Ok, so how it works:
NUMBER is band size. It says how wide is one's chest. It should be measured tight, because fabrics are elastic. Band supports 90% of boobs weight. It makes a shelf for boobs. If it's too big, it won't hold. Bra is bust-holder, not bust-cover.
If you want to hold your boobs in your hands, you catch them from bottom. You don't pull the upper skin, do you? So bra also should keep it all from bottom, by band, not by straps. Straps aren't for support.
If someone has 70 cm under bust, there's no reason to wear band sizes like 85.
Choose smaller size, for 70 cm it's 65. Why? Because fabrics are flexible. Some brands make bras that stretch from 60 cm to 120 cm. How can it support? Right, it can't. So better choose brands that avoid stretchy fabrics. Stretchy fabric can only do harm.
CUP means a difference between band size and your boobs circumference. It should be measured loose and straight, on nipples level. Every inch (2,5 cm) is next letter. So D is only 3"/7,5 cm bigger than A.
Letter means how much boobs protrude from chest. It's like a triangle - and letter is height. Band is baseline. It's why 40D is much bigger than 30D. But it's only wider, not higher.
My favourite image:
How to use SIZE CHART
1. Measure your chest under bust. Do it very tight. Tape isn't flexible, but your bra IS.
Subtract 5 cm and equal down.
Example: your measurement is 77. 77 - 5 = 72 --> 70. Your band size is 70 (32).
2. Measure yourself in bust. Ask somebody for help, if it's a problem. Bend a little and measure straight on nipples level.
Example: your measurement is 99 cm.
3. See your size in chart.
Search 99 in 70 line. It's FF. So your bra size is 70FF.
If it's between, choose bigger cup.
If one boob is bigger and second is smaller, choose size for this bigger.
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