The Evenly Bra Balancer Insert 2-3 Cups (BB3) is a silicone bra filler that mimics the weight of natural breast tissue so that your bra can distribute the weight of your breasts and insert evenly in the cups. BB3 has a thickness that adds a difference of 2-3 cup sizes. This single silicone insert is reusable and waterproof, so you can even wear it to swim! The insert can be worn inside pocketed bras, or inside a regular bra directly against the skin if you have a size difference and want the smaller breast to more evenly fill and fit your bra cups to look like the fuller breast! To find the perfect fit, first, you need a well-fitting bra that should fit your bigger breast best. Then, you select the Evenly Bra Balancer size that matches the difference on your smaller side. A good rule of thumb to determine what THICKNESS you need: 1 cup size = a bra gape of approximately 0.5cm. Once you know which thickness you need, check the Evenly sizing chart to select the number size (based on your UK bra size). If you only have a slight difference of 1 cup then wed recommend the Subtle Shaper instead of Bra Balancer.