Need to convert an inch measurement into millimeters? The calculation is simple, but getting the decimal places right matters, especially when working with tools, hardware, construction plans, or precision components.
To convert inch to mm, multiply the measurement in inches by 25.4. One inch equals exactly 25.4 millimeters. For example, 2 inches equals 50.8 mm, 5 inches equals 127 mm, and 10 inches equals 254 mm. This conversion factor is exact, so you can use it for everyday measurements and technical calculations.
inch to mm Conversion Formula
The standard formula for converting inches to millimeters is:
\[ \text{Millimeters}=\text{Inches}\times25.4 \]
The number 25.4 is not an approximation. It is the internationally standardized relationship between the inch and the millimeter.
For example, suppose you have a metal plate measuring 3.5 inches wide.
\[ 3.5\times25.4=88.9\text{ mm} \]
Therefore, a 3.5-inch-wide plate measures 88.9 millimeters.
How many millimeters are in one inch?
Exact conversion
1 inch = 25.4 mm
The conversion factor is exact, not rounded.
An inch is a unit of length commonly used in the United States, while a millimeter is a metric unit used worldwide in engineering, manufacturing, science, and everyday measurements.
Because 1 centimeter equals 10 millimeters, one inch also equals 2.54 centimeters.
inch to mm calculator
Enter any measurement in inches to see its equivalent in millimeters instantly.
Inches to millimeters
Exact factorInches (in)Decimal places
3
Result in millimeters
25.4 mm
Copy
1 × 25.4 = 25.4 mm
1 in = 25.4 mmReset
inch to mm Conversion Chart
A conversion chart is useful when you need to check several measurements without calculating each one individually.
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.01 | 0.254 |
| 0.1 | 2.54 |
| 0.25 | 6.35 |
| 0.5 | 12.7 |
| 0.75 | 19.05 |
| 1 | 25.4 |
| 1.5 | 38.1 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 12 | 304.8 |
| 24 | 609.6 |
| 36 | 914.4 |
| 48 | 1,219.2 |
| 72 | 1,828.8 |
| 100 | 2,540 |
All values in this table use the exact factor of 25.4 millimeters per inch.
Common inch to mm measurements
Some conversions appear more frequently than others.
- 1 inch: 25.4 mm, often used as a reference measurement.
- 2 inches: 50.8 mm, useful for material dimensions.
- 4 inches: 101.6 mm, common in construction and equipment specifications.
- 6 inches: 152.4 mm, useful for measuring components and fixtures.
- 12 inches: 304.8 mm, equivalent to one foot.
- 36 inches: 914.4 mm, equivalent to one yard.
For a large number of measurements, a calculator or spreadsheet formula is usually faster than consulting a chart.
How to Convert Inches to Millimeters Step by Step
You can convert inches to millimeters manually using three simple steps.
- Identify the measurement in inches. For example, assume a wooden board is 7.25 inches wide.
- Multiply the measurement by 25.4. Calculate 7.25 × 25.4.
- Write the result in millimeters. The answer is 184.15 mm.
The complete calculation is:
\[ 7.25\times25.4=184.15\text{ mm} \]
This method works for whole numbers, decimals, and fractions after converting the fraction into a numerical value.
Example 1: Convert 8 inches to mm
\[ 8\times25.4=203.2\text{ mm} \]
Eight inches equals 203.2 millimeters.
Example 2: Convert 2.75 inches to mm
\[ 2.75\times25.4=69.85\text{ mm} \]
A measurement of 2.75 inches is equivalent to 69.85 millimeters.
Example 3: Convert 0.125 inches to mm
\[ 0.125\times25.4=3.175\text{ mm} \]
This conversion is particularly useful when working with small mechanical components, sheet materials, and machining dimensions.
Fractional inch to mm Conversion Chart
Inches are frequently written as fractions on measuring tapes, rulers, engineering drawings, and woodworking plans. Common examples include 1/8, 1/4, 3/8, and 1/2 inch.
To convert a fractional inch measurement, divide the numerator by the denominator, then multiply the result by 25.4.
| Fractional inches | Decimal inches | 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 |
These values are calculated using the exact international inch definition.
Inch Calculator
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How do you convert 3/8 inch to mm?
First, convert the fraction into a decimal:
\[ 3\div8=0.375 \]
Next, multiply by 25.4:
\[ 0.375\times25.4=9.525\text{ mm} \]
Therefore, 3/8 inch equals exactly 9.525 millimeters.
How do you convert mixed fractions?
A mixed fraction contains a whole number and a fractional part, such as 2 1/2 inches.
