ToolWrenRoman numerals appear on clock faces, book chapters, film copyright years and monarch names — yet most of us only half-remember how they work. Here is the full system, plus the quickest way to convert in either direction.
The seven symbols
Roman numerals are built from just seven letters: I=1, V=5, X=10, L=50, C=100, D=500 and M=1000. You combine them, largest first, adding as you go: MMXV is 1000+1000+10+5 = 2015.
The subtraction rule
The clever part: when a smaller symbol comes before a larger one, you subtract it. So IV is 5−1 = 4, IX is 10−1 = 9, XL is 40 and CM is 900. This keeps numerals compact — without it, 4 would be IIII and 9 would be VIIII. Only certain pairs are valid (I before V or X, X before L or C, C before D or M).
Converting numbers to Roman numerals
To convert a number, break it into thousands, hundreds, tens and units and write each part in turn. 1984 = 1000 (M) + 900 (CM) + 80 (LXXX) + 4 (IV) = MCMLXXXIV. Rather than do it by hand, drop the number into our Roman numeral converter and it appears instantly.
Converting Roman numerals to numbers
Going back, read left to right and add each symbol's value, subtracting whenever a smaller one precedes a larger one. The converter does this both ways, so you can paste MCMLXXXIV and get 1984, or type 1984 and get MCMLXXXIV.
Why they stop at 3999
Standard Roman numerals only go up to 3999 (MMMCMXCIX), because there is no single symbol beyond M and writing four thousand as MMMM is discouraged. There is also no zero — the concept did not exist in the Roman system. For working in other number systems, see the number base converter (binary, octal, decimal, hex).