1. 01Type an integer from 1–3999 on the left and the standard Roman numeral appears on the right instantly.
2. 02Or type a Roman numeral on the right; it is displayed in upper case and converted back.
3. 03If the notation is invalid, read the specific reason below the input box.
4. 04Click an example such as 4, 9, 40, 90, 1994, 2024 or 3999, or the current year, for a quick test, then copy either side's result.

Range

— integers from 1–3999 only; 0, negative numbers, decimals and anything from 4000 up are outside the standard range.

Grammar

— parsing follows the standard unique notation and allows only the six subtractive combinations IV, IX, XL, XC, CD and CM.

Repetition

— I, X, C and M may appear at most three times in a row; V, L and D cannot repeat.

Characters

— any character outside I/V/X/L/C/D/M is an error; case does not affect parsing.

What it does not do

— it does not support overline thousand-multiplier notation, the loose clock-face IIII style or Chinese numeral conversion.

Conversion runs entirely in your browser; the input is neither saved nor uploaded.