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.
Use Roman numeral converter to learn how addition and subtraction work. The converter splits up a Roman numeral to its parts and teaches you how to decode it letter by letter.
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 = IↃ = D
 

Thousands  1000 = CIↃ = ↀ 2000 = CIↃCIↃ 3000 = CIↃCIↃCIↃ 4000 = CIↃIↃↃ 5000 = IↃↃ = ↁ 6000 = IↃↃCIↃ 7000 = IↃↃCIↃCIↃ 8000 = IↃↃCIↃCIↃCIↃ 9000 = CIↃCCIↃↃ  10000 = CCIↃↃ = ↂ 20000 = CCIↃↃCCIↃↃ 30000 = CCIↃↃCCIↃↃCCIↃↃ 40000 = CCIↃↃIↃↃↃ 50000 = IↃↃↃ 60000 = IↃↃↃCCIↃↃ 70000 = IↃↃↃCCIↃↃCCIↃↃ 80000 = IↃↃↃCCIↃↃCCIↃↃCCIↃↃ 90000 = CCIↃↃCCCIↃↃↃ  100000 = CCCIↃↃↃ 500000 = IↃↃↃↃ 
Million  1000000 = CCCCIↃↃↃↃ

There are archaic forms of Roman numbers starting from 500. The system starts with CIↃ being one thousand. Adding C and Ↄ on both sides multiplies the figure by 10. Halving the numeral (leave out the C's on the left) divides the number by 2. Thus, CCIↃↃ is 10×1000 = 10000 and IↃↃ is a half of that, 5000.
The archaic forms can be written in two alternative ways, as shown in the image below.
Use Roman numeral converter to understand modern and archaic Roman numerals. The converter shows you letter by letter what a Roman numeral is made of.
See also: Roman numerals complete list (13,999,999,999)