Roman numeral converter
Numbers to Roman numerals and back, with the six legal subtractions enforced — because IC for 99 looks perfectly reasonable and was never written by anyone. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Only six subtractions, and IC is not one
The subtractive rule is not general. You may write IV, IX, XL, XC, CD and CM, and nothing else. 99 is XCIX, not IC. 49 is XLIX, not IL.
Both of the wrong ones look entirely sensible — more economical, even — and neither was ever used. A converter that quietly accepts IC is reading a notation nobody wrote.
It stops at 3,999
MMMCMXCIX. Beyond that, standard notation runs out: the Romans used an overline meaning "multiply by a thousand", and plain text cannot carry an overline.
So a converter offering you 10,000 as MMMMMMMMMM is inventing a notation. There is no zero either — the system never needed one, and its absence is why arithmetic in Roman numerals is so awkward.
And clock faces use IIII, which is not an error
Look at almost any clock with Roman numerals and four is IIII, not IV. That convention is older than the subtractive rule, it has persisted for centuries, and explanations for it range from visual balance with the VIII opposite to superstition about the letters IV.
This page reads IIII correctly, gives you 4, and tells you the standard form is IV — rather than refusing something that is on a million clock faces.