Roman Numerals Explained: How the System Works

Roman numerals represent numbers using letters rather than digits, and once you know the seven symbols and one subtraction rule behind them, reading and converting any Roman numeral becomes straightforward.

The Seven Symbols

The Subtractive Notation Rule

Most values are built by writing symbols in decreasing order and adding them together (VI = 5 + 1 = 6). But when a smaller symbol appears immediately before a larger one, it’s subtracted instead of added: IV means 5 − 1 = 4, not 1111. This subtractive rule only applies to specific pairs: IV (4), IX (9), XL (40), XC (90), CD (400), and CM (900).

Worked Example

Common Mistakes

• Assuming repetition is unlimited. A symbol is never repeated more than three times in a row (III is fine, IIII is not, it’s written IV instead).
• Applying the subtraction rule too broadly. Only specific smaller-before-larger pairs subtract (IV, IX, XL, XC, CD, CM); other combinations don’t follow the same pattern.
• Reading numerals left to right without checking for subtractive pairs. Skimming past an “IV” as if it were “I” then “V” separately produces the wrong total.

Use the Roman Numeral Converter

The Roman Numeral Converter converts between Roman numerals and standard numbers instantly, in both directions.