Roman Numeral Converter
A Roman numeral converter translates whole numbers (1 to 3999) into standard-form Roman numerals using subtractive notation (like IV for 4), and parses numerals back into numbers, validating that they are well-formed. Subtractive notation, writing IV instead of IIII for 4, became the standard form sometime after ancient Rome, which is why a well-formed modern Roman numeral never repeats a symbol more than three times in a row — a rule this tool enforces when validating a Roman-to-number conversion.
How it works
- Choose a direction: number to Roman, or Roman to number.
- Enter a whole number from 1 to 3999, or a Roman numeral.
- Read the converted result.
Formula Greedy subtractive-notation encoding (M, CM, D, CD, C, XC, L, XL, X, IX, V, IV, I)
Frequently asked questions
What is the largest number this supports?
3999 (MMMCMXCIX) — the largest value classical Roman numerals represent without extra notation like an overline for thousands.
Why does "IIII" for 4 not work?
Standard form uses subtractive notation (IV), not four repeated symbols. This tool validates against standard form and rejects non-standard variants.
Is there a Roman numeral for zero?
No — Roman numerals have no symbol for zero, so 0 (and negative numbers) cannot be converted.
Are lowercase letters accepted for Roman-to-number?
Yes — input is case-insensitive (mcmxciv works the same as MCMXCIV).
Does this validate malformed numerals like "VV" or "IC"?
Yes — the result is checked by converting it back to a numeral and confirming it matches standard form exactly.