Roman numeral converter

How it works

1. Type a number, or type a numeral.
2. It works out which direction you meant from what you typed.
3. One per line converts a list — useful for a set of dates.
4. Anything that is not a valid numeral is named rather than silently accepted.

Frequently asked questions

Why is IIII rejected?

Because the standard form of 4 is IV, and accepting both would mean the same number had two spellings. The check is done by converting back: if the numeral does not produce the same text the number does, it is not standard. Clock faces do use IIII, by a tradition several centuries old and no better explained than “it balances the VIII opposite”.

What is the largest number?

3,999 — MMMCMXCIX. Above that the Romans drew a bar over a numeral to multiply it by a thousand, which is not something a plain text box can represent, so the converter stops where the plain letters do.

Is there a zero?

No. The system has no symbol for nothing, which is exactly why doing arithmetic in it is so painful and why positional notation replaced it.

Why not IM for 999?

Subtraction only goes one step down: I before V and X, X before L and C, C before D and M. So 999 is CMXCIX, not IM. It looks like a reasonable shortcut and is not one, which is why the converter says so rather than guessing.

Is anything uploaded?

No. It runs in your browser, so what you type stays on your computer.