A hot tub and a spa are essentially the same thing, but their origins differ slightly. A hot tub is traditionally a wooden tub that holds enough water for two or more bathers. The water is heated, though original models did not have jets that propelled water for a massaging feel. When manufacturers began making tubs from other materials, such as plastic and fiberglass, they began labeling the tubs spas to differentiate between their product and the older, wooden versions. The two now feature many of the same amenities, and the terms are generally considered interchangeable.
Originally, the difference between a hot tub and a spa was the choice of material, but the features of the two tubs quickly drew a clear line between them. The more modern spas featured seating, which older, wooden tubs did not have. They also featured jets that propelled water at users to create a massaging action. New fiberglass versions began featuring filters that could help keep the water of the tub cleaner for longer periods of time. Wooden hot tub makers quickly caught on, however, and these features became standard on both wooden hot tubs and fiberglass spas.
One difference that has generally remained consistent over the years is the shape of the two tubs. A wooden hot tub is usually a round structure, whereas fiberglass spas come in a variety of shapes but are often square. The fiberglass within the spa can be molded into ergonomic seats with strategically placed jets in them, and many spas feature a light fixture at the bottom of the tub for use at night. Fiberglass is easily molded, so limitless shapes are possible. Wooden hot tubs are less versatile; they are usually round or square and have simple bench seats submerged in the water. Wood is more difficult to cut and shape, so ergonomic seats are less likely in a wooden hot tub.
The line between a hot tub and a spa was further blurred by the aesthetics of fiberglass spas. While they were fiberglass on the inside, they were usually built with a wooden skirt around the fiberglass to provide a more traditional look. The wooden skirt also helps hide the internal pipes and insulation for the spa. Wooden hot tubs are usually made of cedar planks because of their ability to resist rot and mold, and some fiberglass spas also use cedar boards to provide aesthetics. Many fiberglass spas today use a composite material for a skirt rather than wood.
