How to Buy a Strapless Bra
Measuring Your Bust Size
Get a measuring tape. A measuring tape is necessary in order to accurately measure your bust size. The flexibility of the measuring tape will allow you to measure the circumference of your bust. 
You can go to your local pharmacy, like Walgreens or CVS, or a fabric store to find a measuring tape. You can also try visiting a Target or a Walmart; both stores have sewing sections.
Measure your band size. Measure across the bottom of your band, directly under the bust and across your ribcage, by bringing the measuring tape around your back to the front. Make sure the measuring tape is level and close to your body. 
Round to the nearest whole number. If your measurement is an even number, add four inches. If it is odd, then add five inches. The band size is the sum of this calculation. For example, if you measured 32 inches (81.3 cm), then your band size is 36. If you measured 29 inches (73.7 cm), then your band size is 34.
Measure your cup size. Wrap the measuring tape loosely around the fullest part of your chest, i.e., nipple level. Round to the nearest whole number. 
Calculate your bust size. Subtract your band size (the first measurement) from your cup size (the second measurement). The difference determines your bra size, where each inch represents a cup size. 
For example, if your band measurement is 34 and your cup measurement is 36, the difference is 2. Therefore your cup size is a B cup. As a whole, your bra size would be 34B, where 34 refers to the size of your band. 
Visit a lingerie store. If you do not want to or do not have the time to calculate your own bra size, most lingerie stores can calculate it for you.
Visit a lingerie store, like Victoria’s Secret, at your nearest mall, outlet, or shopping center. They can professionally measure your bra size for you.