Need to convert inches into millimeters but aren’t sure which formula to use? The process is straightforward, and you can calculate the result accurately without complicated mathematics.
To convert in to mm, multiply the measurement in inches by 25.4. One inch equals exactly 25.4 millimeters under the international inch definition. For example, 2 inches equals 50.8 mm, 5 inches equals 127 mm, and 10 inches equals 254 mm. The same formula works for whole numbers, decimals, and fractional inches.
in to mm Conversion Formula
The standard formula for converting inches to millimeters is:
\[ \text{Millimeters}=\text{Inches}\times25.4 \]
This conversion factor is exact, not an approximation.
An inch is a unit of length used in the United States customary measurement system and the British imperial system. A millimeter is a metric unit equal to one-thousandth of a meter.
Because the metric system uses powers of ten, millimeters are particularly convenient for measuring small objects, mechanical components, and precise dimensions.
How many millimeters are in one inch?
EXACT CONVERSION
1 inch = 25.4 mm
1 in = 2.54 cm = 0.0254 m
The international inch was standardized at exactly 25.4 millimeters in 1959 through an agreement among several English-speaking countries.
This exact relationship is used in engineering, manufacturing, construction, product specifications, and technical drawings.
Inches to Millimeters Converter
Use this interactive converter to calculate a measurement in millimeters instantly.
Enter a measurement in inches
in
25.4 mm
Formula: inches × 25.4 Copy result
0.5 in1 in2 in5 in10 in
How to convert inches to mm step by step
- Write down the measurement in inches.
- Multiply the measurement by 25.4.
- Express the answer in millimeters.
- Round the result only if your application allows it.
For example, convert 7.5 inches to millimeters:
\[ 7.5\times25.4=190.5\text{ mm} \]
Therefore, 7.5 inches equals exactly 190.5 millimeters.
in to mm Conversion Chart
The following table provides common inch-to-millimeter conversions for quick reference.
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.1 | 2.54 |
| 0.25 | 6.35 |
| 0.5 | 12.7 |
| 0.75 | 19.05 |
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 7 | 177.8 |
| 8 | 203.2 |
| 9 | 228.6 |
| 10 | 254 |
| 12 | 304.8 |
| 15 | 381 |
| 20 | 508 |
| 24 | 609.6 |
| 36 | 914.4 |
| 48 | 1,219.2 |
| 60 | 1,524 |
| 72 | 1,828.8 |
| 100 | 2,540 |
All values are calculated using the exact conversion factor of 25.4.
Common inch conversions
Some frequently searched measurements include:
- 1 inch to mm: 25.4 mm
- 2 inches to mm: 50.8 mm
- 3 inches to mm: 76.2 mm
- 4 inches to mm: 101.6 mm
- 6 inches to mm: 152.4 mm
- 8 inches to mm: 203.2 mm
- 10 inches to mm: 254 mm
- 12 inches to mm: 304.8 mm
These conversions are useful when reading product dimensions, measuring materials, or switching between imperial and metric specifications.
How to Convert Fractional Inches to Millimeters
In woodworking, construction, machining, and hardware measurements, inches are frequently expressed as fractions.
Examples include 1/2 inch, 1/4 inch, 3/8 inch, and 5/16 inch.
To convert a fractional inch measurement into millimeters, first divide the numerator by the denominator. Then multiply the decimal result by 25.4.
For example, converting 3/8 inch:
\[ \frac{3}{8}=0.375 \]
\[ 0.375\times25.4=9.525\text{ mm} \]
Therefore, 3/8 inch equals exactly 9.525 mm.
Fractional inches to mm chart
| Fraction (in) | Decimal (in) | Millimeters |
|---|---|---|
| 1/64 | 0.015625 | 0.396875 |
| 1/32 | 0.03125 | 0.79375 |
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.7625 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.9375 |
| 3/8 | 0.375 | 9.525 |
| 7/16 | 0.4375 | 11.1125 |
| 1/2 | 0.5 | 12.7 |
| 9/16 | 0.5625 | 14.2875 |
| 5/8 | 0.625 | 15.875 |
| 11/16 | 0.6875 | 17.4625 |
| 3/4 | 0.75 | 19.05 |
| 13/16 | 0.8125 | 20.6375 |
| 7/8 | 0.875 | 22.225 |
| 15/16 | 0.9375 | 23.8125 |
Converting mixed fractions
A mixed fraction combines a whole number and a fraction, such as 2 1/2 inches.
