Your hairline's circumference is determined by measuring the distance around your head. Beginning at the forehead, place the measuring tape at your hairline, and then bring it behind your ears. Back up to the beginning, wrap the tape around the other ear's nape.
The tape must be applied to the ears at the highest position possible—it cannot be tucked behind the ear. Make sure the tape is snug, but not so tight that you experience pressure on your head. The wig will feel stiff as a result.