Roman Numeral Converter — Arabic ⇄ Roman, Both Directions

Real-world use cases

• Read a movie or book copyright year off the credits

  Films and many books still stamp the copyright year in Roman numerals — you'll see MCMXCIX flash by at the end of a 90s movie. Workflow: pause on the credits, type the numeral into the Roman field (case doesn't matter, mcmxcix works), and read 1999 back in the Arabic field. The breakdown line confirms it: M + CM + XC + IX. Handy when you're cataloguing a DVD collection or dating an old reprint where the only year is the Roman copyright line.

• Number book chapters or appendix sections like a publisher

  Front matter (preface, foreword, table of contents) is conventionally paginated in lowercase Roman — i, ii, iii, iv — while appendices and parts often use uppercase. If you're laying out a manuscript and need part XVII or page xxiv, type the Arabic number, copy the numeral, and paste it into your document. The repeat-rule validation saves you from shipping XXXX (invalid) when you meant XL, a mistake that slips past a quick eyeball.

• Teach the subtractive system in a maths lesson

  Showing a class why 4 is IV and not IIII is far more convincing live than on a static slide. Type 4, 9, 40, 90, 400, 900 via the preset chips and let students watch the breakdown reveal the subtractive pair each time (4 = IV, 90 = XC). Then ask them to predict 1994 before you type it, and reveal M + CM + XC + IV. The strict error messages ("only IV IX XL XC CD CM are valid") turn common student mistakes into teachable moments.

• Check a tattoo or engraving before it's permanent

  People immortalise birth years and anniversaries in Roman numerals on jewellery, tattoos and plaques — and a wrong numeral is expensive to fix. Before the engraver cuts, paste the proposed numeral into the Roman field. If it lights up red with "not a canonical Roman numeral", you just saved a permanent mistake. If it cleanly resolves to the right year in the Arabic field, you're good to commit.

• Build numbered list styles or super-bowl-style branding

  Designers reach for Roman numerals for event editions (Super Bowl LVIII), monarch and pope names (Louis XIV), and CSS `list-style-type: upper-roman`. When you need the numeral for a specific edition number — what's the 59th Super Bowl? — type 59, copy LIX, drop it into the mockup. Vinculum mode also covers the rare oversized cases (years like 2024 are trivial; large tallies are not) without forcing you to hand-stack symbols.

Common pitfalls

• Writing four as IIII. It reads as 4 to a human and shows up on clock faces, but it violates the subtractive rule (you may repeat I at most three times). Canonical is IV. This tool rejects IIII as non-canonical so you don't ship it into a document or engraving where the standard form is expected.

• Inventing subtractive pairs like IL for 49 or IC for 99. Only six pairs are legal: IV IX XL XC CD CM. 49 is XLIX (40 + 9), 99 is XCIX (90 + 9). A subtracting symbol must be a power of ten and sit before the next two larger symbols only — IL and IC break both halves of that rule.

• Repeating V, L or D. These five-based symbols never double up — VV is not 10 (that's X) and LL is not 100 (that's C). The parser flags any repeated V/L/D immediately. If you find yourself wanting VV, the symbol you actually need already exists one rung up.

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