Free Roman Numeral Converter 2026 — Numbers 1–3999 (Both Directions)

How Roman numerals work

Roman numerals use seven letters: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Numbers are formed by addition (II=2, XX=20) or by subtraction when a smaller letter precedes a larger one (IV=4, IX=9, XL=40, XC=90, CD=400, CM=900). Each letter can repeat at most three times in a row, which is why 4 is IV, not IIII.

Standard Roman notation supports 1 to 3999. Numbers ≥ 4000 require an overline notation (V̄ = 5000) that's hard to type and rarely needed today, so this converter caps at 3999.

Canonical vs informal Roman

Strict canonical form follows the rules above. 'Informal' Roman (sometimes called 'archaic') allows things like IIII for 4 — you'll see this on clock faces, where the symmetry is preferred. This converter requires canonical form for input and produces canonical form on output. So 'IIII' is rejected; type 'IV' instead.

The decoder validates by re-encoding: parse the Roman, get the number, encode that number, and check it matches the input. Anything that doesn't round-trip is rejected. This catches 'VV' (incorrectly = 10), 'IIX' (incorrectly = 8), and other non-standard forms.

Where Roman numerals appear today

Movie copyright dates (MCMXCIX = 1999), Super Bowl numbering, monarch and pope sequences (Henry VIII, John XXIII), book chapters and outlines, clock faces, and stylized year markers. They've largely fallen out of arithmetic use because Arabic-numeral arithmetic is far easier — try multiplying MCXLVII by IV and you'll see why.