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Most, if not all, online clothing websites have a chart that explains their sizing measurements. Sometimes dresses will run bigger or smaller than your typical size, so you will want your measurements handy to check against the website's sizing guide. It's a good idea to shop at the same websites, because you're more likely to already know what size works best for you. Once you know your measurements you'll need to check out the different sizing at different stores. Many stores and many brands operate on their own sizing when it comes to dresses. A lot of times you can check the label to find where your measurements fall.  At Target, for example, a small at Target (in numeric size: 0 or 2) has the bust of 85.09 cm to 86.39, waist measurements of 66.04cm to 67.31cm, and hip measurements of 91.44 cm to93.98 cm.  At Top shop, an American size 6 has a bust of 87 cm, a waist of 69.2 cm, and a hip measurement of 91.5, which is smaller than the generic size chart. Sometimes the best way to find out the dress sizes at different stores is to ask a salesperson. You won't be the first person to be confused and salespeople know that most stores have a different sizing method for dresses. As long as you know your measurements they should be able to help you.
Always check the sizing guide when shopping online. Check the sizing at each store. Ask.