Difference

Cup=A-B

A cup

B cup

C cup

D cup

E cup

7.5-10CM

10-12.5CM

12.5-15CM

15-17.5CM

17.5-20CM

2.8-3.9inch

3.8-4.9inch

4.8-5.9inch

5.9-6.9inch

6.9-7.9inch

CUP/SIZE

A Upper bust(cm)

B Under bust(cm)

Waist(cm)

Hip(cm)

70A

32A

75-80CM

29.5-31.5(inch)

65-70CM

25.6-27.6(inch)

60-70CM

23.6-27.6(inch)

75-85CM

29.5-33.5(inch)

70B

32B

77-82CM

30.3-32.3(inch)

75A

34A

80-85CM

31.5-33.5(inch)

70-75CM

27.6-29.5(inch)

65-75CM

25.6-29.5(inch)

80-90CM

31.5-35.4(inch)

75B

34B

82-87CM

32.3-34.3(inch)

75C

34C

85-90CM

31.5-35.4(inch)

80A

36A

85-90CM

31.5-35.4(inch)

75-80CM

29.5-31.5(inch)

70-80CM

27.6-31.5(inch)

85-95CM

33.5-37.4(inch)

80B

36B

87-92CM

34.3-36.3(inch)

80C

36C

90-95CM

35.4-37.4(inch)

85B

38B

92-97CM

36.3-38.2(inch)

80-85CM

31.5-33.5(inch)

75-85CM

29.5-33.5(inch)

90-100CM

35.4-39.4(inch)

85C

38C

95-100CM

37.4-39.4(inch)

Example

upper bust:85cm , under bust:73cm , cup size:upper-under(85cm-73cm=12cm) , 12cm is B cup,under  bust(73cm) is 75 size.
answer: your bra size is 75/34B