Swim Hats Chart SizeCircumference+XS17.5"/44.5cmBest fit for babies and toddlers for a snug fitS20"/49cmOnly available in the revisable hatsOne Size21.5"/54.6cm XL23.5"/59.7cmBest fit for above-average size adult head Sizing Tips To ensure you get the right size, try this easy method. If you don’t have a measuring tape, use a