Plot No. 41 · Everyday & Shopping
Roman Numeral Converter
Convert numbers to Roman numerals and Roman numerals back to numbers, both ways at once. The converter builds a numeral by subtracting the largest value symbol it can at each step (M, CM, D, CD and so on), and reads a numeral back by adding symbol values while subtracting any that sit before a larger one.
Numbers run from 1 to 3999. Roman input accepts the letters I, V, X, L, C, D and M.
A number must be a whole value from 1 to 3999. A Roman numeral must follow standard form, so IIII and VX read as invalid.
The link saves your inputs so you can bookmark or share this exact result.
How the Roman numeral conversion works
To turn a number into a numeral, the converter repeatedly subtracts the largest value it can from a fixed table of symbols (M for 1000, CM for 900, D for 500, CD for 400 and so on down to I for 1), adding each symbol it uses to the output. The number 2026 becomes MM for 2000, then XX for 20, then VI for 6, giving MMXXVI.
To read a numeral back, it adds each symbol's value while subtracting any symbol that sits before a larger one, which is how IV reads as 4 and IX as 9. It then checks the result is valid by re-encoding it and comparing to your input, so non standard strings such as IIII or VX are rejected as invalid.
How to use the Roman numeral converter
- Type a whole number from 1 to 3999 to see it as a Roman numeral.
- Type a Roman numeral using I, V, X, L, C, D and M to decode it to a number.
- Both conversions update at once, so you can convert in either direction.
Worked examples
Converting the year 2026
Inputs: Number 2026
Result: MMXXVI, built from MM for 2000, XX for 20 and VI for 6.
Decoding a numeral
Inputs: Roman MCMLXXXIV
Result: 1984. M is 1000, CM is 900, LXXX is 80 and IV is 4.
Limitations and common mistakes
Edge cases and limitations
- Standard Roman numerals only go from 1 to 3999, so numbers outside that range show as out of range.
- There is no Roman symbol for zero or for negatives, so only positive whole numbers convert.
- Overline notation for very large numbers is not supported, which is why the upper limit is 3999.
Common mistakes
- Writing four of the same symbol in a row, such as IIII for 4. The correct form is the subtractive IV.
- Using an invalid subtractive pair like VX or IL. Only I, X and C subtract, and only from the next one or two symbols up.
Frequently asked questions
How do you write a number in Roman numerals?
Break the number into thousands, hundreds, tens and units, then map each part to its symbols. 2026 is MM for 2000, XX for 20 and VI for 6, giving MMXXVI.
What is the largest number this converter handles?
Standard Roman numerals run from 1 to 3999, written MMMCMXCIX. There is no symbol for zero and no agreed standard notation above 3999 without overlines, so the converter caps at 3999.
Why does my Roman numeral show as invalid?
The reader rejects strings that break the rules, such as IIII, VX or a stray letter. Use I, V, X, L, C, D and M with at most three repeats of I, X, C or M and correct subtractive pairs like IV or IX.
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