In The Swim 15 Mil 12 ft. x 12 ft. Floating Solar Blanket Spa and Hot Tub Cover 15-mil thick square spa and hot tub solar blanket cover Thousands of tiny air bubbles work to transfer solar heat into your spa Reduces chemical loss through evaporation, saving time and money on chemicals and rebalancing Blanket is square and can be easily cut or trimmed to fit irregularly shaped spas and hot tubs Prevents heat loss due to evaporation