Inches to Millimeters Converter
Convert inches to millimeters instantly. Free in to mm converter with exact 25.4 formula and reference table.
What Is a Inches to Millimeters Converter?
Converting inches to millimeters is fundamental in engineering, manufacturing, and CNC machining where metric tooling is standard. Hardware stores stock both metric and imperial fasteners, and knowing the exact conversion prevents buying the wrong bolt size. The conversion is exact (1 inch = 25.4 mm), which is why it's one of the most reliable unit conversions in precision work.
The Conversion Formula
The formula to convert inches to millimeters is:
Millimeters = Inches × 25.4
The reverse formula is: Inches = Millimeters ÷ 25.4
Inches to Millimeters Reference Table
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.0625 in | 1.59 mm |
| 0.125 in | 3.17 mm |
| 0.25 in | 6.35 mm |
| 0.375 in | 9.52 mm |
| 0.5 in | 12.70 mm |
| 0.625 in | 15.88 mm |
| 0.75 in | 19.05 mm |
| 0.875 in | 22.22 mm |
| 1 in | 25.40 mm |
| 1.5 in | 38.10 mm |
| 2 in | 50.80 mm |
| 3 in | 76.20 mm |
| 4 in | 101.60 mm |
| 5 in | 127.00 mm |
| 6 in | 152.40 mm |
| 8 in | 203.20 mm |
| 10 in | 254.00 mm |
| 12 in | 304.80 mm |
| 18 in | 457.20 mm |
| 24 in | 609.60 mm |
How to Use This Converter
- Enter your inches value in the input field above.
- View the result — the millimeters equivalent appears instantly.
- For manual conversion, use the formula: Millimeters = Inches × 25.4.
- Check the reference table above for 20 pre-calculated common values.
Conversion Tips
- Multiply inches by 25.4 for exact millimeters. For quick math: multiply by 25. So 0.5 inches is about 12.5mm (actual: 12.7mm). Close enough for most measurement tasks.
- Phone and tablet screen sizes are quoted in inches but glass thickness, bezel width, and component sizes use millimeters. A screen protector for a 6.1-inch phone is usually 68mm wide.
- Precision machining tolerances are typically specified in thousandths of an inch (mils) in the US and hundredths of a millimeter in metric. One mil = 0.0254mm. Misinterpreting the unit system ruins parts.
Related Conversions
- Millimeters to Inches Converter
- Inches to Centimeters Converter
- Centimeters to Inches Converter
- Feet to Meters Converter
- Meters to Feet Converter
All conversions are calculated client-side in your browser. No data is sent to any server. For the authoritative source on unit definitions, see the National Institute of Standards and Technology (NIST).
Frequently Asked Questions
How many millimeters are in an inch?
Exactly 25.4 millimeters. This is a precise definition, not an approximation — it was established by international agreement in 1959.
How do I convert fractional inches to mm?
Convert the fraction to a decimal first, then multiply by 25.4. Example: 3/8 inch = 0.375 × 25.4 = 9.525 mm. Common fractions: 1/16" = 1.59mm, 1/8" = 3.18mm, 1/4" = 6.35mm, 1/2" = 12.7mm.
What is 1/4 inch in millimeters?
One quarter inch (1/4") equals exactly 6.35 millimeters. This is one of the most common conversions in hardware and engineering.
Why do engineers prefer millimeters over inches?
Millimeters allow specifications without fractions or decimals of small numbers. Instead of saying '0.125 inches' or '1/8 inch', an engineer says '3.175 mm' — a whole number is easier to read and less prone to error on technical drawings.
Results are calculated in your browser using standard formulas and are provided for reference only. For critical or high-stakes decisions, verify results with an authoritative source.
Free Tools Alert
Join 10,000+ creators. Get our newest productivity tools, templates, and calculators directly to your inbox every month.
No spam. One-click unsubscribe.