Roman Numeral Converter
Convert decimals to Roman numerals and Roman numerals to decimals. Supports 1 to 3999 with full subtractive-notation validation. Browser-based.
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What is the Roman Numeral Converter?
A Roman numeral converter translates between modern decimal numbers (42) and the seven-symbol Roman system (XLII). The Romans used I, V, X, L, C, D, M for 1, 5, 10, 50, 100, 500, and 1000, with subtractive pairs like IV and IX for numbers just below the next symbol. Modern uses are mostly decorative: clock faces, chapter headings, copyright years.
How to use the Roman Numeral Converter
- 1
Type a decimal number
Enter any whole number between 1 and 3999 in the left field. The Roman equivalent appears on the right instantly.
- 2
Or type a Roman numeral
Drop a Roman string into the right field (lowercase is fine, it auto-uppercases). The decimal value appears on the left.
- 3
Watch for validation errors
If you type something like IIII or VV, the converter flags it as invalid. The standard rules are strict: no symbol repeats more than three times, and only specific subtractive pairs are allowed.
- 4
Copy the value you need
Each field has a copy icon next to it. Grab whichever form you need for your document, design, or homework.
Frequently Asked Questions
What number range is supported?
1 to 3999. Romans had no symbol for zero, and numbers above 3999 normally need an overline (a line over a symbol multiplies it by 1000), which most fonts can't render correctly. The 1-to-3999 range covers every practical use: years, chapter numbers, clock faces, movie credits.
How does validation work?
The converter parses your input against standard Roman rules: no symbol repeats more than three times in a row (so IIII is invalid, four is IV), and only six subtractive pairs are allowed (IV, IX, XL, XC, CD, CM). Anything that fails these checks shows an error instead of a wrong number.
What are the subtractive rules?
When a smaller value sits before a larger one, you subtract: IV is 4 (5 minus 1), IX is 9, XL is 40, XC is 90, CD is 400, CM is 900. These are the only six valid subtractive pairs in standard notation. Anything else (like IL for 49 or IC for 99) is non-standard and flagged as invalid.