How Roman Numerals Work: Rules and Easy Conversion
Roman numerals still turn up on clock faces, in book chapters and outlines, in the names of monarchs and sequels, and in the credits of some films and television shows. Many people can read the simple ones, such as III or XII, but get stuck on something like MCMXCIV.
The system is small and consistent. This guide lists the symbols, explains the rules that decide when to add and when to subtract, gives a place-by-place method for writing any number, and shows the errors to watch for. Every example is checked by converting in both directions.
The seven symbols
Roman numerals use seven letters, each with a fixed value:
| Symbol | I | V | X | L | C | D | M |
|---|---|---|---|---|---|---|---|
| Value | 1 | 5 | 10 | 50 | 100 | 500 | 1,000 |
The system is not positional like our everyday digits. The value of a letter does not depend on which column it sits in, only on what stands next to it. There is also no symbol for zero.
When to add and when to subtract
Read from left to right. When a symbol is followed by one of the same or lower value, you add it: VI is 5 + 1 = 6, and XVIII is 10 + 5 + 1 + 1 + 1 = 18. When a smaller symbol comes before a larger one, you subtract it: IV is 5 − 1 = 4.
In the standard modern form, subtraction is limited to six pairs:
- IV = 4 and IX = 9
- XL = 40 and XC = 90
- CD = 400 and CM = 900
Only I, X and C can be placed in front of a larger symbol, and each can subtract only from the next two symbols up. A symbol is repeated at most three times in a row, so 3 is III but 4 is IV, and 30 is XXX but 40 is XL. The symbols V, L and D are never repeated and never subtracted.
Some clock faces write 4 as IIII, and old inscriptions are not always consistent. The rules above describe the standard form used in modern writing.
Writing any number, place by place
The simplest reliable method is to split the number into thousands, hundreds, tens and ones, write each part with this table, and join them from left to right.
| Place | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|
| Ones | I | II | III | IV | V | VI | VII | VIII | IX |
| Tens | X | XX | XXX | XL | L | LX | LXX | LXXX | XC |
| Hundreds | C | CC | CCC | CD | D | DC | DCC | DCCC | CM |
| Thousands | M | MM | MMM |
Worked example, 1994: one thousand is M, nine hundred is CM, ninety is XC and four is IV. Joined together: MCMXCIV.
Worked example, 2026: two thousand is MM, no hundreds, twenty is XX and six is VI, giving MMXXVI. And 444 is four hundred (CD), forty (XL) and four (IV): CDXLIV.
Because the thousands row stops at MMM, the largest number the standard rules can write is MMMCMXCIX, which is 3,999. Larger values were written with extensions, such as a bar over a numeral to multiply it by 1,000, but everyday use rarely needs them.
Reading a numeral
Go left to right and compare each symbol with the one after it. If the next symbol is larger, subtract the current one; otherwise add it.
Worked example, MCMXCIV: M (1,000) is followed by a smaller C, so add 1,000. C (100) is followed by a larger M, so subtract 100. M adds 1,000. X (10) is followed by a larger C, so subtract 10. C adds 100. I (1) is followed by a larger V, so subtract 1. V adds 5. The total is 1,000 − 100 + 1,000 − 10 + 100 − 1 + 5 = 1,994.
A faster habit is to look for the six pairs and read each as a block: M, CM, XC, IV gives 1,000 + 900 + 90 + 4.
Common mistakes
- IL for 49. I may only subtract from V and X. Write 40 and 9 separately: XL then IX, giving XLIX.
- IC for 99. The same rule applies, so 99 is XC and IX: XCIX.
- IIX for 8. Never put two small symbols before a larger one. Eight is VIII.
- XXXX for 40. Four of the same symbol in a row breaks the three-repeat rule. Use XL.
- Treating it as positional. XI is 11, not 1 and 1 in two columns. Each letter is a value, not a place.
A quick check is to convert your answer back. Reading XLIX gives XL (40) plus IX (9) = 49, which confirms the result. The Roman numerals converter converts in both directions and is handy for dates, chapter numbers and checking your work.
- Roman Numerals Converter: Type a whole number to see its Roman numeral, or type a Roman numeral such as MCMXCIV to see the number. The answer updates as you type.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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