Which inch formats can I enter?
Enter a decimal such as 1.375, a fraction such as 3/8, or a mixed number such as 1 3/8.
Fractional Inches Converter
Convert mixed or decimal inches and millimeters, rounding to a chosen inch fraction.
Formula and conventions
millimeters = inches × 25.4 exactly. For the nearest selected fraction, round inches × denominator to a whole tick.
Worked example
1 3/8 in = 1.375 in × 25.4 = 34.925 mm exactly.
The millimeter value uses the entered inch value. The selected nearest fraction is labeled as approximate when it differs from that value.
Frequently asked questions
Does the nearest fraction change the exact conversion?
No. The millimeter result uses the entered inch value. The selected fraction is a separate nearest-mark approximation.
Why is my fraction simplified?
Equivalent fractions are reduced for readability, and a rounded fraction that reaches a whole inch carries into the whole-number part.
Official sources
- NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically
- NIST Handbook 44-2026, Appendix C: General Tables of Units of Measurement
Reviewed .