Inches to Millimeters converter
Quick reference
About length units
Length units split into two families: metric (millimeter through kilometer, each a factor of ten apart) and US/imperial (inch, foot, yard, mile, with historical ratios like 12 inches to a foot and 5,280 feet to a mile). The bridge between them was fixed by international agreement in 1959: one inch equals exactly 25.4 millimeters, which makes every metric-imperial length conversion exact rather than approximate. Runners meet this constantly (a 5K is 3.11 miles, a marathon 26.22), as do builders working from plans drawn in the other system.
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Frequently asked questions
What is the exact formula for Inches to Millimeters?
Multiply the Inches value by 25.4. This comes from the exact unit definitions, not a rounded approximation.
Is this conversion exact?
The underlying ratio is exact where a legal definition exists (for example, 1 inch = 25.4 mm exactly). Displayed results are rounded to six decimal places for readability.
Can I convert Millimeters back to Inches?
Yes. Use the reverse converter linked below, or just swap which field you type into: the live converter recalculates either direction.