🛁 Spa/Hot Tub Volume Calculator

The geometry, in gallons

Both shapes work out cubic feet and multiply by 7.48, the gallons in a cubic foot:

round        gal = π × (dia/2)² × avg depth × 7.48
rectangular  gal = length × width × avg depth × 7.48

A 6 ft round tub 3 ft deep comes to 84.8 cubic feet, or 635 gallons. The rectangular defaults of 7 × 7 × 3 ft give 1,100 gallons. Those are the volumes of the space the shell occupies, filled to the brim.

Why your tub holds much less than that

A spa is not a bucket. Moulded seats, a lounger, the raised footwell edges and the water line sitting below the rim all take volume out of the box:

• Portable acrylic spa with seats: expect roughly 40 to 50 percent of the geometric figure. That 7 × 7 ft tub is a 400 to 550 gallon spa in practice, which is what the maker's spec sheet will say.
• Plain tiled in-ground spa or a stock tank: straight walls and a flat floor, so the geometry is close to right.
• Use the water depth, measured to the fill line partway up the skimmer, not the height of the shell.

Frequently asked questions