To write a number as a Roman numeral, break it into thousands, hundreds, tens and units, then spell each part with the seven Roman letters. So 2026 is MM (2000) + XX (20) + VI (6) = MMXXVI. The Roman numeral converter does it both ways, with validation.
That is the shape of it. Here are the rules so you can read and write any numeral with confidence.
The seven letters
Every Roman numeral is built from seven letters, each a fixed value:
| Letter | I | V | X | L | C | D | M |
|---|---|---|---|---|---|---|---|
| Value | 1 | 5 | 10 | 50 | 100 | 500 | 1000 |
You combine them to make any number from 1 to 3999.
The two rules: add and subtract
Roman numerals work mostly by adding letters from largest to smallest. So VIII is 5 + 1 + 1 + 1 = 8, and CLXII is 100 + 50 + 10 + 1 + 1 = 162.
The exception is the subtraction rule, used to avoid writing four of the same letter in a row. When a smaller letter sits directly before a larger one, it is subtracted:
- IV is 5 − 1 = 4 (not IIII)
- IX is 10 − 1 = 9
- XL is 50 − 10 = 40
- XC is 100 − 10 = 90
- CD is 500 − 100 = 400
- CM is 1000 − 100 = 900
Only those six subtractive pairs are valid. You never write IL or IC, for example; 49 is XLIX, not IL.
How to write any number
Work through the number from largest place to smallest:
- Thousands: one M per thousand (up to MMM for 3000).
- Hundreds: use C, CD, D, CM as needed.
- Tens: use X, XL, L, XC.
- Units: use I, IV, V, IX.
Worked example, 1994:
- 1000 → M
- 900 → CM
- 90 → XC
- 4 → IV
Put together: MCMXCIV. The Roman numeral converter does this instantly and checks that a numeral you type is valid.
Reading a numeral back
To read one, scan left to right and add each letter, unless a letter is smaller than the one after it, in which case subtract it. So MMXXVI is 1000 + 1000 + 10 + 10 + 5 + 1 = 2026.
Why the limit is 3999
Standard numerals stop at 3999 because there is no single letter beyond M for a thousand. Larger values once used an overline above a letter to multiply it by a thousand, but plain text cannot show that, so most converters, including this one, cover 1 to 3999. There is also no symbol for zero, since the Romans had no need for one.
To explore how other number systems work, see how to convert binary to decimal.