How the hot tub volume calculator works
Round spas use 3.1416 times the radius squared. Rectangular spas use length times width. Either area is multiplied by the water depth.
Seats, footwells and moulded loungers all reduce the real capacity, so treat the result as an upper limit and subtract around 20 percent for a heavily moulded shell.
The formula
Round: 3.1416 x radius squared x depth. Rectangular: length x width x depth
Measure: Diameter or length and width, water depth.
Worked example
A 7 foot round spa has an area of 38.5 square feet. At 2.5 feet of water that is 96.2 cubic feet.
That works out to roughly 720 gallons, or 2,725 litres before allowing for seats.
Tips before you measure
Measure the water depth in the footwell and again over a seat, then use the average.
Record the final figure on the cabinet door. You will need it every time you dose.
Hot Tub / Spa pool volume FAQs
How many gallons is a typical hot tub?
Most four to six seat tubs hold 350 to 500 gallons once seats are allowed for. Swim spas run much higher.
Should I subtract for the seats?
Yes. A moulded shell can cut the geometric figure by 15 to 25 percent.
Can I use litres instead?
Yes. Pick metres or centimetres in the unit selector and read the litres result.
Other pool shapes
Not quite the right outline? The main pool volume calculator covers all 20 shapes with the same units and formulas.