Quick Answer: Inches to Millimeters
One inch equals 25.4 millimeters (1 in = 25.4 mm).
Conversely, 1 mm = 0.0393700787 in.
Formula: mm = in × 25.4
Worked examples
- 0.25 in = 6.35 mm
- 0.5 in = 12.7 mm
- 1 in = 25.4 mm
Method and limitations
To convert inches to millimeters, multiply the inch value by 25.4. Calculations use full-precision constants; displayed values are rounded to at most 9 significant figures. Results are rounded for display only. Full precision is kept internally, so long chains of conversions do not accumulate rounding error. Where US customary and imperial units share a name (for example fluid ounces and gallons), this page uses the US customary definition and says so explicitly.
Each factor is exact by definition, derived from exact definitions, or a documented conventional value — see the methodology and sources.
Conversion Formula
To convert inches to millimeters, multiply the inch value by 25.4.
Complete Guide to Inches to Millimeters Conversion
What is a Inches?
An inch (in) is an imperial unit used primarily in the US for everyday and industrial measurements. Fractional inches (1/2", 1/4") are common in American hardware and construction.
What is a Millimeters?
A millimeter (mm) is a metric unit providing greater precision than fractional inches. It is used worldwide in engineering, manufacturing, and scientific applications.
How the Conversion Works
The conversion is exact: 1 inch = 25.4 mm by international definition. This allows perfect conversion between the two systems without rounding errors.
Why This Conversion Matters
Converting inches to millimeters is essential for international manufacturing, 3D printing, CNC machining, and any field requiring precise metric specifications from imperial measurements.
Step-by-Step Conversion Guide
Follow these steps to convert inches to millimeters manually:
Start with your value in inches
Example: You want to convert 1 in
Apply the conversion formula
Use the formula: mm = in × 25.4
Calculate the result
1 in = 25.4 mm
Verify your answer
Use our calculator above to double-check your manual calculation.
Quick Mental Math Tip
To convert inches to millimeters, multiply the inch value by 25.4.
Common Conversions
| Inches (in) | Millimeters (mm) | Millimeters → Inches |
|---|---|---|
| 0.25 in | 6.35 | 0.25 mm = 0.00984252 in |
| 0.5 in | 12.7 | 0.5 mm = 0.019685 in |
| 1 in | 25.4 | 1 mm = 0.0393701 in |
| 2 in | 50.8 | 2 mm = 0.0787402 in |
| 3 in | 76.2 | 3 mm = 0.11811 in |
| 4 in | 101.6 | 4 mm = 0.15748 in |
| 5 in | 127 | 5 mm = 0.19685 in |
| 6 in | 152.4 | 6 mm = 0.23622 in |
| 10 in | 254 | 10 mm = 0.393701 in |
| 12 in | 304.8 | 12 mm = 0.472441 in |
Did You Know?
The inches and millimeters are among the most commonly converted units worldwide, with millions of daily searches.
This converter is used by thousands of people daily for quick reference. Bookmark it for instant access.
Popular Inches to Millimeters Conversions
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When You'll Need This Conversion
Ordering Metric Parts
Convert American specifications to metric when ordering parts from international suppliers.
Drill Bit Sizing
Find equivalent metric drill bits when fractional inch sizes are unavailable or vice versa.
Pipe & Tube Fittings
Convert plumbing and tubing dimensions when mixing imperial and metric components.
3D Printing
Convert model dimensions from inches to millimeters for metric-based slicing software.
Pro Tips for Accurate Conversions
Multiply by 25 for estimates
Using 25 instead of 25.4 is easier for mental math: 2 in × 25 = 50 mm (actual: 50.8 mm).
Common fractional conversions
1/8" = 3.175 mm, 1/4" = 6.35 mm, 3/8" = 9.525 mm, 1/2" = 12.7 mm, 3/4" = 19.05 mm, 1" = 25.4 mm.
Close equivalents
1" ≈ 25 mm, 2" ≈ 50 mm, 4" ≈ 100 mm. These approximations work well for rough estimates.
Common Mistakes to Avoid
Forgetting to convert fractions first
You cannot directly multiply fractions like "3/8" by 25.4. Convert to decimal first.
3/8" = 0.375" → 0.375 × 25.4 = 9.525 mm.
Using 25 for precision work
Using 25 instead of 25.4 causes a 1.6% error that matters in manufacturing and engineering.
For precision: 1" = 25.4 mm exactly, not 25 mm.
History & Background
The 25.4 mm definition of the inch was established internationally in 1959 to standardize measurements between imperial and metric systems in manufacturing and trade.
Global Usage Today
While millimeters are the global standard, many American industries still specify dimensions in inches. The exact 25.4 conversion enables seamless international manufacturing.
Frequently Asked Questions
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Conversion factors on this page are either exact by definition, derived from exact definitions, or conventional values in documented use. Each is listed with its source in our conversion methodology and sources, which cites the BIPM SI Brochure, NIST Special Publication 811 and the relevant IEC or national standards. Results are rounded for display only; calculations use full precision.