Roman Numeral Converter
Convert numbers to Roman numerals and back
Roman Numeral Converter tool
Roman Numeral Converter
Type an Arabic integer or a Roman numeral — the other updates instantly. Covers 1 through 3999.
Roman Numeral Converter: key facts
- What it does
- Convert numbers to Roman numerals and back
- Category
- Converters
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
About the Roman Numeral Converter
Roman numerals appear in places decimal numbers do not: movie copyright years (MMXXIV), clock faces, Super Bowl titles (Super Bowl LVIII), chapter headings in formal documents, and the regnal numbers of monarchs. Converting them manually means memorizing M=1000, D=500, C=100, L=50, X=10, V=5, I=1 and applying the subtractive rule — IV is 4 because I before V means subtract. This converter handles both directions in real time.
Trivia players verifying the year on an old film title, students writing a term paper in formal numbering, designers adding chapter numbers in a book layout, and anyone who wants to know what year MCMLXXXIV is all use a Roman numeral converter. The two-way interface accepts an integer in the left box and shows the Roman form instantly, or accepts a Roman string in the right box and shows the decimal integer — useful when reading an inscription or a tattooed year and wanting the decimal equivalent.
The tool handles the full classical range of 1 to 3999 (I to MMMCMXCIX). Inputs outside this range or containing invalid Roman characters are flagged with clear error messages rather than silently producing a wrong result.
How to use it
- To convert a number to Roman: type an integer (1–3999) in the left input box. The Roman numeral appears on the right instantly.
- To convert Roman to integer: type the Roman numeral string (e.g., MMXXIV) in the right input box. The decimal integer appears on the left.
- Invalid input (out-of-range numbers or unrecognized Roman characters) shows a red error message under the relevant field.
- Use Copy to send the result to your clipboard.
Greedy subtraction, and the thirteen values
Converting a number to Roman numerals works greedily against a descending table of thirteen values. Crucially that table includes the six subtractive pairs — CM, CD, XC, XL, IX, IV — alongside the seven single letters, which is what makes the greedy approach produce correctly-formed numerals rather than strings of repeated letters.
Reading numerals back is simpler: sum the letter values, subtracting instead of adding whenever a letter is followed by a larger one. That single rule handles every subtractive pair without special cases.
The supported range runs to 3,999, because there is no standard way to write 4,000 with these seven letters — representing larger values historically required an overline denoting multiplication by a thousand, which has no plain-text equivalent.
- 1994 is MCMXCIV — M, then CM for 900, XC for 90, and IV for 4.
- 2026 is MMXXVI.
- 4 is IV rather than IIII, though clock faces conventionally use IIII for visual balance.
Why use this version
- Real-time two-way conversion means you can work in either direction without toggling a mode or clicking Convert.
- Input validation catches both out-of-range integers and invalid Roman character sequences, so you get an error rather than a silently wrong result.
- Pure JavaScript lookup-table logic — no external libraries — keeps the tool fast and entirely private, running with no network calls.
- Covers the full canonical range 1–3999, which is the standard limit for classical Roman numerals before the system breaks down.
The rules, and what the system cannot do
Well-formed numerals follow a few conventions: only I, X, C, and M may repeat, and at most three times; only powers of ten may be subtracted, and only from the next two higher values, which is why IX and IV are valid while IL and IC are not. Historical inscriptions break these rules freely — standardisation is a much later imposition than most people assume.
Two things the system lacks entirely. There is no zero, and no way to express one: the concept arrived in Europe with Hindu-Arabic numerals much later, and its absence is one reason Roman numerals are hopeless for calculation. There is also no notation for fractions in the same scheme, and no positional structure — which is why arithmetic was done on an abacus and the numerals used only to record results.
They survive where the association with permanence or formality is the point: clock faces, book front matter, monument inscriptions, film copyright dates, regnal and papal names, and the numbering of recurring events. For anything to be computed with, they are decorative.
Frequently Asked Questions
Why do Roman numerals stop at 3999?
The classical system uses M as the largest single symbol (1000), and conventionally repeats symbols no more than three times (MMM = 3000). The next step would require a symbol for 5000 that the standard system does not include. Extended systems exist for larger numbers (e.g., a bar over a letter multiplies by 1000) but are rarely used.
What is the subtractive rule in Roman numerals?
When a smaller value symbol appears before a larger one, it is subtracted rather than added. IV = 4 (5 − 1), IX = 9 (10 − 1), XL = 40, XC = 90, CD = 400, CM = 900. Only specific pairs are valid — IL (49) is not standard Roman, for example, which is why the converter validates the input.
Is MMXXIV correct for the year 2024?
Yes. MM = 2000, XX = 20, IV = 4, total 2024. You will see this on film copyright notices and formal publications. The tool confirms it: enter 2024 and the result is MMXXIV.
Are Roman numerals case-sensitive?
Conventionally Roman numerals are written in uppercase (MCMLXXXIV), but this converter accepts lowercase input (mcmlxxxiv) and normalizes it to uppercase for output.
