Roman Numeral Converter
Convert numbers into Roman numerals and Roman numerals back into numbers. Type a number like 2024 to get MMXXIV, or a numeral like MCMXCIV to get 1994 — the tool works out which direction you mean. It validates numerals strictly, so a malformed sequence such as IIII or IC is flagged with an explanation rather than quietly mis-converted. The standard range is 1 to 3999.
How to use this tool
- Type a whole number between 1 and 3999 to convert it to a Roman numeral.
- Or type a Roman numeral to convert it back to a number.
- The tool detects which direction you mean automatically.
- For numbers, it shows how the numeral breaks down piece by piece.
- Invalid numerals are rejected with a message explaining what is wrong.
The formula
Roman numerals combine seven letters with an additive rule, using subtraction for the four cases IV, IX, XL, XC, CD and CM. Converting a number greedily takes the largest value that fits, repeatedly.
I=1 V=5 X=10 L=50 C=100 D=500 M=1000
subtractive: IV=4 IX=9 XL=40 XC=90 CD=400 CM=900
value = sum of symbols, subtracting a smaller before a larger- additive
- Symbols add up: XXVII = 10+10+5+1+1 = 27
- subtractive
- A smaller before a larger subtracts: IX = 10−1 = 9
A numeral is valid only if it is the canonical way to write its value. IIII and IC are rejected because 4 is IV and 99 is XCIX.
Worked examples
A year
- Given
- 2024
- Result
- MMXXIV
Two thousands (MM), twenty (XX), and four (IV) combine to MMXXIV.
A subtractive numeral
- Given
- MCMXCIV
- Result
- 1994
M (1000) + CM (900) + XC (90) + IV (4) = 1994, showing three subtractive pairs.
Catching an error
- Given
- IIII
- Result
- Rejected
Four is written IV, not IIII, so the tool flags it as invalid rather than returning 4.
Frequently asked questions
Standard Roman numerals cover 1 to 3999. There is no symbol for zero and no widely agreed standard for numbers above 3999 in plain text — larger values historically used an overline (a vinculum) to multiply by a thousand, which does not translate cleanly to ordinary characters.
Some clock faces do show IIII for four as a stylistic and visual-balance choice, but it is not the standard mathematical form. In proper Roman numeral notation four is IV, so the converter treats IIII as invalid to keep conversions unambiguous.
When a smaller symbol appears before a larger one, it is subtracted: IV is 5−1 = 4, IX is 10−1 = 9, CM is 1000−100 = 900. Only six such pairs are valid (IV, IX, XL, XC, CD, CM), and each may appear once, which is why sequences like IC or VX are not allowed.
No. You can type numerals in either case — mcmxciv or MCMXCIV — and the tool reads them the same way, converting to uppercase in the result. Only the letters M, D, C, L, X, V and I are accepted.
Plenty of places: clock faces, book chapters and page prefaces, the names of monarchs and popes, film sequels and copyright years, and the Super Bowl. They persist wherever a classic or formal look is wanted, even though everyday arithmetic long since moved to Arabic numerals.
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