Here are some expert tips on how to get the best fit

The Steps in Choosing a Size

Determine Your Band Size

• Measure the distance around your ribcage directly under the bust
• The tape measure should rest snuggly and smoothly against the skin
• Add five inches to this measurement to determine the band size

Determine the Cup Size

• Measure around the fullest part of the bust, preferably while
  wearing an unpadded bra
• Hold the tape measure straight but not too tight
• The difference between this measurement and the Band size is your
  cup size

For in-between measurements, always try sizes above and below and adjust hooks to find a comfortable fit.