Roman Numeral Converter
Convert a number into Roman numerals or read Roman numerals as a number, with validation that explains exactly what it rejected. Range 1 to 3,999.
What the validator accepts
The input is read either as an integer or as Roman numerals, whichever it looks like, and the reading row says which. Subtractive forms are required where they belong: IV before V and X, IX, XL, XC, CD and CM in their proper places. Additive forms that a strict ruleset would reject - IIII for four, VV for ten - are flagged rather than silently accepted, because those spellings do appear on clocks and coinage while still being wrong for any other context.
The range stops at 3,999. Beyond it, Roman notation needs an overline to multiply values by a thousand, and plain text has no reliable way to carry that bar - a converter that emitted MMMM would be producing notation nobody reads as four million. Stating the limit beats generating something that looks authoritative and is not.
Usage example
A client sends a copyright line reading MMXXIV and you need to confirm the year. Paste it: the number row reads 2024, the reading row confirms it was parsed as Roman, and the checked row lists the tokens it consumed - which makes a typo like MMXXLI visible as a rejected character rather than a plausible wrong year. Paste 1926 going the other way to get the numeral for a title page.
This is one of the few converters where the output is destined for typography rather than code, so check the surrounding style before publishing: for a footnote or a section number on a page you control, the slug you pair it with comes from the slug generator, and any body copy it lands in can be length-checked with the character counter.
Frequently asked questions
Why does it reject IIII as four?
It does not reject it outright - it accepts the value but reports the additive spelling as non-standard, because IIII is used on clock faces while IV is correct everywhere else. The state row makes the distinction explicit so you can choose deliberately.
Can it convert dates written as month and day?
It converts a single integer per input. To render a date such as 24 December as XXIV XII, convert each part separately - combining them automatically would require choosing a separator convention the output format does not enforce.
What about numbers above 3,999?
Out of range, and the state row says so rather than returning an approximation. Values that large were historically written with an overline, which plain HTML text cannot express unambiguously, so refusing is more honest than emitting MMMM.
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