Why is the range limited to 1 to 3999?

Standard Roman numerals only use the letters I, V, X, L, C, D and M. The largest value expressible without overlines or other extensions is 3999 (MMMCMXCIX). Zero and negative numbers have no Roman representation.

What is the subtractive rule?

A smaller numeral placed before a larger one is subtracted: IV is 4 and IX is 9. Only I, X and C may be used subtractively, and only before the next one or two larger symbols, which is why 4 is IV and never IIII in standard form.

Does the parser reject invalid numerals?

Yes. It rejects strings that do not round-trip, such as IIII, VX or IC. A valid numeral must re-encode to exactly the same text, so malformed input is flagged rather than silently accepted.

How do I write a year like 1999?

1999 is MCMXCIX: M is 1000, CM is 900, XC is 90 and IX is 9. The tool builds this automatically by repeatedly subtracting the largest possible value-symbol pair.

Are lowercase Roman numerals supported?

Yes. The parser uppercases your input before reading it, so mcmxcix and MCMXCIX both decode to 1999. Output is always uppercase, the conventional form.

Roman Numeral Converter

Roman numerals encode numbers with seven letters — I, V, X, L, C, D, M — using addition and a limited subtractive rule. This converter turns any integer from 1 to 3999 into a correctly-formed numeral and parses numerals back to integers, rejecting malformed input.

How it works

Encoding uses a greedy table of value-symbol pairs, including the subtractive pairs, processed from largest to smallest:

1000 M   900 CM   500 D   400 CD
 100 C    90 XC    50 L    40 XL
  10 X     9 IX     5 V     4 IV
   1 I

For each pair, the algorithm appends the symbol while the remaining value is greater than or equal to that pair’s value, then subtracts it. Decoding scans the string left to right: if a symbol’s value is less than the next symbol’s value it is subtracted, otherwise it is added. To validate, the decoded integer is re-encoded and compared to the original text — anything that does not match exactly (such as IIII or IC) is reported as invalid.

Example and tips

The year 2024 becomes MMXXIV: two M’s for 2000, XX for 20, and IV for 4. Note that the subtractive rule is strict — 99 is XCIX (90 + 9), never IC, because a smaller symbol may only precede the next one or two sizes up. The tool enforces this so the numerals you copy are valid for clock faces, book chapters, film copyright dates, and monument inscriptions.