Hot Tub Volume Calculator
Calculate your hot tub or spa's water volume in gallons from its footprint and average depth, for correct chemical dosing.
How this calculator works
Hot tub volume is length times width times average depth, giving cubic feet — then multiplied by 7.48, the number of gallons in a cubic foot, to get total gallons. This treats the spa as a simple rectangular box, a useful approximation before accounting for molded seats and foot wells.
Formula: Gallons = Length (ft) × Width (ft) × Average depth (ft) × 7.48.
Worked example
A 7 ft × 7 ft square spa with a 36-inch (3 ft) average depth:
- Average depth in feet: 36 ÷ 12 = 3 ft
- Gallons: 7 × 7 × 3 × 7.48 = 1,099.56 gallons
- At $0.01/gallon for fill water, that’s $11.00.
Coverage reference
| Spa footprint | