How Roman numerals work
Seven letters carry the values: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1000. Letters are added from left to right, so XVI is 10 + 5 + 1 = 16. When a smaller letter comes before a bigger one it is subtracted: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. So 1994 is MCMXCIV: 1000 + 900 + 90 + 4.
The converter shows the parts a number is built from, so you can follow the working. A numeral typed in is checked against the standard form: IIII is refused (use IV), and so are odd combinations such as VX or IC.
Questions
Why does it stop at 3999? The standard letters can write up to MMMCMXCIX. Bigger numbers used a bar over a letter to multiply it by 1000, which is outside this tool.
Is there a zero? No. Romans had no symbol for zero, so the first number is 1.
Where are Roman numerals used today? On clock faces, film and book copyright years, chapter and outline headings, monarchs and popes (Elizabeth II), and sporting events such as the Super Bowl. The “This year” button shows the current year as a numeral.