When taking body measurements, you need to use the correct type of tape measure. You will want to use a soft cloth or flexible plastic tape measure, similar to what a seamstress uses. Do not use a metal measuring tape, used in construction (your measurements will be inaccurate). Please don't guess, if you still have your sock packaging you can refer to the packaging size tab. Pay special attention not to pull your measuring tape too tight. Doing so will produce a measurement that is too small.
The most difficult measurement is the bottom measurement. Too often, people measure too high along the bottom of their limbs producing a larger "Bottom" measurement. Ideally, measure 2 inches up from the bottom of your limb and use a pen to mark this level on your limb. Once you know exactly where to measure, use a tape measure to carefully measure your bottom circumference.