How Roman numerals work
Seven letters, one clever subtraction trick, and a fixed reading order. Learn the whole system in a few minutes, then try any number in the converter.
Type a number or a Roman numeral to convert.
The seven symbols
Roman numerals use just seven letters. Learn these and you can read most numerals on sight. Tap any symbol for its own page on where it comes from and the rules for using it, or see them all on the symbols reference.
The additive rule
Most of the time you simply add symbols from left to right, largest first. VIII is 5 plus 1 plus 1 plus 1, which is 8. CCC is 100 plus 100 plus 100, which is 300. A symbol never repeats more than three times in a row, which is exactly why the next rule exists.
The subtractive rule
To avoid four repeats, a smaller symbol placed before a larger one is subtracted. So 4 is IV rather than IIII, and 9 is IX rather than VIIII. The same idea scales up: 40 is XL, 90 is XC, 400 is CD and 900 is CM. Only I, X and C subtract, and only from the next one or two larger symbols.
A worked example: how to write 1984
- Split it by place value: 1000, 900, 80, 4.
- 1000 is M.
- 900 is CM (100 before 1000).
- 80 is LXXX (50 plus three tens).
- 4 is IV (1 before 5).
- Join them: MCMLXXXIV.
Common mistakes to avoid
- Writing IIII for 4. The standard form is IV, though clocks often keep IIII by tradition.
- Writing IC for 99. It must be XCIX, because subtraction only crosses one step at a time.
- Repeating a symbol four times, such as XXXX for 40. Use XL instead.
- Subtracting with V, L or D. Only I, X and C are ever placed before a larger symbol.
What about numbers over 3999?
Standard Roman numerals stop at 3999 (MMMCMXCIX). For larger values the Romans drew a bar, called a vinculum, over a numeral to multiply it by one thousand. A barred V means five thousand. It is rarely needed today, which is why most converters, including this one, focus on 1 to 3999.
Frequently asked questions
What are the seven Roman numeral letters?
I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. Every Roman numeral is built from these seven symbols.
What is the subtractive rule?
When a smaller symbol appears before a larger one, it is subtracted. IV is 4 (5 minus 1) and IX is 9 (10 minus 1). Only I, X and C are used this way, and only before the next one or two larger symbols.
Why is there no zero in Roman numerals?
The Roman system is additive and had no concept of a placeholder zero. Counting starts at I, and there is simply no symbol for nothing.
What is the largest standard Roman numeral?
3999, written MMMCMXCIX. Larger values use an overline, called a vinculum, that multiplies a numeral by one thousand.