Roman Numeral Converter

Features

• Both directions, detected automatically (or forced)
• Several values at once, separated by spaces, commas or Chinese punctuation
• Non-standard forms are flagged along with the standard spelling
• A breakdown of each result (M = 1000 + X = 10 + …) for checking
• Strict validation of the subtractive rule: only IV, IX, XL, XC, CD, CM
• Too many repeats of a symbol (IIII, for example) is called out
• Copy a single row, or all of them

How to use

1. Type a number or a Roman numeral (several are fine)
2. The result and its breakdown appear immediately
3. Switch Direction if you want to force one way
4. Use the copy button on a row to take just that result

FAQ

Why can it not convert above 3999?

Because standard Roman numerals stop at 3999 (MMMCMXCIX). Larger numbers need an overline meaning "times a thousand" (V̅ = 5000), which is a typographic mark: it renders differently across fonts and systems and cannot be carried reliably in plain text. So this refuses rather than assembling `MMMM`, which is neither standard nor accepted by the systems you would be typing it into.

Why do clocks use IIII for 4 instead of IV?

That is a clockmaking tradition (the "watchmaker's four"). Explanations vary — visual symmetry with VI, or needing one fewer V in the mould. It is a craft convention, not the standard: in documents and forms, 4 is IV. This tool judges by the standard, so it flags IIII and tells you IV is standard — but if you are looking at a clock face, IIII is right for that context.

Is IIX 8 or 10?

This tool gives 10, by the standard pairwise reading (I + IX), and notes that it is not standard. Roman writers did use "double subtraction" (IIX = 8, XXC = 180), but that usage is not in the modern rules, and the two readings give different numbers — so the tool has to pick one and say which, not guess silently. For 8, the standard spelling is VIII.

Can I convert a batch at once?

Yes. Separate with spaces, English commas, Chinese commas or enumeration commas; dozens at a time is fine. Each value becomes its own block that can be copied on its own, which is handy for a list of years or chapter numbers.