Quick answer
Millimetres = inches × 25.4. A 3/8 inch size is 9.525 mm, or 0.9525 cm, and 0.375 as a decimal.
Enter a size in inches (fractions like 3/8 or decimals). The calculator returns millimetres, centimetres and the decimal. Use this sae to metric converter to check your own figures.
Related tools: Length Converter: Convert between metric and imperial lengths. Feet and Inches Calculator: Add or subtract two lengths given in feet and inches. Scale Conversion Calculator: Convert between model and actual lengths at a 1:N scale.
How to use the SAE to Metric Converter
- Type size in inches, for example 3/8 or 1 1/4 or 0.5.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Size in inches | Text, such as 3/8 or 1 1/4 or 0.5 | — |
Formula and method
Formula
mm = inches × 25.4
IEEE-754 double precision; shown to at most 6 decimal places.
Accepts fractions, mixed numbers and decimals. The nearest standard metric fastener size is not chosen for you.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Size in inches = 3/8.
| Result | Value |
|---|---|
| Millimeters | 9.525 mm |
| In centimeters | 0.9525 |
| Decimal inches | 0.375 |
Example 2
Inputs: Size in inches = 0.5.
| Result | Value |
|---|---|
| Millimeters | 12.7 mm |
| In centimeters | 1.27 |
| Decimal inches | 0.5 |
Example 3
Inputs: Size in inches = 1 1/4.
| Result | Value |
|---|---|
| Millimeters | 31.75 mm |
| In centimeters | 3.175 |
| Decimal inches | 1.25 |
The first line is the headline result (millimeters). Below it the sAE to Metric Converter lists in centimeters and decimal inches, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Wrenches and bolts
SAE sizes are fractional inches. A 3/8 inch socket is 9.525 mm, close to but not equal to a 10 mm socket. Using the wrong system rounds off fasteners, so match the size exactly.
Common fractions
1/4 in = 6.35 mm, 1/2 in = 12.7 mm, 5/8 in = 15.875 mm, 3/4 in = 19.05 mm and 1 in = 25.4 mm.
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Size in inches: left empty shows "Enter size in inches." beside that field and no result.
Choosing between the SAE to Metric Converter and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Length Converter](/converters/length-converter/) | value, from and to | Mile (mi) → Kilometre (km) |
| [Feet and Inches Calculator](/converters/feet-inches-calculator/) | first length: feet, first length: inches and second length: feet | Result |
| [Scale Conversion Calculator](/converters/scale-conversion/) | scale denominator N, convert and length | Converted length |
| [Volume Converter](/converters/volume-converter/) | value, from and to | US gallon → Litre (L) |
| [Weight Converter](/converters/weight-converter/) | value, from and to | Pound (lb) → Gram (g) |
Background: unit conversion
Many customary units are defined exactly in terms of metric ones: since the 1959 International Yard and Pound Agreement an inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg (NIST SP 811 lists these). So most length and mass conversions are exact apart from the rounding of the displayed result.
To convert, multiply the value by the source unit's factor to reach the base unit, then divide by the target unit's factor; the unit table on this page lists every factor used. Temperature and fuel economy are the exceptions, because they are not simple multiples.
Assumptions and limitations
- Positive sizes only
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for size in inches to confirm this.
Frequently asked questions
What is 1/2 inch in mm?
Half an inch is 12.7 mm.
Is a 10 mm wrench the same as 3/8 inch?
No, 3/8 inch is 9.525 mm.
Can I enter mixed numbers?
Enter an improper fraction or decimal.
What does the SAE to Metric Converter calculate?
Fractional or decimal inches to millimeters. It returns Millimeters, In centimeters and Decimal inches.
What formula does the SAE to Metric Converter use?
mm = inches × 25.4. Accepts fractions, mixed numbers and decimals.
What do I need to enter in the sAE to Metric Converter?
You enter Size in inches.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Positive sizes only.
What is an example of a sAE to Metric result?
With size in inches 3/8, the millimeters is 9.525 mm.
Where does the method come from?
The method follows: International Yard and Pound Agreement (1959): 1 in = 25.4 mm.
Related calculators
- Length Converter — Convert between metric and imperial lengths.
- Feet and Inches Calculator — Add or subtract two lengths given in feet and inches.
- Scale Conversion Calculator — Convert between model and actual lengths at a 1:N scale.
- Volume Converter — Convert litres, gallons, cups, spoons and more.
- Weight Converter — Convert kilograms, pounds, ounces, stone and more.
- Energy Converter — Convert joules, calories, kWh, BTU and more.
Sources
- International Yard and Pound Agreement (1959): 1 in = 25.4 mm
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.