Roman Numeral Converter.
Convert numbers to Roman numerals and back, anywhere from 1 to 3999. Type in either box and the other updates; malformed numerals like IIII or IL are rejected rather than silently "fixed." Everything runs in your browser.
How Roman numerals work.
Seven letters carry values: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. You write the largest values first and add them up — except for six subtractive pairs where a smaller letter before a larger one means "subtract": IV=4, IX=9, XL=40, XC=90, CD=400, CM=900.
Those rules make the notation strict: 4 is IV, never IIII; 49 is XLIX, never IL. This converter validates by round-trip — it only accepts a numeral if re-encoding the value reproduces exactly what you typed — so non-canonical forms are flagged instead of quietly accepted.
The classic system has no zero and tops out conventionally at 3999 (MMMCMXCIX); larger numbers historically used a bar over a letter to mean ×1000, which this tool doesn't cover.