Roman Numeral Converter

How to use

1. Type a number from 1 to 3999, or a Roman numeral such as MMXXIV — the tool detects which one you entered and converts in the right direction.
2. See the Roman numeral and its additive breakdown — for example, 1994 becomes M + CM + XC + IV, showing how each symbol contributes.
3. Use the example buttons to try common values, and the symbol table to check the seven letters and the six valid subtractions.

About this tool

Roman numerals combine seven letters — I, V, X, L, C, D and M — worth 1, 5, 10, 50, 100, 500 and 1,000. To write a number, this converter walks a list of thirteen values in descending order (1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1) and subtracts the largest one that still fits, appending its symbol each time. That greedy pass is what produces the canonical form and nothing else: 1994 becomes M + CM + XC + IV = MCMXCIV, and the breakdown is printed on screen so you can follow every step.

Reading a numeral back is where most converters get sloppy, because plenty of malformed strings still add up to something. This one validates first, against the canonical regular expression M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3}), which encodes the real rules: I, X, C and M may repeat at most three times; V, L and D never repeat; and only six subtractive pairs exist — IV, IX, XL, XC, CD and CM. So IIII is rejected (it must be IV), IC is rejected (99 is XCIX), VX and XXXX are rejected too. Only after a numeral passes does the tool scan it left to right, taking two-letter pairs before single letters, and add the values up. Every number from 1 to 3999 round-trips exactly.

The 1 to 3999 range is not an arbitrary limit: the system has no zero, no negative numbers and no fractions, and 3999 (MMMCMXCIX) is the largest value the seven standard letters can express without the medieval overline that multiplied a symbol by a thousand. In practice the tool is used for the things Roman numerals survived into — dating a monument, a film copyright or a book preface, numbering chapters, volumes, popes, monarchs and Super Bowls, reading a clock face, checking a tattoo or engraving before it becomes permanent, and homework. Conversion and validation run 100% in your browser; nothing is sent anywhere.

The formula

Symbols: I = 1 · V = 5 · X = 10 · L = 50 · C = 100 · D = 500 · M = 1000. Subtractive pairs: IV = 4 · IX = 9 · XL = 40 · XC = 90 · CD = 400 · CM = 900. To convert a number, subtract the largest value in the list 1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1 that fits and append its symbol, repeating until zero. Validation uses the canonical pattern M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3}). Examples: 1994 = MCMXCIV, 2026 = MMXXVI, MMXXIV = 2024.

Frequently asked questions