To measure the most accurate length, be sure to wear a heel that is the same height as you plan to wear with your dress.  The dress length is taken from the top of the shoulder to the point where it touches the floor.  For a strapless or off the shoulder style, the length is taken from the top of the neckline to the floor. 

For more information on how to take your measurements click HERE or go to: https://www.windsorstore.com/blogs/support/how-to-measure .