First, convert the fraction into a decimal:
\[ 2+\frac12=2.5 \]
Then multiply by 25.4:
\[ 2.5\times25.4=63.5\text{ mm} \]
This approach also works for measurements such as 1 3/4 inches, 3 5/8 inches, and 4 7/16 inches.
How to Convert mm Back to Inches
Sometimes you need to reverse the calculation, especially when a technical drawing uses millimeters but your measuring tape displays inches.
To convert millimeters to inches, divide by 25.4:
\[ \text{Inches}=\frac{\text{Millimeters}}{25.4} \]
For example, convert 100 mm to inches:
\[ 100\div25.4\approx3.937\text{ in} \]
So 100 millimeters is approximately 3.94 inches when rounded to two decimal places.
| Millimeters | Inches (approx.) |
|---|---|
| 1 | 0.03937 |
| 5 | 0.19685 |
| 10 | 0.39370 |
| 20 | 0.78740 |
| 25 | 0.98425 |
| 50 | 1.96850 |
| 100 | 3.93701 |
| 200 | 7.87402 |
| 500 | 19.68504 |
| 1,000 | 39.37008 |
Remember that the reverse conversion requires division, not multiplication.
Inches vs. Millimeters: Understanding the Difference
Although inches and millimeters both measure length, they belong to different measurement systems.
| Feature | Inch (in) | Millimeter (mm) |
|---|---|---|
| Measurement system | Imperial / US customary | Metric / SI |
| Unit symbol | in or ″ | mm |
| Equivalent in mm | 25.4 mm | 1 mm |
| Equivalent in cm | 2.54 cm | 0.1 cm |
| Typical applications | Screens, furniture, construction | Engineering, machining, science |
| Common subdivisions | Fractions and decimals | Decimal metric units |
Why are millimeters used for precise measurements?
Millimeters allow small dimensions to be expressed conveniently without relying on fractions.
For example, a component measuring 0.125 inches can be described as 3.175 mm.
Metric measurements are also easy to scale:
- 10 millimeters = 1 centimeter
- 100 millimeters = 10 centimeters
- 1,000 millimeters = 1 meter
However, millimeters are not inherently more accurate than inches. Measurement accuracy depends on the instrument, calibration, technique, and tolerance.
Both systems can express highly precise measurements.
Practical Applications of in to mm Conversion
Inch-to-millimeter conversion is useful whenever measurements from different systems need to be compared.
Engineering and manufacturing
Mechanical engineers often work with dimensions expressed in both inches and millimeters.
For example, a component specified as 0.5 inches thick has a nominal thickness of 12.7 mm.
This conversion is useful for interpreting CAD drawings, measuring manufactured parts, and comparing technical specifications.
However, converting a dimension does not automatically make an imperial part interchangeable with a metric part. Thread pitch, manufacturing tolerances, and nominal sizing must also be considered.
Construction and woodworking
Builders and carpenters frequently encounter fractional inch measurements on rulers, tape measures, and building plans.
A wooden board measuring 3/4 inch thick converts to:
\[ 0.75\times25.4=19.05\text{ mm} \]
This is particularly useful when working with metric tools or international material specifications.
Keep in mind that nominal lumber dimensions may differ from actual dimensions.
Computer monitors and televisions
Display sizes are commonly advertised in inches, even in countries that primarily use metric measurements.
For example:
| Screen size | Diagonal in mm |
|---|---|
| 13.3 inches | 337.82 |
| 15.6 inches | 396.24 |
| 24 inches | 609.6 |
| 27 inches | 685.8 |
| 32 inches | 812.8 |
| 55 inches | 1,397 |
| 65 inches | 1,651 |
These values represent the screen’s diagonal measurement, not its width or height.
Actual screen dimensions depend on the aspect ratio.
