Roman Numeral Converter

The thirteen building blocks

The converter works from a list of seven single letters plus six subtractive pairs. To encode a number it walks down the list from 1,000, appending a symbol and subtracting its value as many times as it fits before moving to the next entry. This greedy procedure always produces the canonical form, the one with the fewest symbols that follows the usual rules.

Worked examples: 1994 and 3999

For 1994: one M fits (994 left), D does not, so CM is taken (94 left); C and L do not fit, XC does (4 left); X, IX and V are too big, IV finishes it. Result: MCMXCIV. For 3999 the same walk yields MMM, CM, XC and IX, giving MMMCMXCIX, the longest numeral the tool can produce at nine letters. 2026 is MMXXVI, and 1666 is MDCLXVI, the only year that uses every letter exactly once in descending order.

How the reverse direction is validated

Parsing a numeral starts with a character check: anything other than M, D, C, L, X, V and I fails immediately. The letters are then summed left to right, subtracting a letter whenever the one after it is larger. Finally the total is encoded again and compared with what you typed; if the two strings differ, the input is rejected. That round trip is what filters out non-standard spellings.

Limits and quirks to know about

• The range stops at 3999 because the tool has no overline (vinculum) notation for thousands, so 4000 and beyond cannot be written, and there is no numeral for zero.
• Decimals such as 3.5 are refused; the number field only accepts whole numbers.
• When one box becomes invalid, the other keeps its last good value rather than clearing, so a half-typed numeral does not wipe the number.
• Clock-face IIII and additive forms like VIIII are rejected, as are the single Unicode characters such as Ⅻ (U+216B); type ordinary letters.
• Spaces before or after a numeral are ignored, but spaces inside it are not.

Where you still meet Roman numerals

• Copyright years in film and television credits, such as MCMXCIV for 1994.
• Super Bowl and Olympiad numbering: LVIII is 58.
• Monarchs and popes (Louis XIV, Elizabeth II) and generational suffixes like III.
• Front-matter page numbers in books, printed in lowercase (i, ii, iii), which the tool accepts and uppercases.
• Outline headings, clock faces and building cornerstones.