How Roman numerals work
The whole system in one read: seven symbols, two rules, and a handful of pairs to memorise.
The seven symbols
Roman numerals use seven letters, each with a fixed value. Everything else is built by combining these.
| Symbol | Value |
|---|---|
| I | 1 |
| V | 5 |
| X | 10 |
| L | 50 |
| C | 100 |
| D | 500 |
| M | 1,000 |
Rule one: add from large to small
Write the biggest values first and add as you move right. VIII is 5 + 1 + 1 + 1 = 8. LX is 50 + 10 = 60. MCC is 1000 + 100 + 100 = 1200. As long as each symbol is the same size or smaller than the one before it, you simply sum them.
Rule two: subtract when small comes first
To avoid four of the same letter in a row, a smaller symbol placed before a larger one is subtracted. IV is 5 minus 1, which is 4. IX is 10 minus 1, which is 9. XL is 50 minus 10, which is 40. Only I, X and C subtract, and only before the next one or two larger symbols. You will never see IL or IC; those are written XLIX and XCIX.
Reading a longer numeral
Split it at each place value. Take MCMXCIV: M is 1000, CM is 900, XC is 90, IV is 4. Add them and you get 1994. Working left to right in groups is exactly how the converter on this site parses your input.
Common mistakes
- Writing IIII for four instead of IV, except on clock faces where IIII is traditional.
- Repeating V, L or D. These never double up, so 10 is X, not VV.
- Over-subtracting, such as writing IC for 99. The correct form is XCIX.
- Forgetting that there is no zero and no fraction in the standard system.
Frequently asked questions
What is the rule for Roman numerals?
Write symbols from largest to smallest and add them up. When a smaller symbol comes before a larger one, subtract it instead. No symbol repeats more than three times in a row, which is why four is IV, not IIII.
Why is 4 written as IV?
To avoid writing four identical letters (IIII), the Romans put a smaller symbol before a larger one to mean 'one less'. So IV is one before five, which is four. The same idea gives IX for nine and XL for forty.
What symbols can be subtracted?
Only I, X and C are used for subtraction, and only before the next one or two larger symbols. I goes before V and X, X goes before L and C, and C goes before D and M. You never subtract V, L or D.
Common questions
A few specific questions come up again and again once the rules click. Each has its own worked answer:
Put it into practice
Last updated July 22, 2026. Reviewed for accuracy against the standard rules for Roman numerals.