Conversion Result
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Millimeters (mm)
About This Converter
- What it does
- Converts inches to millimeters with reverse conversion.
- Input
- Length in inches (in)
- Output
- Millimeters (mm); reverse inches value
- Formula
- mm = inches × 25.4 (exact)
- Last updated
About Inches to Millimeters Conversion
Inches and millimeters are both used for small, precise measurements, but they belong to different measurement systems. Millimeters are part of the metric system and are standard in engineering, manufacturing, and scientific work worldwide. Inches remain common in the United States for woodworking, plumbing, and electronics. The conversion between them is exact: 1 inch = 25.4 mm.
Conversion Formula
mm = inches × 25.4
One inch is defined as exactly 25.4 millimeters. Multiply inches by 25.4 to get millimeters; divide by 25.4 to get inches from millimeters.
Worked Example
Convert 2.5 inches to mm: 2.5 × 25.4 = 63.5 mm. A half-inch bolt (0.5 in) = 0.5 × 25.4 = 12.7 mm.
Common Use Cases
- Machining: converting US imperial bolt sizes (1/4 in, 3/8 in) to metric mm for metric tools
- 3D printing: models designed in inches exported to a metric slicer — convert all dimensions to mm
- Bicycle parts: wheel sizes in inches and tire widths in mm (e.g. 2.1 in = 53.3 mm)
- Woodworking: US hardwood lumber thickness (4/4 = 1 inch = 25.4 mm) to metric for EU suppliers
- Screen pixel density: phone screens in inches converted to mm for PPI calculations
Frequently Asked Questions
1 inch = 25.4 mm exactly. This is a fixed international definition.
2 × 25.4 = 50.8 mm.
0.5 × 25.4 = 12.7 mm.
0.25 × 25.4 = 6.35 mm.
0.375 × 25.4 = 9.525 mm.
25 ÷ 25.4 = 0.984 inches, approximately 1 inch.
6 × 25.4 = 152.4 mm.
The international inch was redefined in 1959 as exactly 25.4 mm under the International Yard and Pound Agreement, to align US and UK imperial systems with the metric system.
Important Notes
- 1 inch = 25.4 mm exactly — no rounding or approximation.
- Common inch fractions in mm: 1/4″ = 6.35, 3/8″ = 9.525, 1/2″ = 12.7, 3/4″ = 19.05, 1″ = 25.4.
- For fractions of an inch: divide by denominator first (e.g. 1/8 in = 0.125 in = 3.175 mm).
- 1 foot = 12 in = 304.8 mm; 1 yard = 36 in = 914.4 mm.