How it works
Roman numerals use seven letters: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1000. Values are added from left to right, except when a smaller numeral comes before a larger one, which means subtract: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900.
Standard notation covers 1 to 3999 (MMMCMXCIX). The converter checks that numerals follow the standard rules, so forms like IIII or IC are flagged with a suggestion.
Formula
Subtractive pairs: IV 4 · IX 9 · XL 40 · XC 90 · CD 400 · CM 900
Questions people ask
Why is there no Roman numeral for zero?
The Roman system was built for counting and recording quantities and had no positional notation, so it never needed a zero symbol.
Why do some clocks show IIII instead of IV?
It is a long-standing clockmaking tradition, often said to balance the dial visually against VIII. Standard notation uses IV.
How are numbers above 3999 written?
Historically with a bar (vinculum) over a numeral to multiply it by 1,000, so V̄ = 5,000. That extension is not standardized and is not supported here.
What year is MCMLXXXIV?
M (1000) + CM (900) + LXXX (80) + IV (4) = 1984.