How to use it
- Pick the direction: To Roman turns an ordinary number into Roman notation, To Arabic reads Roman numerals back as a number.
- Type the value: a number from 1 to 3999, or the letters I, V, X, L, C, D, M.
- The answer appears in large type, and under it the breakdown — which parts it is built from.
- The Copy button puts the finished answer on your clipboard.
How Roman numerals are written
The Roman system is not positional: a number is assembled from seven base symbols, written left to right from larger to smaller, and added up. To avoid four identical symbols in a row, the numbers 4, 9, 40, 90, 400 and 900 use subtractive pairs, where the smaller symbol stands in front of the larger one and is taken away from it. So 4 is IV, 9 is IX, 40 is XL, 90 is XC, 400 is CD and 900 is CM. The same symbol repeats no more than three times, and V, L and D are never doubled at all.
This converter works both ways and checks the notation strictly. When reading Roman numerals it assembles the number, converts that number back into Roman form and compares the result with what you typed. If the two forms disagree — IIII instead of IV, say, or IC instead of XCIX — the notation is wrong, and you get a hint instead of an answer. All of it runs in the browser.
Why 3999 is the ceiling
In the classical notation, without an overbar, the largest symbol is M for 1000, and it may be repeated only three times, which caps the thousands at 3000. Add 999 (CMXCIX) and you get the limit: 3999, written MMMCMXCIX. Larger numbers need a bar over the symbol to multiply it by a thousand. This tool has no bar notation, so the range stops at 1–3999.
The base symbols
| Symbol | Value | Example |
|---|---|---|
| I | one | III is 3 |
| V | five | VIII is 8 |
| X | ten | XIV is 14 |
| L | fifty | LX is 60 |
| C | a hundred | CC is 200 |
| D | five hundred | DCC is 700 |
| M | a thousand | MM is 2000 |
Questions
How do I write a number in Roman numerals?
Leave the mode on To Roman and type a number from 1 to 3999. The answer appears at once in large type, with the breakdown by place underneath. 2024 becomes MMXXIV, and 3999 becomes MMMCMXCIX.
How do I read a Roman numeral back as a number?
Switch to To Arabic and type the letters I, V, X, L, C, D, M. The tool parses the notation and shows the number: MCMLXXXIV is 1984, MMXXIV is 2024.
Does anything I type get sent anywhere?
No. The conversion is done by JavaScript on the page itself, so nothing reaches the server. We never see the number or the Roman numeral you entered, and nothing is stored.
Why can I not enter a number above 3999?
Because in the standard notation, without a bar over the symbol, M repeats at most three times, which puts the limit at 3999 (MMMCMXCIX). Anything larger needs the overbar form, which this tool does not have.
Why are IIII and IC treated as errors?
Because a symbol repeats three times at most, and only pairs one place apart subtract: IV, IX, XL, XC, CD, CM. IIII is properly written IV, and 99 is XCIX, not IC. Malformed notation gets a hint rather than an answer.