Convert it into decimal inches:
\[ 2+\frac12=2.5 \]
Then multiply by 25.4:
\[ 2.5\times25.4=63.5\text{ mm} \]
This method also works for measurements such as 1 3/8, 4 5/16, and 8 5/8 inches.
Understanding Inches and Millimeters
Although inches and millimeters both measure length, they belong to different measurement systems.
What is an inch?
An inch is a unit of length in the US customary and imperial measurement systems. Its standard symbol is in, although the double-prime symbol (″) is also commonly used.
There are 12 inches in one foot and 36 inches in one yard.
The international inch was standardized at exactly 25.4 millimeters in 1959, establishing a consistent relationship with the metric system.
NIST
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What is a millimeter?
A millimeter is a metric unit of length equal to one-thousandth of a meter.
Its standard symbol is mm.
The metric system is based on decimal relationships, making it straightforward to convert between millimeters, centimeters, and meters.
- 1 millimeter = 0.1 centimeter
- 10 millimeters = 1 centimeter
- 100 millimeters = 10 centimeters
- 1,000 millimeters = 1 meter
The millimeter is particularly useful for measuring small objects and dimensions requiring greater numerical detail.
Inches vs. millimeters
| Feature | Inches | Millimeters |
|---|---|---|
| Symbol | in | mm |
| Measurement system | Imperial / US customary | Metric / SI |
| Unit size | Larger | Smaller |
| Common subdivisions | Fractions and decimals | Decimal subdivisions |
| Typical applications | Construction, screen sizes, US hardware | Engineering, manufacturing, scientific measurement |
| Equivalent length | 1 in | 25.4 mm |
Neither unit is inherently more accurate. Measurement accuracy depends on the instrument, method, and precision used.
How to Convert mm to Inches
Sometimes you need to reverse the conversion, especially when working with metric components and inch-based drawings.
To convert millimeters to inches, divide the measurement by 25.4.
\[ \text{Inches}=\frac{\text{Millimeters}}{25.4} \]
For example, to convert 100 mm to inches:
\[ 100\div25.4\approx3.9370\text{ in} \]
Therefore, 100 millimeters is approximately 3.937 inches.
Other common reverse conversions include:
| Millimeters | Inches (approx.) |
|---|---|
| 1 | 0.03937 |
| 5 | 0.19685 |
| 10 | 0.39370 |
| 25 | 0.98425 |
| 50 | 1.96850 |
| 100 | 3.93701 |
| 200 | 7.87402 |
| 500 | 19.68504 |
| 1,000 | 39.37008 |
Remember that converting inches to millimeters requires multiplication, while converting millimeters to inches requires division.
Where inch to mm Conversion Is Used
Converting between inches and millimeters is common in industries that use both imperial and metric dimensions.
Engineering and manufacturing
Mechanical engineers and machinists frequently work with drawings containing dimensions in different units.
For example, a machine component with a specified diameter of 0.375 inches has a metric diameter of 9.525 mm.
Precision measuring instruments, including digital calipers and micrometers, often support both inches and millimeters.
However, a converted dimension should not automatically be treated as an interchangeable manufactured size. Thread specifications, tolerances, and engineering standards may differ.
Construction and woodworking
Carpenters and builders regularly encounter inch-based measurements on tape measures, lumber specifications, and construction drawings.
A board measuring 3/4 inch thick has a mathematical thickness of 19.05 mm.
But nominal lumber sizes do not always represent actual finished dimensions. Check the product’s stated dimensions before cutting or assembling materials.
Plumbing and pipe measurements
Pipes, fittings, and valves may have sizes expressed in inches or millimeters.
A nominal 1-inch pipe does not necessarily have an actual inside or outside diameter of 25.4 mm.
Pipe sizing systems, including Nominal Pipe Size (NPS) and Diameter Nominal (DN), use standardized designations that are not simple physical length conversions.
For this reason, always consult the relevant pipe specification when selecting compatible fittings.
Screws, bolts, and fasteners
Fasteners are commonly described using either imperial or metric dimensions.
An inch-based bolt may use Unified Thread Standard (UTS) specifications, while a metric bolt may use ISO metric screw threads.
Converting a bolt’s diameter into millimeters does not make it equivalent to a metric-threaded bolt.
Thread pitch, diameter tolerance, and thread profile must also match.
Electronics and screen measurements
Televisions, monitors, tablets, and smartphones often use inches to describe screen size.
