Convert inches to millimeters accurately for engineering, manufacturing, and precision work. 1 inch = 25.4 mm exactly. Free calculator with real-world examples.
1 inch equals 25.399986284007404 millimeter
Formula: mm = in × 25.4000
The inch-to-millimeter conversion is fundamental in precision industries worldwide. With 1 inch equaling exactly 25.4 millimeters, this relationship bridges imperial and metric measurement systems. Engineers, manufacturers, and technicians rely on this conversion daily for component specifications, technical drawings, and quality control across automotive, aerospace, electronics, and construction sectors.
Precision in this conversion directly impacts product quality, safety, and international trade compliance. A single millimeter error in manufacturing can render components unusable—for example, a tolerance error of just 1 mm on a 100-inch assembly means a 1% deviation that could cause mechanical failure. Since 1 inch = 25.4 mm exactly, using approximate conversions in aerospace or medical device manufacturing can have catastrophic consequences and regulatory violations.
Aircraft components require extreme precision where even 0.1 mm deviations can affect aerodynamics and structural integrity. Engineers work with specifications in inches from legacy US designs but must manufacture to metric tolerances for international supply chains. Every fastener, panel, and structural element demands exact conversion to ensure parts fit perfectly in assembly.
Circuit board layouts, component spacing, and chip design specifications often originate in inches but must be manufactured in millimeters. PCB manufacturers worldwide require metric dimensions for CNC drilling, routing, and placement accuracy. A 0.01-inch error translates to 0.254 mm, which can short circuit traces or prevent component placement.
| inches | millimeters |
|---|---|
| 1 in | 25.399986 mm |
| 5 in | 126.999931 mm |
| 10 in | 253.999863 mm |
| 50 in | 1269.999314 mm |
| 100 in | 2539.998628 mm |
Formula: mm = in × conversion factor
The inch is a widely used unit for measuring length. It is commonly used in various applications and industries worldwide.
The millimeter is a widely used unit for measuring length. It is commonly used in various applications and industries worldwide.
This inch to millimeter converter uses high-precision conversion factors to ensure accurate results for professional and academic use.
Our calculator provides results with up to 10 decimal places for maximum accuracy.
The 25.4 factor comes from the 1959 international agreement defining 1 yard = 0.9144 meters exactly. Since 1 yard = 36 inches, dividing 914.4 mm by 36 gives precisely 25.4 mm per inch. This exact mathematical relationship was deliberately chosen to create a permanent, unchanging bridge between imperial and metric systems, ensuring that conversions never become outdated or approximate.
Multiply the inches by 25, then add 0.4 times the original number. For example: 8 inches → (8 × 25) + (8 × 0.4) = 200 + 3.2 = 203.2 mm. Alternatively, use the 'multiply by 2.54 and shift decimal' method: 8 inches × 2.54 = 20.32, then add a zero: 203.2 mm. For rough estimates, multiply by 25 (8 inches ≈ 200 mm) which gives within 1.6% accuracy.
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