To understand the principle of size for a bra, it is necessary to be based on 2 essential measures: the bust measurement and the determination of the cup.
You have several solutions to know its size in a discreet way: either you find the bra that fits her best and you go to look for the size on the label at the level of the staples at the back, or you subtly question her (in this period, she will undoubtedly understand the purpose of the question but it will nevertheless remain a surprise).
Last method, you slip her that you have just heard about a study which specifies that 80% of the women choose badly the size of their bra, that you read a lot on the subject and that you became an expert. You can offer to take the measurements yourself, or you can explain to her the procedure to follow to make sure she's wearing the right size. And we'll explain everything to you below to make sure you understand.
First of all you have to take 2 measurements, making sure that the tape measure (flexible) is neither too loose nor too tight, that it lies flat and that the measurement is done horizontally.
The chest circumference corresponds to the measurement taken at the point of the breasts.
The underbust corresponds to the measurement taken just below the chest, at the level of the rib cage.