How to Convert Inches to Millimeters
Inches and millimeters are two of the most commonly used units for precise measurements. Inches are part of the imperial system, still widely used in the United States, the United Kingdom, and Canada for construction, woodworking, and everyday measurements. Millimeters belong to the metric system and are standard in engineering, manufacturing, and science worldwide. Converting between inches and mm is essential for anyone working with technical drawings, imported hardware, screen specifications, or paper sizes like A4 (210 x 297 mm = 8.27 x 11.69 inches). The exact conversion factor is 1 inch = 25.4 mm, or equivalently, 1 mm = 0.0393701 inches. This is not an approximation — it is the precise international standard since 1959. To convert inches to mm, multiply by 25.4. Common reference points include: 1 inch = 25.4 mm exactly, 0.5 inch = 12.7 mm, 0.25 inch = 6.35 mm (1/4 inch), 0.125 inch = 3.175 mm (1/8 inch), and 12 inches = 304.8 mm (1 foot). This free inches to mm converter provides instant results in both decimal inch and millimeter formats. The fractional inch display (e.g., 1/8", 1/4", 3/8") is especially valuable for woodworkers, machinists, and anyone using imperial measuring tools. Enter a value in either field for bidirectional conversion — switch between inches to mm and mm to inches without reloading. Common use cases include converting fastener sizes (1/4-20, 3/8-16 inch to mm equivalents), translating paper dimensions (Letter 8.5 x 11 inch = 215.9 x 279.4 mm), working with 3D printer tolerances, checking screen sizes and phone dimensions, sizing jewelry, and converting woodworking plans between imperial and metric measurements.
Why Use UtilDaily's Inches to MM Converter?
Inches and millimeters coexist in engineering, woodworking, and manufacturing worldwide. A single misreading — confusing 1 inch with 25 mm, for example — can ruin a part, waste material, or cause a safety issue. Our converter uses the exact international standard (1 inch = 25.4 mm, per NIST SP 811) and shows fractional inch results so you can read values directly off an imperial tape measure or caliper.
- Fractional inch display — shows results as practical fractions (1/8", 1/4", 3/8", etc.) used in woodworking, machining, and construction
- Exact conversion — uses the internationally defined 1 inch = 25.4 mm for perfectly accurate results, not approximations
- Bidirectional conversion — switch between inches to mm and mm to inches in one tool
- Paper size reference — quickly convert Letter, Legal, A4, and other standard paper sizes
- Engineering precision — handles decimal places for technical drawings, 3D printing tolerances, and machining specs
- No signup or download — free browser-based tool with instant results on any device
Frequently Asked Questions
How many mm is 1 inch?
1 inch equals exactly 25.4 millimeters. This is not an approximation — it is the precise international standard defined by the International Yard and Pound Agreement of 1959. This exact relationship is the foundation for all inches-to-mm conversions: to convert inches to mm, multiply by 25.4; to convert mm to inches, divide by 25.4.
How many mm is 1/2 inch?
1/2 inch equals exactly 12.7 millimeters. The calculation is 0.5 × 25.4 = 12.7 mm. This is a common size for drill bits, pipe fittings, and woodworking joints. In fractional terms, 12.7 mm equals exactly 1/2 inch.
How many mm is 1/4 inch?
1/4 inch equals exactly 6.35 millimeters. The calculation is 0.25 × 25.4 = 6.35 mm. This is one of the most common measurements in woodworking, plumbing, and construction.
How many mm is 3/8 inch?
3/8 inch equals exactly 9.525 millimeters. The calculation is 0.375 × 25.4 = 9.525 mm. This is a common size for socket wrenches, drill bits, and pipe fittings.
How many mm is 1 inch?
1 inch equals exactly 25.4 millimeters. This is the international standard used worldwide for engineering, manufacturing, and scientific applications. To convert any inch value to mm, multiply by 25.4.
How many mm is 2 inches?
2 inches equals exactly 50.8 millimeters. The calculation is 2 × 25.4 = 50.8 mm. This measurement is common in plumbing (2-inch pipes), screen sizes (some older monitors), and photography (2-inch lens filters).
How many mm is 8.5 inches (Letter paper width)?
8.5 inches (US Letter paper width) equals exactly 215.9 millimeters. The calculation is 8.5 × 25.4 = 215.9 mm. Combined with the 11-inch height (279.4 mm), US Letter paper measures 215.9 × 279.4 mm. This is slightly wider and shorter than A4 paper (210 × 297 mm).
How many mm is 11 inches (Letter paper height)?
11 inches (US Letter paper height) equals exactly 279.4 millimeters. The calculation is 11 × 25.4 = 279.4 mm. Combined with the 8.5-inch width (215.9 mm), US Letter paper measures 215.9 × 279.4 mm. A4 paper is 210 × 297 mm — slightly narrower but taller.
How do I convert inches to mm?
Multiply the inch value by 25.4 to get millimeters. For example: 1 inch × 25.4 = 25.4 mm. Alternatively, you can divide by 0.0393701. The multiplication by 25.4 method is preferred because 25.4 is the exact defined conversion factor. For a quick mental estimate, multiplying by 25 gives a result within about 1.6% of the exact answer.
What common objects are measured in inches?
Many everyday items use inch measurements: TV and monitor screens (15", 24", 32", 55"), wheel rims (15", 17", 18"), pipe diameters (1/2", 3/4", 1"), screws and bolts (#6, #8, #10, 1/4-20, 3/8-16), lumber dimensions (2x4, 2x6, 4x4), screen resolutions in pixels per inch (PPI), and US paper sizes (Letter 8.5x11, Legal 8.5x14, Tabloid 11x17).
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