Holding a soft tape measure horizontally, pull it all the way around the ribcage just below the chest (also known as the underbust). The tape measure should feel snug.

Grab a string (or even a shoelace), a pen and a ruler. Pull the string around the ribcage horizontally just below the chest—similar to using a tape measure—and use the pen to mark where one end of the string meets the other side. Then, lay the string on a flat surface and use a ruler to determine the underbust measurement.

Use the chest measurement from steps 1 or 2 to find the bra size on the chart at the top of the screen. If the measurement falls between sizes on the chart, opt for the larger size—a slightly larger bra will be more comfortable than a bra that's too tight.
*Extended-size (+) sports bras differ only in circumference, not length or style.

Two fingers should fit between the strap and the body. If it's too loose, try a smaller size. If it feels or looks like it's digging into the shoulders, go up a size.

Make sure the band isn't too tight—two fingers should fit comfortably between the band and the body. Also, raise arms over the head to make sure the band doesn't ride up—it should fit straight across the ribs.

Go ahead … jump! There should be minimal bounce in the chest area when jumping, and the chest should feel covered and secure. If there is too much bounce to move around comfortably or there is spillage at the underarm or neckline, try a women's-size bra.