Use your hands to smooth out the dress so it as flat as possible, with the front of the dress facing up. Make sure any ruffles or details on the bottom and straps of the dress lay flat. Take a measuring tape made for garments and place one end on the top of one of the straps. Extend the measuring tape horizontally from the top of the strap to the bottom edge. Note where the bottom edge hits on the measuring tape and record the measurement. Most dresses are at least 30 inches (76 cm) in length and can be as long as 62 inches (160 cm) in length. Measuring a strapless dress must be done when you have it on. Place one end of the measuring tape at the center of your collarbone and then extend the tape down to the bottom edge of the dress to get the correct measurement. You may need a friend to help you hold the measuring tape in place.

Summary:
Lay the dress flat on the floor or a countertop. Place the measuring tape on the top strap of a dress with straps. Measure from the top to the bottom edge of the dress. Put on the dress and measure from the hollow of your neck if it is strapless.