Roman Numeral Converter

Features

• Both directions with live updating
• Strict validation rejecting malformed numerals
• Symbol-by-symbol breakdown of the conversion
• Covers 1 to 3,999 in standard notation
• One-click conversion of the current year

How to use it

1. Type a number from 1 to 3999 on the left.
2. Or type a Roman numeral on the right.
3. The other box updates immediately.
4. The table below shows how the value decomposes.

The rules of Roman numerals

Seven symbols carry all the values: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1,000. Numerals are written largest to smallest and added together, so MMXXVI is 1000 + 1000 + 10 + 10 + 5 + 1 = 2026.

Subtractive notation is the complication. Placing a smaller symbol before a larger one subtracts it, so IV is 4 rather than IIII. But only six subtractive pairs are valid, IV, IX, XL, XC, CD and CM, and only powers of ten may be subtracted, from the next two values up. That rules out IL for 49, which must be written XLIX. This converter enforces those rules, so pasting in a malformed numeral produces an error rather than a silently wrong number.

Standard notation stops at 3,999, because expressing 4,000 would need four Ms. Larger values historically used a vinculum, an overbar multiplying a numeral by 1,000, but that convention was never fully standardised and does not render reliably in plain text, so this tool caps at MMMCMXCIX. There is also no zero: the concept simply had no symbol in the system, which is one reason Roman numerals are hopeless for arithmetic.