Roman Numeral Converter
Free Roman numeral converter — type a number or a Roman numeral into either field and the other updates instantly, both directions, with standard-form validation.
How it works
- Type a number (1-3999), or a Roman numeral
- The other field updates automatically
- Non-standard Roman numerals (like IIII instead of IV) are flagged, not silently accepted
Frequently asked questions
Why does it reject IIII for 4?
Standard Roman numeral form uses subtractive notation (IV, not IIII) — this converter checks that any Roman numeral you type round-trips back to itself, so non-standard forms are flagged rather than silently misread.
Why does it stop at 3999?
Standard Roman numeral notation (using I, V, X, L, C, D, M) has no way to represent 4000 or higher — larger numbers historically used a line drawn over a numeral to mean ×1000, which isn't practical to render as plain text.
Related tools
From the blog
Update history
- 2026-09-02 Launched: two-way Roman numeral converter (1-3999) with standard-form validation that rejects non-canonical numerals. Entirely client-side.