Pipes, bolts, and fasteners
Fasteners and plumbing components may use imperial or metric sizing systems.
For instance, a 1/4-inch measurement converts mathematically to 6.35 mm.
However, a 1/4-inch bolt is not necessarily interchangeable with an M6 bolt. Their thread specifications differ.
Likewise, nominal pipe sizes do not always represent the actual inside or outside diameter.
Always verify the relevant standard before selecting replacement components.
How Accurate Is the 25.4 Conversion Factor?
The value 25.4 is exact because it forms part of the internationally adopted definition of the inch.
According to the National Institute of Standards and Technology, the international inch equals precisely 25.4 millimeters.
NIST
+1
This means the conversion itself introduces no approximation.
For example:
\[ 0.125\times25.4=3.175\text{ mm} \]
The result is exact if the original value is exact.
However, a measured length may contain uncertainty. A ruler reading of 2.5 inches does not necessarily indicate the same measurement precision as a calibrated digital micrometer reading.
How many decimal places should you use?
The appropriate rounding depends on the task.
| Application | Typical display precision |
|---|---|
| General household measurements | 1 mm |
| Basic woodworking | 0.1 mm |
| Detailed technical drawings | 0.01 mm |
| Precision machining | As specified by tolerance |
These are illustrative conventions, not universal requirements. Engineering tolerances and measuring instrument capabilities should determine the actual precision.
For instance, 3/8 inch equals exactly 9.525 mm. Rounding that to 9.5 mm may be acceptable for an approximate measurement but unsuitable for a tightly controlled component.
Common Mistakes When Converting Inches to Millimeters
Even with a simple formula, several mistakes can lead to incorrect results.
Multiplying by 2.54 instead of 25.4
The number 2.54 converts inches to centimeters, not millimeters.
For example:
- 5 inches = 12.7 centimeters
- 5 inches = 127 millimeters
Confusing these factors produces a result ten times too small.
Treating fractional inches as decimal inches
A measurement of 1/4 inch means 0.25 inches, not 0.4 inches.
Similarly, 3/8 inch equals 0.375 inches.
Convert the fraction into a decimal before multiplying by 25.4.
Rounding too early
Suppose a part measures 0.375 inches.
The exact conversion is 9.525 mm.
If you round the inch value to 0.38 before converting, the result becomes 9.652 mm, introducing an unnecessary difference.
Retain the original precision throughout the calculation and round only the final result when appropriate.
Confusing nominal sizes with physical dimensions
Some hardware and construction materials use standardized names that do not match their actual measured dimensions.
A nominal 1-inch pipe, for example, does not necessarily have a physical diameter of exactly 25.4 mm.
Always distinguish between a mathematical unit conversion and a standardized product size.
Quick Mental Tricks for Converting Inches to Millimeters
When a calculator is unavailable, you can estimate inch measurements using simple arithmetic.
Multiply by 25 for a quick estimate
Since 25.4 is close to 25, multiplying inches by 25 gives a useful approximation.
For example:
\[ 8\times25=200\text{ mm} \]
The exact result is:
\[ 8\times25.4=203.2\text{ mm} \]
The estimate is 3.2 mm lower than the exact result, an error of approximately 1.57%.
This shortcut is useful for rough comparisons, not precision work.
Split the multiplication into two parts
For an exact mental calculation, multiply by 25 and then add 0.4 times the original inch measurement.
For example, converting 6 inches:
\[ 6\times25=150 \]
\[ 6\times0.4=2.4 \]
\[ 150+2.4=152.4\text{ mm} \]
This method produces the exact mathematical conversion without directly multiplying by 25.4.
Final Thoughts on in to mm Conversion
Converting inches to millimeters requires just one formula: multiply the number of inches by 25.4.
This exact conversion applies to whole numbers, decimal measurements, and fractional inches. It is useful for engineering drawings, woodworking projects, electronics, construction, and everyday product dimensions.
For the reverse calculation, divide millimeters by 25.4.
When precision matters, preserve the original measurement accuracy, avoid premature rounding, and check whether the dimensions refer to actual measurements or nominal industry sizes.
The key relationship is simple: 1 inch = exactly 25.4 millimeters.