Guides / Number systems

How Roman Numerals Work

Roman numerals use seven letters to write numbers. They still appear on clocks, book chapters, film dates and monuments, so it helps to know how to read them.

The seven letters

I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. Larger numbers are built by adding letters together, so VI is 6 and XV is 15.

The subtraction rule

When a smaller letter comes before a larger one, it is subtracted. IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. Only these six pairs use subtraction.

Reading 1994

Split the number into thousands, hundreds, tens and units: 1000 is M, 900 is CM, 90 is XC and 4 is IV. Together they make MCMXCIV. Read it left to right, adding each part.

Limits

Traditional Roman numerals have no zero and no symbol for numbers above 3999 in standard form. Repeating a letter is limited to three times, so 4 is IV rather than IIII. Our converter follows these standard forms.

Try it yourself

The Roman Numeral Converter converts in both directions and rejects forms that break the rules.