General tools
Roman numeral converter
Type a number or a Roman numeral and it converts live, both ways, from 1 to 3999. Everything runs in your browser - nothing is uploaded.
How it works
Roman numerals build numbers from seven letters - I (1), V (5), X (10), L (50), C (100), D (500), and M (1000) - written largest to smallest and added together, with the subtractive trick (IV for 4, IX for 9, XL for 40, and so on) keeping any letter from repeating more than three times. This converter goes both ways: type a number from 1 to 3999 to see its Roman form, or type a Roman numeral to read its value, and the other box updates as you type.
It also checks your input. Numbers outside the 1 to 3999 range (the range writable with standard letters) and malformed numerals like IIII or VV are flagged with a short explanation instead of a wrong answer. Everything runs in your browser and nothing is sent to a server, so it is handy for clock faces, book chapters, movie sequels, copyright years, and homework alike.
Example. Type 2026 and you get MMXXVI; type MCMXciv (case does not matter) and it reads 1994. Enter 4000 or a stray IIII and the tool explains why it is not valid rather than guessing.
FAQ
How do Roman numerals work?
You write the letters from the largest value to the smallest and add them up, so MMXVI is 1000 + 1000 + 10 + 5 + 1 = 2016. When a smaller letter sits before a larger one it is subtracted, which is how IV becomes 4 and CM becomes 900. No basic letter is repeated more than three times in a row.
Why is the range limited to 1 to 3999?
The standard letters only reach 3999 (MMMCMXCIX). Larger numbers historically used a bar over a letter to multiply it by 1000, a notation that is hard to type and rarely needed, so this tool sticks to the everyday 1 to 3999 range. There is also no Roman numeral for zero or for negative numbers.
Does capitalisation matter?
No. You can type Roman numerals in lower or upper case - mcmxciv and MCMXCIV both read as 1994 - and the result is shown in conventional upper case. Spaces around the value are ignored too.
Why does it reject something like IIII?
IIII breaks the rule that a letter cannot repeat four times; the correct form for 4 is IV. The converter validates the numeral against the standard subtractive rules and explains the problem rather than returning a misleading number.