Today, the terms hot tub and spa are used interchangeably. The word "spa" is often associated with a health resort destination or a commercial establishment where patrons can get aesthetic services, massages, facials, etc. Thus, the term hot tub is more specific in describing that hot, bubbly tub, and has experienced a resurgence in usage to distinguish it from a day spa.
Small hot tubs measure about 3-4 feet high by 5 feet across and hold approximately 500 gallons of water. Larger hot tubs are around 4-5 feet high and 6 feet in diameter. They hold about 850 gallons of water.
Remember Will Ferrell and Rachel Dratch's "hot tub luh-vahs" on Saturday Night Live? They often hung out at the Welshley Arms Motel's hot tub, often harrassing other guests.