Inches to Millimeters Conversion Guide for Engineering and DIY
Most of the world uses millimeters for precision measurements in engineering, manufacturing, and construction. The US still works largely in inches. When you're sourcing hardware from overseas, reading a technical drawing, using a 3D printer, or installing imported fixtures, you'll hit this conversion constantly.
Use the Length Converter for instant conversion between any length units. This article covers the exact formula, a full inch-to-millimeter reference table, and the practical situations where this conversion matters most.
The Exact Conversion
1 inch = 25.4 millimeters (exactly)
This is not an approximation — since 1959, the inch has been defined as exactly 25.4 mm by international agreement. The conversion is precise in both directions:
- Inches to millimeters: multiply by 25.4
- Millimeters to inches: divide by 25.4
Mental shortcuts:
- 1 inch ≈ 25 mm (close enough for rough estimates)
- 10 mm ≈ 0.4 inches (2/5 of an inch)
- 25 mm ≈ 1 inch
- 50 mm = 2 inches exactly (50 ÷ 25.4 = 1.9685, so very close but not exact)
For any precision work, use the exact factor of 25.4 or the Length Converter.
Inches to Millimeters Reference Table
Whole inches
| Inches | Millimeters |
|---|---|
| 1/16" | 1.588 mm |
| 1/8" | 3.175 mm |
| 3/16" | 4.763 mm |
| 1/4" | 6.35 mm |
| 5/16" | 7.938 mm |
| 3/8" | 9.525 mm |
| 7/16" | 11.113 mm |
| 1/2" | 12.7 mm |
| 9/16" | 14.288 mm |
| 5/8" | 15.875 mm |
| 11/16" | 17.463 mm |
| 3/4" | 19.05 mm |
| 7/8" | 22.225 mm |
| 1" | 25.4 mm |
| 1-1/4" | 31.75 mm |
| 1-1/2" | 38.1 mm |
| 2" | 50.8 mm |
| 2-1/2" | 63.5 mm |
| 3" | 76.2 mm |
| 4" | 101.6 mm |
| 5" | 127 mm |
| 6" | 152.4 mm |
| 8" | 203.2 mm |
| 10" | 254 mm |
| 12" (1 ft) | 304.8 mm |
Decimal inches to millimeters
| Decimal inches | Millimeters |
|---|---|
| 0.010" | 0.254 mm |
| 0.020" | 0.508 mm |
| 0.025" | 0.635 mm |
| 0.050" | 1.27 mm |
| 0.100" | 2.54 mm |
| 0.125" | 3.175 mm |
| 0.250" | 6.35 mm |
| 0.500" | 12.7 mm |
| 0.750" | 19.05 mm |
| 1.000" | 25.4 mm |
Where This Conversion Comes Up Most Often
Fasteners and hardware
Hardware is where the inch/mm conflict causes the most practical frustration. US hardware stores stock bolts, screws, and nuts in imperial sizes (1/4"-20, 5/16"-18, 3/8"-16). European and Asian hardware uses metric threads (M6, M8, M10). The diameter and thread pitch are both different, so the parts are not interchangeable.
Key reference for bolts:
- M5 (5mm) ≈ close to 3/16" but not interchangeable
- M6 (6mm) ≈ close to 1/4" but not interchangeable
- M8 (8mm) ≈ close to 5/16" but not interchangeable
- M10 (10mm) ≈ close to 3/8" but not interchangeable
None of these are exact matches. Imperial and metric fasteners look similar but have different thread pitches and tolerances. Always use the correct size — using a close-but-wrong substitute can strip threads or cause joint failure under load.
3D printing
Most 3D printers (particularly those based on the RepRap design standard, like Prusa and Creality models) use millimeters as their native unit. CAD software used in the US often works in inches. When exporting a design from an inches-based CAD file to an STL for a metric printer, the scale must be correct: 1 inch in the design must export as 25.4 mm in the STL.
