Cap size is the quiet reason a wig looks natural or looks wrong. A good style cannot save a cap that slides, squeezes, or sits in the wrong place.
Measure circumference
Wrap a soft tape around your hairline, behind the ears, and around the nape. This is the main cap-size number.
Add support measurements
Measure front hairline to nape, ear to ear over the crown, temple to temple around the back, and nape width. These numbers help with custom or lace wigs.
Measure with your wig foundation
If you will wear braids, a wig cap, or a grip underneath, measure with that setup or leave enough room for it.
Bottom line
Measure before buying. Cap size controls comfort, security, and realism more than most beginners realize.