Roman Numeral Converter

What is a Roman numeral converter?

A Roman numeral converter translates between the everyday numbers we write (1, 2, 3…) and the Roman numeral system still seen on clock faces, film credits and monument cornerstones. The system uses just seven letters: I (1), V (5), X (10), L (50), C (100), D (500) and M (1,000). Most numbers are built additively — write letters in descending order and add them up, so VII is 5+1+1=7. Six pairs also work subtractively, placing a smaller letter directly before a larger one to mean “one less”: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). This converter reads either direction and does the work instantly.

How to use it

1. Type into the single box. Enter digits (like 1994) or Roman letters (like MCMXCIV) — the converter detects which one you typed and works out which direction to convert.
2. Check the result and what was read. The converted value appears underneath, along with a note showing exactly what the tool read from your input, so you can confirm it matched what you meant.
3. Copy, clear, or try an example. Use the copy button to grab the result, the clear button to start over, or the example button to see a sample conversion.

The 1 to 3,999 limit, and why it's honest

Standard Roman numerals only run from 1 to 3,999, and the ceiling is not arbitrary. Every subtractive pair works by placing a smaller letter in front of a larger one — IV, IX, XL, XC, CD, CM — but there is no single letter for 5,000, so there is nothing to subtract a thousand from to write 4,000. MMMM is a workaround some people use, not something the standard system provides, and the overline convention (a vinculum, meaning “multiply by a thousand”) is a separate convention again. This tool implements neither, and says so plainly when a number falls outside the range it supports.

There is also no zero and no negative numbers in Roman numerals — not a limitation of this tool, but a fact about the system itself. Rome had no symbol for nothing and no sign for “less than nothing”, so typing 0 or −5 gets its own explanatory message rather than a generic error.

The converter reads input permissively and then judges it. Type IIII — what a clock face actually shows — and it reads 4, then tells you the standard form is IV. Type IC and it reads 99, then tells you the standard form is XCIX. Refusing to read anything but textbook-perfect numerals would be less useful and no more correct; plenty of people typing into this box copied a numeral off a building or a film reel, not out of a style guide. Lowercase works, spaces and full stops between letters are ignored, and Roman numerals pasted as their own Unicode characters (Ⅻ, from the Number Forms block) work too, because they decompose to plain ASCII letters. One consequence is worth knowing: pasting Ⅰ next to Ⅶ gives the letters “IVII”, which the subtractive rule reads as 6, not 8 — which is exactly why the tool always shows what it actually read. The archaic signs ↀ, ↁ and ↂ do not decompose and are outside the standard form, so they get their own message. On the number side, grouping separators are accepted (1,994 or 1.994 or 1 994) but a real decimal like 12.5 is refused rather than silently becoming 125.

The whole domain is only 3,999 numbers — small enough to check completely rather than sample. Every value from 1 to 3,999 was converted to a Roman numeral and back, and confirmed to return the original number; all 3,999 outputs were confirmed distinct from each other; every output was confirmed to match the form this tool calls standard; and the sequence was confirmed to strictly increase, so no two numbers can ever produce an ambiguous result. That is 60 assertions in total, and none of it depended on agreeing with another library — brute force over the entire space is a stronger check than that.

One quirk worth knowing: 1666 is MDCLXVI, which uses each of the seven letters — M, D, C, L, X, V, I — exactly once, in descending order. It is the only number that does, since using all seven exactly once leaves no other arrangement.

Why is it free?

This page is static and does all its conversion in your browser — there is no server call, no file upload, and nothing to process on our end, so there is no cost to recover with a fee. No account, no sign-up, and no watermark on anything you copy out. It is free because running it costs nothing beyond hosting a page.