Roman numerals
Type a number or a Roman numeral: it converts as you type, in both directions, and automatically flags malformed numerals.
| Subtractive pair | Value | Rule |
|---|---|---|
| IV | 4 | I is subtracted only from V and X |
| IX | 9 | never IIII or VIIII |
| XL | 40 | X is subtracted only from L and C |
| XC | 90 | never LXXXX |
| CD | 400 | C is subtracted only from D and M |
| CM | 900 | never DCCCC |
How the Roman numeral system works
Seven letters cover everything: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000. Values add up from left to right, with one exception: when a smaller symbol comes before a larger one, it's subtracted. A symbol repeats at most three times in a row, and V, L, and D never repeat at all — which is why IIII, VV, and XXXX aren't valid Roman numerals. This tool flags them right away instead of quietly giving you a wrong answer.
Why the limit is 3999
In standard notation the largest writable number is MMMCMXCIX, i.e. 3999: anything bigger would need four Ms in a row, which the three-repetition rule forbids. For huge numbers the Romans used the "vinculum," a bar drawn over a letter that multiplies its value by a thousand. That's also why Roman numerals today show up on clock faces, in Super Bowl editions (Super Bowl LVIII), movie sequels, kings and popes (Louis XIV), and book chapters — but never in actual arithmetic.