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Roman numeral converter

Type a number and see its Roman numeral, or type a numeral and see the number. It works both ways as you type, across the range the numerals actually cover, 1 to 3999.

A malformed numeral is flagged rather than half-read, so IIII does not quietly become 4. Nothing is sent anywhere.

From 1 to 3999.

How the numerals work

Seven letters carry fixed values: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. You add them from largest to smallest, so MMXXIV is 1000 plus 1000 plus 10 plus 10 plus 4. When a smaller letter sits directly before a larger one it is subtracted instead, which is why 4 is IV and 9 is IX, rather than IIII. A letter repeats at most three times, and there is no zero and no single letter past M, so the system stops at 3999.

Converted in your browser. Nothing is sent anywhere.

Questions

Is anything I type sent anywhere?
No. Every tool here runs in the page itself. There is no server that receives your grades, your essay or your sources, and you can check it in your browser's network tab: using a tool produces no request carrying what you typed.
Why does it stop at 3999?
Because the plain numerals do. There is no zero, no single letter past M for 1000, and the standard way to write larger numbers, a bar over a letter for a thousand times its value, is not agreed on or easy to type. So the tool covers 1 to 3999 rather than inventing notation.
What makes a numeral valid?
The canonical subtractive form: a letter repeats at most three times, and only I, X and C subtract, each only before the next one or two sizes up. IIII, VX and IC are not valid, and the tool says so instead of guessing what you meant.
Is it case-sensitive?
No. You can type mcmxciv or MCMXCIV; both read as 1994. The output is always in capitals, which is the usual way to write numerals.

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