For years the most exciting thing about picking out a new pair of boots was the “heel” style – and whether you wanted black, brown and if you were really adventurous – cordovan. That is no longer the case. Now you pick out your next pair of boots by “shaft” length. (The height of the shaft measured from the middle of the arch up the outsole of the boot to the top of the boot shaft in inches. (Definition taken from Zappos measurement guide).
Ankle – boot shaft height is 3″ to 8 3/4″
Mid-Calf – boot shaft height is 9″ to 13 1/2″
Knee-High – Boot shaft height is 13 3/4″to 17 3/4″
Over – the Knee – Boot shaft height is 18″ or higher
This table is taken from Nordstrom’s Measuring Guide. They use a women’s size 8 1/2 to measure their boot shaft heights.