The seven symbols and how they combine
Roman numerals use seven letters: I for 1, V for 5, X for 10, L for 50, C for 100, D for 500 and M for 1000. Symbols are written from largest to smallest and added: VIII is 5 + 3 = 8 and MDCLXVI is 1666. To avoid four of the same symbol in a row, a smaller symbol placed before a larger one is subtracted. Only six such pairs are used in the standard form: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900).
Writing any number place by place
The easy way to convert is to split the number into thousands, hundreds, tens and units and write each part on its own, then join them. 1994 is 1000 + 900 + 90 + 4, so M + CM + XC + IV = MCMXCIV. A zero place is simply left out: 2006 is MM + VI = MMVI. The table under the result shows this split for whatever you type, which makes it a handy check for homework.
Why the limit is 3999
In the standard form no symbol is repeated more than three times, and there is no symbol above M, so the largest number you can write is MMMCMXCIX, 3999. There is also no Roman numeral for zero or for negative numbers. Larger values were historically written with a bar over a numeral to multiply it by 1000 and with other notations, but these were never standardised, so the converter keeps to 1 to 3999.
Common mistakes the converter catches
Four symbols in a row, as in IIII or XXXX, should be IV and XL. V, L and D are never repeated, so VV is X. Only I, X and C can be subtracted, and only from the next two larger symbols, so 99 is XCIX and not IC, and 49 is XLIX and not IL. Pieces must also come in the right order: IXI or MCMC are not valid. When the converter can work out what you meant, it suggests the standard spelling with its value.
Years, dates and where Roman numerals appear
Roman numerals still appear in film and TV copyright years, book chapters and front matter, monarchs and popes (Elizabeth II), recurring events such as the Olympic Games and the Super Bowl, outlines, and on clock and watch faces. Use This year for the current year, or the date converter for a birthday or anniversary written day, month and year, such as XVIII.X.MMXXVI for 18 October 2026. Check engravings and tattoos letter by letter before they are made permanent.