Roman numeral converter

How it works

Roman numerals are written with seven letters: I, V, X, L, C, D and M for 1, 5, 10, 50, 100, 500 and 1000. A smaller letter before a larger one subtracts it, so IV is 4 and XC is 90; otherwise the values add, so VI is 6. To convert a number, the tool walks down the value table, taking as many of each step as fit: 1994 takes M (1000), then CM (900), then XC (90), then IV (4). In the other direction it reads each letter, subtracting when the next letter is larger and adding otherwise, and checks the result against the canonical pattern so strict forms such as IIII or IC are refused. Only whole numbers from 1 to 3999 are accepted.

Worked example

1994 becomes MCMXCIV: M covers 1000, CM covers 900, XC covers 90 and IV covers 4, and 1000 + 900 + 90 + 4 = 1994. Reading MCMXCIV back gives the same 1994, because each subtractive pair is in its canonical place.

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