Type a value into either the Arabic-number field or the Roman-numeral field - the other field updates live.
If the value you enter is out of range or the Roman numeral is not in standard form (for example IIII instead of IV), the tool names exactly what is wrong instead of failing silently. Copy the result to your clipboard with one click.
Roman Numeral Converter
Convert a number between Roman numerals and standard Arabic numbers, right in the browser. Type a value into either field and the other updates live, so a single tool covers both directions with no Convert button to click.
Key features
- Arabic to Roman conversion using a fixed value/symbol lookup table, from 1000/M down to 1/I including the subtractive pairs like 900/CM and 4/IV.
- Roman to Arabic conversion that validates the input as shortest-form notation, so a non-canonical form such as IIII for 4 is flagged rather than silently accepted.
- Two linked fields that update live on every keystroke - edit the Arabic-number field or the Roman-numeral field and the other refreshes at once.
- A worked example on load, so the page shows a real result before you type anything.
- A Copy result button that puts the current value on the clipboard for pasting into a document, spreadsheet, or chat message.
Supported range
Accepts whole numbers from 1 to 3999, the range standard subtractive Roman notation represents. That notation has no way to write zero, negative numbers, or values above 3999, so inputs outside this range return an inline error instead of a wrong answer.
The same shortest-form rule that flags IIII for 4 (see the FAQ below) applies to every symbol, not only I. VIIII, a literal but non-canonical way to write 9, is rejected for the same reason: the ones-place pattern in Roman Numeral Converter allows at most three I characters after an optional V, so a fourth I fails the match. The thousands place has its own limit - MMMM is rejected because the pattern caps M at three repetitions, so a number that would need a fourth M (4000 or higher) already falls outside the 1-3999 range and gets the same treatment as any other out-of-bounds value. A canonical numeral at the very top of the range, MMMCMXCIX for 3999, still converts correctly because it never repeats a symbol more times than the shortest form allows.
How the conversion works
Conversion runs locally from a fixed lookup table. The Arabic-to-Roman direction walks the table from largest symbol to smallest, subtracting each value as many times as it fits. The Roman-to-Arabic direction first checks the text against the canonical shortest-form pattern, then adds up the symbol values, subtracting when a smaller symbol sits before a larger one.
Roman Numeral Converter accepts a decimal in the Arabic-number field because the field itself does not block a decimal point. Typing 1994.9 or 1994.1 both convert to the same result, MCMXCIV, because the tool truncates the fractional part before running the lookup-table arithmetic - it drops the decimal rather than rounding to the nearest whole number, and it shows no error for the dropped fraction. A value that truncates into the 1-3999 range is accepted this way with no warning; only a value that truncates outside that range, such as 0.9 (which truncates to 0), returns the same "Enter a whole number from 1 to 3999" error as any other out-of-range input. Remove the decimal point before typing if you need an exact whole-number conversion, since Roman Numeral Converter has no way to flag that a fraction was silently dropped.
Privacy
Nothing is uploaded and no account is required. All arithmetic runs in the page's own JavaScript, so your input stays on your device and is not stored beyond the page session.
Frequently Asked Questions
What does Roman Numeral Converter do?
Convert a number between Roman numerals and standard Arabic numbers, right in the browser.
What range of numbers does Roman Numeral Converter accept?
Accepts whole numbers from 1 to 3999, the range the standard subtractive Roman notation represents.
Does Roman Numeral Converter handle numbers larger than 3999?
Does not support the vinculum/overline extension for numbers above 3999 - only the standard subtractive notation.
Does the Roman numeral field accept lowercase letters?
Yes. Roman Numeral Converter converts whatever you type to uppercase before checking it, so mcmxciv or McMxCiV both convert to 1994, the same as typing MCMXCIV.
Why does clearing the Roman field show a different message than clearing the Arabic field?
An empty Arabic-number field truncates to zero and fails the same range check as any out-of-range number, so it shows "Enter a whole number from 1 to 3999." An empty Roman-numeral field is checked for emptiness first, so Roman Numeral Converter shows a dedicated message with a worked example instead: "Enter a Roman numeral, e.g. MCMXCIV."
Does the Copy result button show an error if nothing is copied?
No. The Copy result button shows no success message and no error message either way - it does nothing when there is no valid result yet, such as right after typing an invalid numeral, or when your browser does not support clipboard access. Check your clipboard directly to confirm the copy worked.