Most dresses come with a standard length measurement but some designers offer a custom hollow to hem. Measure from the edge of the left shoulder across to the prominent neck bone located at the center of the back of the neck continuing to the edge of the right shoulder.

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How to get body measurements for a dress. For a dress with sleeves measure from the top of the shoulder seam down to the hem of the dress. Ask them to extend the tape measure along the front of your body passing over your bust and reaching down to your desired hemline. It is in this order for the vertical measurements i assume 1 body length. Then the horizontal measurements. Stand with your back straight and your legs hip width apart. 3 shoulder to shoulder length.
Measure your dress length from your shoulder to your desired hemline. Your hollow to hem measurement is determined by measuring from the center of your collarbone hollow to the bottom hem of your dress. Sleeve sizes are always in whole numbers. Round up to the nearest whole number if needed. Donot take this too tight. Place the tape on top of the shoulders.
Neck round measure around the base of the neck slightly loose. Have a friend measure from the center of your back across your shoulder down to your elbow and then to your wrist for your full sleeve measurement. We highly recommend that you have a formal wear professional perform this measurement as it can be a bit tricky. Most dresses are at least 30 inches 76 cm in length and can be as long as 62 inches 160 cm in length. Have a friend hold the end of the tape measure at the top of your shoulder. Most sleeve measurements fall between 32 and 39 inches.