Roman Numeral Converter

What this tool does

Roman Numeral Converter writes whole numbers as Roman numerals using standard subtractive notation and reads numerals back as numbers. It validates the canonical form, explains why a numeral such as IIII or IC is non-standard, and supports the vinculum (overline) for values above 3,999.

Roman numerals are still everywhere: on clock faces, monarch and pope names, film copyright dates, book chapters, Super Bowl numbers and tattoos. They use seven letters, I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1,000, added from left to right, with one twist: a smaller letter placed before a larger one is subtracted, so IV is 4 and CM is 900. The rules for which subtractions are allowed are easy to forget, which is why mistakes like IC for 99 or MIM for 1999 appear on real buildings.

This converter follows the standard modern form. Numbers from 1 to 3,999 are written with the six permitted subtractive pairs (IV, IX, XL, XC, CD, CM), so 1994 becomes MCMXCIV and 2026 becomes MMXXVI. Strict validation accepts only that canonical form and explains the problem with anything else, while lenient mode reads historical additive forms such as IIII. For larger values you can enable the vinculum, a line over a letter that multiplies it by 1,000, written with the Unicode combining overline. Each line is detected and converted separately, entirely in your browser.

How to use Roman Numeral Converter

1. Enter numbers or Roman numerals in the Input box, one per line, or click Sample.
2. Keep Mode on “Detect per line”, or force one direction.
3. Leave “Strict validation” on to check numerals, or switch it off to read additive forms such as IIII.
4. Turn on the overline notation for numbers from 4,000 to 3,999,999.
5. Optionally choose lowercase numerals or “Show input = result”, then copy the output.

Tips for better results

• Only I, X and C are used subtractively, and only before the next two larger letters: I before V and X, X before L and C, C before D and M.
• V, L and D never repeat, and no letter appears more than three times in a row in standard form.
• Clock faces often show 4 as IIII; switch off strict validation to read those.
• Lowercase numerals (i, ii, iii) are the convention for preface page numbers in books.

Common use cases

• Working out the year in a film’s closing credits, such as MCMXCIX.
• Writing a date in Roman numerals for a wedding invitation or engraving.
• Checking a numeral before it is used in a tattoo or inscription.
• Numbering outline sections, appendices or event editions.

Common mistakes to avoid

• Writing 99 as IC or 1999 as MIM. The standard forms are XCIX and MCMXCIX.
• Expecting a Roman numeral for zero or negative numbers; the Romans had neither. The tool uses N (nulla) for zero, as medieval scholars did.
• Writing 4,000 as MMMM in contexts that expect standard numerals, which stop at 3,999.