Roman Numeral Converter (1 to 3999, both directions)
Type a number 1-3999 to get the Roman numeral, or paste Roman numerals to convert back. Strict validation rejects non-canonical forms.
How it works
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.
Frequently asked questions
›Why isn't IIII accepted as 4?
Canonical Roman writes 4 as IV. IIII appears on some clock faces but is not standard arithmetic notation. Use IV.
›What's the largest Roman numeral?
Without overlines, 3999 = MMMCMXCIX. Numbers above that need an overline notation we don't support.
›Can I have zero?
Romans had no zero. The notation starts at 1 (I).
›What does CMXLIV mean?
CM=900, XL=40, IV=4. Total = 944. Subtractive pairs (CM, XL, IV) plus their components.
›How do I read MMXXVI?
MM=2000, XX=20, VI=6. Total = 2026.
›Why are clock 4s sometimes IIII?
Tradition. Watchmakers historically preferred IIII for visual symmetry with VIII (both four characters). It's not 'correct' canonically.
›Are Roman numerals base-10?
They have decimal-grouped letters (I/V for 1s, X/L for 10s, C/D for 100s, M for 1000s) but use addition and subtraction rather than positional notation. So they're decimal-influenced but not positional.
›Does the data leave my browser?
No. Conversion runs locally.
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