What is a Roman numeral converter?
A Roman numeral converter changes an ordinary number into Roman numeral notation, or reads a Roman numeral and returns its number. Roman numerals appear on clock faces, book chapters, film sequels, copyright dates, monarch names, events and formal outlines. This tool converts both directions using the common modern Roman numeral rules.
Enter a whole number from 1 through 3,999 to see its Roman form, or type a Roman numeral from I through MMMCMXCIX to see its value. The inputs are checked as you type, and every conversion happens in your browser. Your entries are not uploaded or stored.
How to use the converter
- Type a whole number between 1 and 3,999 in the left field to convert it to Roman numerals.
- Type a Roman numeral in the right field to convert it to a number.
- Use the standard letters I, V, X, L, C, D and M.
- Correct any message shown for an invalid number or numeral.
For example, 2026 becomes MMXXVI. The numeral XLII becomes 42, while CMXCIX becomes 999. The converter accepts lowercase input too, then changes it to uppercase for clear display.
How Roman numerals work
Roman numerals combine letters with fixed values: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1,000. Usually, values are written from largest to smallest and added together. For example, VIII is 5 + 1 + 1 + 1, or 8.
Some smaller symbols before larger symbols use subtraction. IV is 4 because I comes before V, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. These pairs avoid writing the same symbol four times in a row. Modern standard notation normally repeats I, X, C and M no more than three times consecutively.
Why validation matters
Not every arrangement of Roman letters is standard. For example, IIX is not the usual way to write 8; VIII is correct. The converter validates the conventional modern forms rather than merely adding letter values, which helps spot mistyped numerals. Very large Roman values can use historical overline conventions, but those rules vary and are outside this tool's 1–3,999 range.