Many women think bras are just uncomfortable and painful to wear, but a well-fitting bra can be quite comfortable and actually make you look better. The problem is that most women are wearing a bra that's the wrong size. Many women either don't know how to measure themselves or think they know their size because it is the size they have always worn. Women gain weight, lose weight, have kids and generally change shape over time and they need to change their bra size accordingly. Measuring yourself regularly is the best way to insure a proper fit.
Put on a supportive non-padded bra. Tighten the shoulder straps until the fullness of your breasts are half way between your shoulder and your elbow.
Wrap the measuring tape around your chest just under the breasts. The tape measure needs to be parallel to the floor for the measurement to be accurate. Keep your arms down at your sides and exhale.
Round the number of centimetres you get to the nearest whole number. Add 10 to the number if it is even. If the number is odd, add 13. This is your band size.
Wrap the measuring tape around the fullest part of your breasts. The tape measure needs to be parallel to the floor again. Do not pull the tape measure tight. Round the number of centimetres you get to the nearest whole number.
Subtract the measurement you got for your band from the measurement you got across your breasts. The difference is your cup measurement. If the difference is less than 2.5 cm (1 inch) the cup size is AA, if the difference is 2.5 cm (1 inch) the cup size is A, if the difference is 5 cm (2 inches) the cup size is B, if the difference is 7.5 cm (3 inches) the cup size is C, if the difference is 10 cm (4 inches) the cup size is D.
Bras fit differently among manufactures and you should always try on a bra before you buy it. It's better and more accurate if you can have someone else measure you. For the best fit, measure yourself and evaluate your bras every six months or so.