The tape should be level and snug, but not tight. A good rule of thumb is to allow enough space for one finger to fit between your head and the tape.
Place the measuring tape around your head, approximately 1/8 to 1/2 inch (0.3–1 cm) above your ears and across the middle of your forehead —exactly where the hat's sweatband will sit. how to know what size hat to buy
Measure 2–3 times to ensure your results are consistent. The tape should be level and snug, but not tight