How the conversion works
To turn a number into Roman form, the converter works from the largest symbol downward, subtracting the biggest value that still fits and writing its letter, then repeating with the remainder. Special two-letter pairs like IV (4) and CM (900) handle the cases where a smaller symbol sits before a larger one to mean subtraction.
Going the other way, each letter is read left to right. A symbol is added to the running total unless it is smaller than the symbol immediately after it, in which case it is subtracted. The tool then re-encodes the result and checks it matches, which rejects malformed inputs.
The symbols and their values
Seven letters cover the whole system, combined with a few subtractive pairs.
- I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000
- IV = 4, IX = 9 (one before five or ten)
- XL = 40, XC = 90 (ten before fifty or a hundred)
- CD = 400, CM = 900 (a hundred before five hundred or a thousand)
Rules that make a numeral valid
Roman numerals follow strict spelling rules, which is why some letter combinations look reasonable but are not allowed. Knowing them helps you spot typos in dates, copyrights and chapter numbers.
- A symbol repeats at most three times in a row, so 4 is IV, not IIII
- Only I, X and C are used subtractively, never V, L or D
- A subtractive letter precedes only the next one or two sizes up
- Larger groups are written before smaller ones from left to right
Range and common mistakes
Standard Roman numerals run from 1 to 3999 because there is no single symbol larger than M, and writing four Ms would break the no-more-than-three rule. There is also no symbol for zero or for fractions.
When converting in the Roman to number direction, the input must be a genuinely valid numeral. Sequences like VX or IIII are flagged as errors rather than guessed at, so double-check spelling if a value is rejected.
Formula
M=1000, D=500, C=100, L=50, X=10, V=5, I=1 (subtractive pairs allowed)Frequently asked questions
- Why is there a limit of 3999?
- Standard Roman numerals have no symbol above M (1000), so the largest value writable with up to three Ms is 3999 (MMMCMXCIX).