ローマ数字コンバーター

What Is the Roman Numeral Converter?

The Roman numeral converter is a free online tool that translates Arabic numbers (1–3999) to Roman numerals and converts Roman numerals back to Arabic numbers. It supports the standard subtractive notation (IV, IX, XL, XC, CD, CM) and validates round-trip accuracy to ensure only valid Roman numeral strings are accepted.

Roman numerals appear in book chapters, clock faces, movie titles, sporting events, and architectural inscriptions. This converter handles the conversion instantly without requiring any knowledge of the numeral rules.

How to Use the Roman Numeral Converter

1. Select Arabic → Roman mode to convert a number like 2024 to MMXXIV.
2. Select Roman → Arabic mode to convert a Roman numeral like MCMXCIX to 1999.
3. Type your value or click a preset to load it instantly.
4. Use the ⇄ swap button to flip the current result back into the input.
5. Click Copy to copy the result to your clipboard.

Features

• Arabic to Roman numeral conversion (1–3999)
• Roman to Arabic numeral conversion with validation
• Swap button to chain conversions
• Preset values for common numbers and edge cases
• Invalid input detection and clear error messages
• Symbol reference table with Latin names
• One-click copy to clipboard

FAQ

What is the largest Roman numeral?

In the standard system, the largest Roman numeral is MMMCMXCIX (3999). Roman numerals do not have a symbol for zero or for numbers above 3999 in the classical notation.

Why does 4 become IV and not IIII?

The subtractive notation rule says that a smaller numeral placed before a larger one is subtracted. So IV = 5 − 1 = 4, and IX = 10 − 1 = 9. This convention became standard in the Middle Ages and is the form used today, though IIII still appears on some clock faces.

How do I convert 2024 to Roman numerals?

2024 = 2000 + 24 = MM + XXIV = MMXXIV. Break the number into its thousands, hundreds, tens, and units, then convert each part using the standard table.

Are Roman numerals still used today?

Yes. Roman numerals are still used for: clock faces, book chapter numbers, movie sequel titles and copyright years, names of monarchs and popes (e.g. King Charles III), Super Bowl numbering, and architectural inscriptions.