Roman Numeral Converter

Turning a number into a numeral is a greedy walk down a thirteen-entry table: take the largest value that still fits, write its symbol, subtract, repeat. Because that table already holds the six subtractive pairs — IV, IX, XL, XC, CD and CM — the result is always the modern shortest form, which is why 2024 becomes MMXXIV in six characters built from four distinct symbols, and why you will never see the IIII that clock faces traditionally use for four. The ceiling of 3999 is structural rather than arbitrary. M is the largest symbol available, and anything beyond needs the vinculum, an overline that multiplies a numeral by a thousand and which no ordinary text field can render. There is likewise no zero, the Romans having had no symbol for it, and no negative numbers, so the entry field starts at 1.

One behaviour is worth stating plainly: choosing Roman Numeral to Number does not give you a text box for typing MCMXCIV. Both modes read the same numeric field and produce the same pair of values; the only thing that changes is which way round the Conversion row prints the arrow. The two statistics underneath count the characters in the numeral and how many different symbols it uses, which is how you can discover that 3888 is the longest of them all at fifteen characters. Fractions are outside the system entirely — Rome handled those in twelfths with their own marks — as are the medieval variants that broke these rules freely.