While standing, measure your calf at the widest point, ensure the tape is firm then with the leg slightly bent. To measure the height measure from the floor to the highest part of the leg (without shoes), right up in to the crease at the back of the knee.
Add 1 cm - 1.5 cm to the height measurement as the boot will lower after wearing in.
Find your foot size on the chart, then the height, then your calf to calculate size.
Please take measurements over breeches (or similar) and socks that you normally wear.