Imagine looking down on your breasts. After a beast augmentation, the beast tissue stretches over the implants. While it is possible to try sizes by placing implants in a bra, the implants are stacked on top of the breasts, creating too much projection and not enough spread of the volume. The rice test allows the volume to spread across the width of the breast more closely mimicking an augmented breast. Your surgeon will add a little bit to the volume of rice you like in order to compensate for compression by your tissues.
First start with an unpadded bra that is the size you'd like to be. Make sure it isn't too structured with padding, but it shouldn't compress your breasts either. Next get a measuring cup to measure the rice. Your surgeon can give you a suggested volume to start with, but it's not a recommendation. After all, you're the one who will choose your implant size with this rice test. Get some sheer knee high nylons or cut off the legs of regular nylons and fill each foot with the rice. Leave some space before knotting the opening so you'll be able to spread the rice across each breast. Cut off the excess nylon so it doesn't create too much bulk. Place the rice bags in the cups of your bra and spread each bag across each breast. Write down the number of ounces you're testing on a piece of paper. Now put on a tight shirt. Holding the number, take a photo of yourself in the mirror or have someone take a photo of you. The photo will give you a more objective view than just looking at yourself in the mirror. If you are happy with that size, add more rice to the stockings and write the new number of ounces. Keep increasing the volume until you find the size that looks too big. The next size down is your choice.
Some people wonder how heavy their implants will feel. To get an idea, take the number of ounces you liked and measure that amount of water into a Ziploc bag. Add three more tablespoons of water to each bag and zip the bags up. Put the bags into your bra to give you an idea of the weight. You can go out and job to see what bouncing feels like with your new implants. Of those who are dissatisfied with their implant size, the vast majority wish they'd gone larger, but not everybody. Stay within your comfort zone. Take your time with the rice test. Don't focus on bra size which is extremely variable and unscientific. Focus on volume. When you decide on your size, wear your rice bags around for a couple of days just to be sure. Remember your size is your choice. We just want you to be happy.
The rice test is a tool to help in choosing implant size, but there's no guarantee as to final cup size, projection, or appearance.