Rule 1: Addition - In Roman numerals, we add the values to get the numeric value. But you must make sure that the left value is equal or greater than the value on the right.
For example, XI = 11. We know that X = 10, and it comes before I (1). Therefore, according to the addition rule, we need to add 10 + 1 = 11 (XI).
Similarly, VII = 7. Once again, V (5) comes before I (1).
Rule 2: Repetition - Symbols should not be written together more than three times. And if it’s written 2 or 3 times, then the values are added. For instance, XXX = 30. In the above example, the symbol ‘X’ (10) repeats three times. Therefore, it’s added (X + X + X = 30).
Similarly, II = 2. The symbol I (1) is written 2 times continuously. Thus, the value will be added (I+I=2).
Rule 3: Subtraction - As the name suggests, we subtract the values. If the numeral on the left is smaller than its right counterpart, we subtract their values.
For example, IV = 4. Here, I (1) is of lesser value compared to that of V (5). So according to the subtraction rule, we need to subtract i.e., 5 (V) - 1 (I) = 4 (IV).
Similarly, IX = 9. In this case, the value of I (1) will be subtracted from X (10) to produce IX (9). So, [10 (X) - 1 (I) = IX (9)]
Rule 4: Limitation Rule - According to this rule, symbols like V, L, and D are not repeated. And symbols like X, I, and C cannot be written more than thrice in a row. Instead, addition and subtraction methods are followed. For e.g., instead of writing 40 as XXXX, we write it as XL(50 - 10 = 40). Similarly, instead of writing 6 as IIIIII, write it as VI(5 + 1 = 6).
Refer to the above table and see the value of each symbol.