Ⅻ Roman Numeral Converter
Convert between Arabic numbers and Roman numerals (1↔I, 4↔IV, 9↔IX, 3999↔MMMCMXCIX). Bidirectional.
Arabic → Roman
Roman → Arabic
About the Roman Numeral Converter
This converter runs entirely in your browser. It converts Arabic numbers (1 to 3999) to Roman numerals and back, using the standard subtractive notation. Both directions are validated: invalid inputs are rejected with a friendly message.
Roman numeral rules
- Symbols —
I=1, V=5, X=10, L=50, C=100, D=500, M=1000. - Additive notation — Symbols are summed left to right when each value is no larger than the previous: VI = 5 + 1 = 6.
- Subtractive notation — A smaller symbol before a larger one subtracts: IV = 5 − 1 = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900. Only these six subtractive pairs are valid.
- Range — Standard Roman numerals represent integers from 1 to 3999 (MMMCMXCIX). The tool rejects 0, negatives, and numbers ≥ 4000.
How to use the converter
- Pick the card for the direction you need.
- Type an Arabic number (1–3999) or a Roman numeral.
- Click Convert to see the result.
Frequently asked questions
Why is the limit 3999? Standard Roman numerals have no symbol for 5000 or above, so 3999 (MMMCMXCIX) is the largest value representable without extension conventions.
Is “IIII” valid? No. The subtractive form “IV” is the correct way to write 4. The converter rejects non-standard patterns.
Are lowercase letters accepted? Yes — the input is normalised to uppercase before parsing.
Is my input stored or uploaded? No. Everything happens in your browser. Your input is never sent to a server.