To measure bust size, follow these steps:

1. Use a soft measuring tape and stand straight with arms relaxed.
2. Measure under the bust: Wrap the tape snugly around your torso just below your breasts. This is your underbust measurement.
3. Measure bust size: Wrap the tape around the fullest part of the bust (usually over the nipples), making sure the tape is parallel to the floor and not too tight or loose.
4. The difference between the bust measurement and the underbust measurement helps determine the cup size.
5. Cup size is typically categorized by the difference in centimeters or inches between bust and underbust measurements.

In summary:

• Underbust measurement = band size base
• Bust measurement - underbust measurement = cup size difference

For example, if the bust is 92 cm and underbust is 76 cm, the difference is 16 cm which corresponds roughly to a C cup. Hence, the bra size would be 75C (using the example sizing system).