Take your actual body measurements as they are more accurate than measuring over your clothes or shoes.
Feet: Use a ruler or tape measure to measure your foot (from the heel to the tip of your longest toe) Be sure to measure both feet. Many people have feet of different sizes. Fit to the larger of the two.
Remember, feet can expand over a quarter size during a full day's walk so ensure you get the right fit.
Having suffered with blisters through the heat generated on long walks I decided to give these a try. I have been on walks over 10 miles and am well pleased with my purchase. They are so comfy and being well ventilated has stopped my feet from heating up to much. Well recommended for day to day use and for long walks as well.