Roman Numeral Converter
Convert both directions between Roman numerals and decimal numbers, rejecting malformed input.
Convert Roman numerals both ways instantly. Free Roman numeral converter with validation for numbers 1–3999.
Calculator Inputs
About Roman Numerals
Convert both directions between Roman numerals and decimal numbers, rejecting malformed input.
Enter either side: a decimal 1–3999 converts greedily through subtractive pairs (CM before D, IV before V), while a numeral is validated against strict Roman grammar first — so IIII or VX are rejected, not misread — then scanned left to right, subtracting smaller values that precede larger ones.
Classical Romans had no zero and no standard form above 3999 (M with bars); this tool covers the conventional 1–3999 range. Everything runs in your browser.
Where it is used
Clock faces, book chapters, outlines, copyright dates on films, and classroom exercises.
How to use the Roman Numerals
- Number (1–3999, optional) — enter the value (e.g. 2026).
- Numeral (optional) — enter the value (e.g. MMXXVI).
- Calculate — press the Calculate button to see the result instantly above.
Formula
Greedy subtraction with subtractive pairs; reverse by signed left-to-right scan with strict validation.
Examples
Example
Frequently Asked Questions
- Why is IIII wrong but IV right?
- Standard form allows at most three repeats and requires subtractive pairs for 4 and 9 at each magnitude. Clocks traditionally break this rule with IIII — a famous exception.
- Is there a Roman zero?
- No. The system has no zero and no place value, which is exactly why positional decimal replaced it for arithmetic.
- How are numbers above 3999 written?
- With a vinculum (bar) multiplying by 1,000. There is no ASCII standard for it, so this tool caps at 3999.