For example, a 15.6-inch laptop display has a diagonal measurement of:
\[ 15.6\times25.4=396.24\text{ mm} \]
This is the screen’s diagonal, not its width or height.
Actual screen dimensions depend on the aspect ratio.
3D printing and CAD software
Computer-aided design (CAD) applications and 3D printing workflows frequently involve switching between inches and millimeters.
A model designed as 4 inches long should measure 101.6 mm when correctly converted.
Unit mismatches can produce dramatically incorrect model sizes, particularly when importing files between programs with different default measurement settings.
Precision, Rounding, and Measurement Accuracy
Although the conversion factor is exact, the precision of the original measurement determines how accurately the result represents a real object.
For example:
\[ 1.2345\times25.4=31.3563\text{ mm} \]
The mathematical result is exact for an input of exactly 1.2345 inches.
However, if 1.2345 inches was obtained using an instrument with limited accuracy, the converted number does not become more accurate simply because it contains additional decimal places.
How many decimal places should you use?
The appropriate number of decimal places depends on the application.
| Application | Typical reporting precision |
|---|---|
| General household measurements | Nearest 1 mm |
| Basic woodworking | Nearest 0.5 or 1 mm |
| General mechanical measurements | 0.1 or 0.01 mm |
| Precision machining | According to drawing tolerances |
| Scientific measurements | Based on measurement uncertainty |
These are illustrative practices, not universal requirements.
For critical components, follow the specified tolerance rather than choosing an arbitrary number of decimal places.
Does rounding change the conversion?
Rounding changes the displayed numerical result, not the underlying conversion factor.
For example:
1/16 inch = 1.5875 mm exactly.
Rounded to different precisions:
- Nearest whole millimeter: 2 mm
- One decimal place: 1.6 mm
- Two decimal places: 1.59 mm
- Three decimal places: 1.588 mm
Rounding too early can introduce errors when performing additional calculations.
Keep the full precision during intermediate steps and round the final answer only when necessary.
Common inch to mm Conversion Mistakes
Most conversion errors come from using the wrong operation, misunderstanding fractions, or confusing nominal sizes with actual measurements.
Multiplying by 2.54 instead of 25.4
The factor 2.54 converts inches into centimeters, not millimeters.
For example:
- 5 inches = 12.7 cm
- 5 inches = 127 mm
Confusing centimeters and millimeters produces a tenfold error.
Treating fractional inches as decimal inches
The measurement 1/4 inch is 0.25 inches, not 0.4 inches.
Similarly, 3/8 inch is 0.375 inches, not 0.38 inches exactly.
Always divide the numerator by the denominator before converting.
Confusing feet and inches
A measurement of 5 feet 6 inches does not mean 5.6 inches.
First, convert feet into inches:
\[ (5\times12)+6=66\text{ inches} \]
Then convert into millimeters:
\[ 66\times25.4=1676.4\text{ mm} \]
Assuming rounded sizes are interchangeable
A 1/4-inch diameter is exactly 6.35 mm, not 6 mm.
The difference of 0.35 mm can matter for close-fitting components.
Similarly, a 1/2-inch fastener is not necessarily interchangeable with a 12 mm fastener.
Ignoring measurement uncertainty
A ruler, tape measure, digital caliper, and micrometer have different measurement capabilities.
Converting a roughly measured length into a highly detailed decimal result does not eliminate the original uncertainty.
Why Is One Inch Exactly 25.4 Millimeters?
The relationship between inches and millimeters is based on international measurement standardization.
Before modern standardization, slightly different definitions of the inch were used in different countries and applications.
In 1959, the international yard and pound agreement established the international yard as exactly 0.9144 meters.
Because one yard contains 36 inches:
\[ 0.9144\div36=0.0254\text{ meters} \]
And because one meter contains 1,000 millimeters:
\[ 0.0254\times1000=25.4\text{ mm} \]
This gives the exact relationship:
\[ 1\text{ inch}=25.4\text{ mm} \]
The US National Institute of Standards and Technology (NIST) recognizes this definition in its measurement references.
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The standardized relationship allows manufacturers, researchers, and engineers to convert measurements consistently across systems.
Final Thoughts on inch to mm Conversion
Converting inches to millimeters is straightforward once you remember the exact relationship: 1 inch = 25.4 mm.
Multiply inches by 25.4 to get millimeters, or divide millimeters by 25.4 to convert back into inches.
For fractional measurements, convert the fraction into a decimal first. For precision work, retain sufficient decimal places and follow the applicable measurement tolerances.
Use the conversion calculator or reference charts above whenever you need to check a measurement quickly and accurately.