Medium is 21"-22 3/4" Large is 22 3/4"-24" Serious Sun Protection! Try our Derma Safe sun hats for an additional sun protective bonus. Designed to match the great coverage of our Adventure Hat with a little extra sun protection detail. We've added a layer of sun protection fabric to cover the mesh ventilation panels on the sides of the hat. These sun protection fabric panels are open along the bottom to keep you cool and dry. This allows ventilated, yet complete sun protection coverage above the hairline. These sun hats are designed for people that cannot take any sun exposure on their head due to hair loss, extremely fair skin, or other medical concerns. Hats made from Supplex� fabric. Adjustable sizing. Only 3.3 ounces. 50 UPF (Mesh vents not rated). white with navy, or Sand(gray) with black