Roman Numeral Converter
Convert between Roman numerals and decimal numbers instantly. Supports numbers 1–3999 with validation.
How It Works
This converter supports two modes:
- Decimal → Roman — Enter a number from 1 to 3999. The converter subtracts the largest possible Roman numeral value repeatedly until the number reaches zero.
- Roman → Decimal — Enter a Roman numeral string. The converter reads each character, subtracting its value when a smaller numeral precedes a larger one, and adding it otherwise. The result is validated by converting back to Roman.
Frequently Asked Questions
What are the basic Roman numeral symbols?
The seven basic symbols are: I (1), V (5), X (10), L (50), C (100), D (500), and M (1000). These are combined to form all numbers from 1 to 3999.
How does subtractive notation work?
When a smaller numeral appears before a larger one, it is subtracted. For example, IV = 4 (5 − 1), IX = 9 (10 − 1), XL = 40 (50 − 10), XC = 90 (100 − 10), CD = 400 (500 − 100), and CM = 900 (1000 − 100).
Why is 3999 the maximum?
Standard Roman numerals can only represent numbers up to 3999 (MMMCMXCIX). Numbers 4000 and above require a bar notation (vinculum) which places a line above a numeral to multiply it by 1000, but this is rarely used in modern contexts.
Is the input validated?
Yes. When converting from Roman to decimal, the converter checks that the input uses only valid Roman characters and that it follows proper Roman numeral conventions by converting the result back to Roman and comparing.