How to Convert Inches to Millimeters: The Complete Guide
Master the conversion from Imperial inches to Metric millimeters. Includes simple formulas, common fraction conversions (1/2, 3/4, 1/8), and pipe size charts.
How to Convert Inches to Millimeters: The Complete Guide
In a globalized world, working with both Imperial (Inches) and Metric (Millimeters) systems is a daily reality for engineers, machinists, and DIY enthusiasts. This guide ensures you never drill the wrong size hole again.
Quick Answer (TL;DR)
The Golden Rule:
- 1 inch = 25.4 mm (exactly)
Formulas:
- Inches to mm:
Inches × 25.4 - mm to Inches:
mm ÷ 25.4
Example:
- 2 inches = 2 × 25.4 = 50.8 mm
- 100 mm = 100 ÷ 25.4 = 3.937 inches
Fractional Inches Explained
Most US tools and blueprints use fractions (e.g., 1/2", 3/8"), not decimals. Converting these requires an extra step.
Step-by-Step Conversion
- Convert Fraction to Decimal: Divide numerator by denominator.
- Example: 3/8" -> 3 ÷ 8 = 0.375"
- Multiply by 25.4:
- 0.375 × 25.4 = 9.525 mm
Common Fractions Reference Table
| Fraction | Decimal | Polished Metric (mm) | Precise (mm) |
|---|---|---|---|
| 1/32" | 0.03125 | 0.8 mm | 0.794 |
| 1/16" | 0.0625 | 1.6 mm | 1.588 |
| 1/8" | 0.125 | 3.2 mm | 3.175 |
| 1/4" | 0.250 | 6.35 mm | 6.350 |
| 3/8" | 0.375 | 9.5 mm | 9.525 |
| 1/2" | 0.500 | 12.7 mm | 12.700 |
| 5/8" | 0.625 | 16 mm | 15.875 |
| 3/4" | 0.750 | 19 mm | 19.050 |
| 7/8" | 0.875 | 22 mm | 22.225 |
| 1" | 1.000 | 25.4 mm | 25.400 |
Machining & Precision
In machining (CNC, Lathe), precision matters.
- Rule of Thumb: 1/1000th of an inch (0.001", a "thou") is 0.0254 mm.
- Tolerance Check:
- +/- 0.005" (Standard US tolerance) ≈ +/- 0.13 mm (Metric).
- +/- 0.1 mm (Standard Metric tolerance) ≈ +/- 0.004".
Pipe Sizes: The Confusing Part
Warning: "Nominal Pipe Size" (NPS) does not equal actual measured diameter!
- A 1-inch pipe does not have an outer diameter of 1 inch (25.4mm).
- Actual OD of 1" NPS steel pipe = 1.315 inches (33.4 mm).
When converting piping/plumbing:
- Don't just multiply the name by 25.4.
- Consult a Pipe Schedule Chart for actual dimensions.
Mental Math Tricks
Need to guess in the field without a calculator?
Method 1: The "25" Approximation
- Assume 1 inch ≈ 25 mm.
- 4 inches ≈ 100 mm (Actual: 101.6 mm - 1.6% Error).
- Good for: Rough carpentry, estimating cable lengths.
Method 2: The "Times 200, Divide by 8" (Binary Math)
- Complex but accurate? No, stick to "Times 25" for mental math!
Method 3: Ruler Check
- Most tape measures have both scales.
- 10 cm = just under 4 inches (3.93").
- 30 cm (standard ruler) = just under 12 inches (11.8").
Common Mistakes
1. Rounding Too Early
- Bad: 3/8" -> 0.37 -> 0.37 × 25.4 = 9.398 mm
- Good: 3/8" -> 0.375 -> 0.375 × 25.4 = 9.525 mm
- Tip: Keep at least 3 decimal places for inch calculations before converting.
2. Confusing Layout vs. Component
- In construction, "16-inch centers" (stud spacing) = 406.4 mm.
- Metric construction usually uses 400 mm or 600 mm spacing, not the direct conversion of 406.4. Be careful when mixing grid systems!
Conclusion
Converting inches to millimeters is the bridge between the US and the rest of the world.
- Memorize: 1 inch = 25.4 mm.
- Tools: Use our Length Converter for instant precision.
Whether you're 3D printing a part downloaded from Europe or fixing a Ford truck with metric bolts, this conversion is the single most useful number in engineering.
Last Updated: January 31, 2026
Reading Time: 6 minutes