Roman Numeral Converter
Convert numbers to Roman numerals and back, validated against the canonical subtractive rules.
Every conversion runs locally in your browser. Only the canonical modern subtractive form is accepted — historical variants like "IIII" are rejected.
For informational and educational purposes only — not professional or technical advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Why Roman numerals stop at 3,999
Standard subtractive notation never repeats a symbol more than three times in a row, and there's no accepted symbol above M (1,000) — so the largest representable value is MMMCMXCIX (3,999). Converting to Roman numerals uses a greedy algorithm: repeatedly subtract the largest value whose symbol fits, largest first. Converting back is validated against that same canonical form via pattern matching before summing, which is why entries like "IIII" (a form Roman clockmakers actually used) or "VX" get rejected here even though a naive left-to-right sum would happily produce a number for them.
Frequently asked questions
What are the subtractive notation rules?
Certain smaller numerals placed before a larger one subtract instead of add — like IV (4) instead of IIII, or IX (9) instead of VIIII. Only specific pairs are valid per the canonical rules, which is what this converter validates against.