A common mistake is exporting at 1:1 scale without verifying units. The result is a model that's 25.4× too large or 25.4× too small depending on which direction the error runs.
Check: after slicing, verify that the bounding box dimensions match your expected real-world measurements before printing.
Woodworking and carpentry
American woodworking typically uses imperial measurements — lumber is sold in nominal inches (a 2×4 is actually 1.5" × 3.5"), and plans are written in feet and inches. European woodworking, IKEA furniture, and many imported tools use millimeters.
When adapting a plan from one system to the other, convert all dimensions carefully. A 3/4" plywood thickness = 19.05 mm; suppliers often carry 18mm or 19mm sheet goods, which differ by 1mm or less — usually acceptable, but worth noting.
Common woodworking conversions:
- 3/4" = 19.05 mm (standard plywood, often available as 18mm or 19mm)
- 1/2" = 12.7 mm (often 12mm available)
- 1/4" = 6.35 mm (often 6mm available)
- 1-1/2" = 38.1 mm (standard 2× lumber thickness after surfacing)
Machining and precision engineering
Machining tolerances are often specified in thousandths of an inch (thou or mil) or in hundredths of a millimeter. Converting between these requires care:
- 1 thou (0.001") = 0.0254 mm
- 0.1 mm = 0.00394" ≈ 3.94 thou
A tolerance of ±0.005" = ±0.127 mm. A tolerance of ±0.01 mm = ±0.000394". These are significantly different levels of precision, and the right tolerance depends on the application.
Machined parts specified in inches often have tolerances in fractions of a thou; metric parts in hundredths or tenths of a millimeter. Both can achieve similar real-world precision, but the numbers look very different, and unit confusion can lead to parts that are machined out of tolerance.
Screen and display sizes
Screen sizes are measured diagonally in inches virtually everywhere, regardless of the country. A 27" monitor is a 27" monitor in Germany, Japan, and Brazil. The diagonal measurement converts to:
- 24" monitor = 609.6 mm diagonal
- 27" monitor = 685.8 mm diagonal
- 32" monitor = 812.8 mm diagonal
When calculating whether a monitor fits in a specific enclosure or desk space, you need the physical width and height, not just the diagonal. A 27" 16:9 monitor has physical dimensions of approximately 632 mm × 356 mm (25" × 14").
Pipe and tube sizes
Pipe sizing is one of the most confusing areas of inch/mm conversion, because nominal pipe sizes (NPS) don't correspond to actual dimensions in a simple way. A 1/2" NPS pipe has an outside diameter of 21.3 mm (0.84"), not 12.7 mm. The nominal size is historical and refers to the approximate bore size, not the outside diameter.
For plumbing work across systems, always verify actual OD and ID measurements rather than converting the nominal size. The Length Converter converts the actual measurement accurately — the nominal label is a separate matter.
Working with Fractional Inches
US measurements often use fractions: 3/8", 5/16", 7/32". To convert these to millimeters:
1. Convert the fraction to a decimal: 3/8 = 0.375 2. Multiply by 25.4: 0.375 × 25.4 = 9.525 mm
For measurements like 2-7/16": 1. Convert: 2 + 7/16 = 2 + 0.4375 = 2.4375 2. Multiply: 2.4375 × 25.4 = 61.9125 mm
The fractional-to-decimal step is where errors usually happen. Writing out the decimal explicitly before multiplying reduces mistakes.
Millimeters to Inches: Going the Other Direction
For the reverse conversion, divide by 25.4:
- 10 mm = 10 ÷ 25.4 = 0.3937" ≈ 25/64"
- 20 mm = 20 ÷ 25.4 = 0.7874" ≈ 25/32"
- 100 mm = 100 ÷ 25.4 = 3.937" ≈ 3-15/16"
- 300 mm = 300 ÷ 25.4 = 11.811" ≈ 11-13/16"
The decimal result rarely maps to a clean fraction. For practical purposes, round to the nearest 1/64" for woodworking, or keep the decimal form for machining and engineering.
