About Roman Numeral Converter

Bidirectional converter between Arabic numbers (1–3,999) and Roman numerals. Validates Roman numeral input against the standard pattern (no illegal repetitions like IIII or VV) using a strict regex, and converts Arabic numbers using the standard subtractive notation table (13 entries from M=1000 down to I=1). Supports instant swapping of conversion direction, auto-converts on input, maintains a 10-entry conversion history, and offers CSV export.

• Arabic → Roman: greedy algorithm iterates through 13 value/numeral pairs (M, CM, D, CD, C, XC, L, XL, X, IX, V, IV, I)
• Roman → Arabic: validated against ^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$
• Swap button reverses direction and uses the previous result as the new input
• 14 clickable examples from 1 (I) to 3000 (MMM)
• History tracks last 10 conversions with timestamps, exportable as CSV

Frequently Asked Questions

Why is the limit 3,999?

Standard Roman numerals have no symbol for 5,000 or above. 3,999 (MMMCMXCIX) is the largest number representable without the vinculum (overline notation), which is not supported in standard Unicode text.

Does the converter accept lowercase Roman numerals?

Yes. Input is case-insensitive—“xlii” and “XLII” both convert to 42. The output always uses uppercase.