Roman Numeral Converter

How Roman Numeral Converter Works

Roman numerals are the letter-based number system used by the ancient Romans and still seen today on clock faces, movie copyright years, book chapter numbers, and monarch or event names (e.g. "Super Bowl LVIII"). This tool converts a plain number into its Roman numeral form and back again, live as you type in either box.

Formula & Method

The converter works from a fixed table of value/symbol pairs, from largest to smallest: M=1000, CM=900, D=500, CD=400, C=100, XC=90, L=50, XL=40, X=10, IX=9, V=5, IV=4, I=1. To go from number to numeral, it repeatedly subtracts the largest value that still fits and appends the matching symbol, which naturally produces the standard subtractive form (IV instead of IIII, IX instead of VIIII). To go from numeral to number, it reads the numeral left to right, adding each symbol's value unless a smaller-value symbol comes immediately before a larger one, in which case that smaller value is subtracted instead (the same rule that makes IV mean 4, not 6). Only well-formed standard-form numerals are accepted -- non-standard variants like IIII or VV are rejected rather than guessed at.

Worked Example

The number 1994 breaks down as 1000 + 900 + 90 + 4, giving the Roman numeral MCMXCIV (M + CM + XC + IV). Typing MCMXCIV into the Roman numeral box converts back to 1994, confirming the round trip.

Frequently Asked Questions

Why can't this tool convert numbers larger than 3999?

The standard Roman numeral system has no single symbol for 5,000 or above without adding extra notation (like a bar over a letter to mean ×1000), so 3999 (MMMCMXCIX) is the largest number expressible with the standard symbols this tool uses.

Why does the converter reject "IIII" for 4 instead of accepting it?

IIII is a non-standard (additive) way of writing 4 that appears on some clock faces for stylistic reasons, but the mathematically standard convention -- and the one this tool enforces -- uses the subtractive form IV instead.

Is there a Roman numeral for zero?

No -- the Roman numeral system has no symbol for zero, so this tool only accepts whole numbers from 1 to 3999.

Can I type lowercase Roman numerals like "mcmxciv"?

Yes -- the Roman numeral input is automatically converted to uppercase before it's validated and parsed, so lowercase or mixed-case input works the same as uppercase.