Roman numerals use seven letters — I (1), V (5), X (10), L (50), C (100), D (500) and M (1000) — combined with a subtractive rule (IV = 4, IX = 9, XL = 40, and so on). This converter turns any whole number from 1 to 3999 into its standard Roman form, and reads a Roman numeral back into an ordinary Arabic number.
The classic Roman system has no zero and no simple notation above about 3999, so numbers outside 1–3999, non-integers and malformed numerals are rejected with an error message. Decoding is case-insensitive. Everything runs locally in your browser — nothing is uploaded.