It's well known that the Romans used Latin letters for writing numbers. It is considered that the Roman numeral system is a classic example of nonpositional numeral systems, i.e., numeral systems in which the value of a figure is independent of its position.
We remind you that in the Roman numeral system, I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1000. For example, the number 3 is written as III in Roman numerals. Although everything is not so simple, and it's a nonpositional numeral system because there is an additional rule that modifies the value of a digit according to its place.
That rule forbids the use of the same digit three times in a row. That's why 3 is III, but 4 is IV, and I(1) placed before the larger digit V(5) means subtraction so it's actually -1.
Anticipating the obvious question, we must say that the largest number allowed in this calculator is 3999. For larger numbers, which were used mainly in Medieval times, several different notations were used, including apostrophus and vinculum, but none of which were ever standardized.
Below are two calculators - one for numbers ranging from 1 to 3999 for Roman conversion and one for decimal to Roman conversion.