 200
8 × 0.4 = 3.2
Add them:
200 + 3.2 = 203.2 mm
This gives the exact result because:
25 + 0.4 = 25.4
For rough estimation only, you can think of one inch as approximately 25 mm, but that shortcut should not replace the exact factor when precision matters.
Is 25.4 mm Exactly 1 Inch?
Yes. 25.4 mm is exactly one international inch, not merely a rounded approximation. NIST states that the standard inch has been exactly equivalent to 25.4 mm since the international standardization that took effect in 1959. NIST
This is why reliable conversion tools use:
1 in = 25.4 mm
rather than an approximate factor.
Final inch to mm Reference
The key relationship to remember is:
1 inch = exactly 25.4 millimeters
To convert inch to mm:
mm = inches × 25.4
To reverse the conversion:
inches = mm ÷ 25.4
For common values, 0.5 inch equals 12.7 mm, 1 inch equals 25.4 mm, 5 inches equals 127 mm, 10 inches equals 254 mm, and 12 inches equals 304.8 mm. Using the exact 25.4 factor gives a reliable conversion for whole, decimal, and fractional-inch measurements.
