Roman numerals are formed using seven letters to represent different number values. 
1 = I 
5 = V 
10 = X 
50 = L 
100 = C 
500 = D 
1000 = M 

Roman numerals are written as an addition of number values from highest to lowest, from left to right. The exceptions are fours and nines, which are written as subtractions by putting the lower value first. See below.

Hundreds
100 = C
200 = CC
300 = CCC
400 = CD
500 = D
600 = DC
700 = DCC
800 = DCCC
900 = CM

Tens
10 = X
20 = XX
30 = XXX
40 = XL
50 = L
60 = LX
70 = LXX
80 = LXX
90 = XC

Units
1 = I
2 = II
3 = III
4 = IV
5 = V
6 = VI
7 = VII
8 = VIII
9 = IX

To convert Arabic numerals e.g. 999 into Roman numerals, write the symbols for the number of hundreds: CM, followed by the symbols for the number of tens: XC, followed by the symbols for the number of units: IX. So 999 is written as CMXCIX. 

Sometimes cryptic clues break the conventional use of Roman numerals. Examples: 
49 = IL 
99 = IC 
499 = ID 
999 = IM 

Medieval Roman numerals

Medieval Roman numerals are listed in some dictionaries. They are occasionally used in cryptic clues and the medieval usage is sometimes indicated by thousand, e.g. 200,000 = H. 

1 = I 
5 = A or V 
7 = S or Z 
10 = X 
11 = O 
40 = F 
50 = L 

70 = S 
80 = R 
90 = N or Q 
100 = C 
150 = Y 
160 = T 
200 = H 

250 = E or K 
300 = B 
400 = G or P 
500 = A, D or Q 
1000 = M 
2000 = Z

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