Dress Size Conversion Chart
If you're not sure of the size you should buy, or you're not familiar with your measurements and know only your dress size, this chart may help. Please do understand, though, that the MOST ACCURATE way you can shop without actually trying on clothes before buying is to have your measurements.
Yes, this sounds like a royal pain (in the you know where), but you should only need to do this once and then you'll be able to shop with confidence knowing the clothing you buy will fit just right. :-)
USA Size Chart Reference