I equals 1
V equals 5
X equals 10
L equals 50
C equals 100
D equals 500
M equals 1,000
Roman numerals are usually arranged in descending value and added up from left to right. But when a smaller number is placed before a larger one, the smaller value is subtracted from the larger one to the right.
For example: IV is 4, XL is 40 and CM equals 900. So MCMXLIV is 1944.
There are certain restrictions when subtracting. For example, 45 is written as XLV, not VL. And 49 is XLIX, not IL.
Source: Gerard Michon of -http://www.numericana.com