The Additive Principle
The system for writing Roman Numerals is an additive system. This means to calculate the value of a Roman Numeral, you simply have to add up all the digits in the Roman Numeral!
Examples:
 VIII = 5 + 1 + 1 + 1 = 8
 XXXIII = 10 + 10 + 10 + 1 + 1 +1 = 33
 CLXXVIII = 100 + 50 + 10 + 10 + 5 + 1 + 1 + 1 = 178
 MMX = 1,000 + 1,000 + 10 = 2,010


There are a couple of other rules to follow:
 Roman Numerals are always written with the largest numbers on the left, down to the smallest on the right. For example, you wouldn't write 8 above as IIIV, it's written as VIII.
 Roman Numerals are always written with the shortest number of numerals as possible. For example, you wouldn't write 5 as IIIII, you'd always write it as V.