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Roman numeral converter

Type an Arabic number or a Roman numeral and the other updates live. Standard subtractive notation, 1 to 3999. Local only.

Type in either field. Roman input is case-insensitive. Output uses standard subtractive notation.

What this tool does

Converts between an ordinary (Arabic) number and a Roman numeral, in both directions and live. Type a whole number from 1 to 3999 and its Roman numeral appears instantly; type a Roman numeral and the number appears. Output always uses standard subtractive notation (IV, IX, XL, XC, CD, CM), which is the shortest valid spelling. Everything runs in your browser; nothing is sent anywhere.

Runs entirely in your browser - to verify, open DevTools and watch the Network tab: typing produces zero requests.

How to use it

Put a whole number from 1 to 3999 in the Arabic number field to see its Roman numeral, or type a Roman numeral (upper or lower case) in the Roman numeral field to read its value. The conversion updates on every keystroke. Invalid or non-standard input, such as a number above 3999 or a numeral like IIII, shows an inline message instead of a wrong answer, and suggests the standard spelling where it can. Press Clear to empty both fields.

Common use cases

  • Reading clock faces, book chapters, film sequels, and Super Bowl numbers written in Roman numerals.
  • Formatting a copyright year or a monument date as MMXXIV or MCMXCIV.
  • Numbering outline sections, appendices, or preface pages (the same letters, often shown lowercase as i, ii, iii).
  • Checking homework, crossword, or puzzle answers that involve Roman numerals.
  • Confirming that a numeral for a design, tattoo, or engraving is in correct standard form.

Common pitfalls

  • No zero, negatives, or fractions. Roman numerals cannot express 0, negative numbers, or fractional values. The smallest value this tool accepts is 1, written I.
  • 3999 is the ceiling. Standard numerals stop at MMMCMXCIX. Larger numbers historically used an overline (a vinculum) to multiply a symbol by 1000, which plain ASCII cannot represent, so this tool caps input at 3999.
  • IIII is not standard. Additive spellings like IIII (seen on some clock faces) are not the modern standard form. The tool flags them and suggests the canonical spelling, here IV.
  • Order and adjacency matter. A smaller symbol counts as subtraction only when it sits directly before one of the next two larger symbols: I before V or X, X before L or C, C before D or M. Strings like IC or IIX are not valid standard numerals.

Frequently asked questions

How do you convert a number to a Roman numeral?
Type the number into the Arabic number field and the Roman numeral appears instantly in the field beside it. The tool works from the largest value down: it subtracts thousands (M), then the subtractive pair for nine hundred (CM), then five hundred (D), four hundred (CD), and so on, building the shortest standard string. For example 2024 becomes MMXXIV and 1994 becomes MCMXCIV. It goes both ways, so typing a Roman numeral fills in the number.
What is the largest number in Roman numerals?
In standard form the largest value is 3999, written MMMCMXCIX. That limit exists because the basic symbols stop at M for one thousand, so the most thousands you can stack is three (MMM). Larger values historically used a bar over a symbol to multiply it by a thousand (a vinculum), but that notation is not part of the standard ASCII set, so this tool caps input at 3999 and shows a message for anything higher.
Is there a Roman numeral for zero?
No. The Roman system has no symbol for zero and no way to write negative numbers or fractions; it was built for counting and tallying, not positional arithmetic. The concept of zero as a number reached Europe later, through Indian and Arabic mathematics. If you enter 0 or a negative value, the tool shows a message explaining that only 1 to 3999 is supported.
What is subtractive notation (IV, IX, XL)?
Subtractive notation is the rule that a smaller symbol placed directly before a larger one is subtracted instead of added. So IV is five minus one (4) and IX is ten minus one (9), rather than IIII and VIIII. The six standard subtractive pairs are IV (4), IX (9), XL (40), XC (90), CD (400), and CM (900). This tool always outputs the standard subtractive form, which is the shortest valid spelling.
Why does IIII show an error?
IIII is an additive spelling of four that you sometimes see on clock faces, but it is not the standard modern form, which is IV. This tool validates against standard form: it parses your input, converts the value back to a numeral, and if the result does not match what you typed it flags the difference and suggests the canonical spelling. That catches non-standard spellings like IIII, VIIII, and IC as well as truly invalid strings.
Is the conversion done on my device?
Yes. Every conversion runs in JavaScript in your browser with no network calls, no server, and no data leaving the page. You can open your browser DevTools, watch the Network tab, and confirm that typing produces zero requests. That also means the tool keeps working offline once the page has loaded.

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