To determine your dress size, you must measure your bust, your natural waist and your hips. You should also keep in mind that not all dresses are made the same. A prom dress might require a different size than a sun dress.
To get an accurate size, measure over your underwear, but do not measure over clothes. If you like your clothes to fit less snugly, then let the measuring tape sag a little when taking measurements. When measuring your bust, raise your arms, and wrap the measuring tape around the fullest part of your bust. Measuring your natural waist means measuring the smallest part of your midsection, not your hips.To measure your hips, measure at the fullest part of your bottom. Consult a size chart to find the size that corresponds most closely with your measurements. You may not match up perfectly to one size. In that case, choose the size that fits your largest measurement.
Differences in the way dresses are made primarily are because of the fabric used to make the dress. Material like cotton shrinks, so if you like clothes with more room in them then don't buy cotton clothing in a smaller size. Try the next size up.