Tuomas Salste – Roman numerals
1 = I 2 = II 3 = III 4 = IV = IIII 5 = V 6 = VI 7 = VII 8 = VIII = IIX 9 = IX = VIIII 10 = X  11 = XI 12 = XII 13 = XIII 14 = XIV 15 = XV 16 = XVI 17 = XVII 18 = XVIII 19 = XIX 20 = XX  21 = XXI 22 = XXII 23 = XXIII 24 = XXIV 25 = XXV 26 = XXVI 27 = XXVII 28 = XXVIII 29 = XXIX 30 = XXX  31 = XXXI
32 = XXXII
33 = XXXIII
40 = XL
50 = L
60 = LX
70 = LXX
80 = LXXX
90 = XC
99 = XCIX
 100 = C 200 = CC 300 = CCC 400 = CD 500 = D 600 = DC 700 = DCC 800 = DCCC 900 = CM 1000 = M 
Roman numerals are constructed using additive and subtractive principles.
Addition is the main rule. Simply add up the digits. Example: XXI = 10+10+1 = 21.
Subtraction happens when a smaller digit comes before a larger digit. In that case, deduct the smaller digit from the larger digit. Example: IX = 10−1 = 9. The usual subtractive combinations are: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). Note that other combinations are not generally used, so 99 is not IC, but XCIX = 100−10 + 10−1 = 99.
Subtraction is a shorthand for four successive digits. Thus, IIII=IV, XXXX=XL and so on. Both forms are possible.
Number zero does not exist in Roman numerals.
Thousands  1000 = M 2000 = MM 3000 = MMM 4000 = MMMM 5000 = V 6000 = VM 7000 = VMM 8000 = VMMM 9000 = VMMMM  10000 = X 20000 = XX 30000 = XXX 40000 = XL 50000 = L 60000 = LX 70000 = LXX 80000 = LXXX 90000 = XC  100000 = C = I 200000 = CC = II 300000 = CCC = III 400000 = CD = IV 500000 = D = V 600000 = DC = VI 700000 = DCC = VII 800000 = DCCC = VIII 900000 = CM = IX 

Millions  1000000 = M = X 2000000 = MM = XX 3000000 = MMM = XXX 4000000 = IV = XL 5000000 = V = L 6000000 = VI = LX 7000000 = VII = LXX 8000000 = VIII = LXXX 9000000 = IX = XC  10000000 = X = C 20000000 = XX = CC 30000000 = XXX = CCC 40000000 = XL = CD 50000000 = L = D 60000000 = LX = DC 70000000 = LXX = DCC 80000000 = LXXX = DCCC 90000000 = XC = CM  100000000 = C = M 200000000 = CC = MM 300000000 = CCC = MMM 400000000 = CD 500000000 = D 600000000 = DC 700000000 = DCC 800000000 = DCCC 900000000 = CM 
Milliards (billions)  1000000000 = M 2000000000 = MM 3000000000 = MMM  nn = 1000 × nn nn = 100,000 × nn nn = 1,000,000 × nn 
In order to write large numerals, one draws lines above or around numbers. This causes multiplication as per the table above.
Hundreds  500 = IƆ = D
 

Thousands  1000 = CIƆ = ↀ 2000 = CIƆCIƆ 3000 = CIƆCIƆCIƆ 4000 = CIƆIƆƆ 5000 = IƆƆ = ↁ 6000 = IƆƆCIƆ 7000 = IƆƆCIƆCIƆ 8000 = IƆƆCIƆCIƆCIƆ 9000 = CIƆCCIƆƆ  10000 = CCIƆƆ = ↂ 20000 = CCIƆƆCCIƆƆ 30000 = CCIƆƆCCIƆƆCCIƆƆ 40000 = CCIƆƆIƆƆƆ 50000 = IƆƆƆ 60000 = IƆƆƆCCIƆƆ 70000 = IƆƆƆCCIƆƆCCIƆƆ 80000 = IƆƆƆCCIƆƆCCIƆƆCCIƆƆ 90000 = CCIƆƆCCCIƆƆƆ  100000 = CCCIƆƆƆ 500000 = IƆƆƆƆ 
Million  1000000 = CCCCIƆƆƆƆ

There are archaic forms of Roman numbers starting from 500. The system starts with CIƆ being one thousand. Adding C and Ɔ multiplies the figure by 10. Halving the numeral (leave out the C's on the left) divides the number by 2. Thus, CCIƆƆ is 10×1000 = 10000 and IƆƆ is a half of that, 5000.
The archaic forms can be written in two alternative ways, as shown in the image below.
See also: Roman numerals complete list (13,999,999,999)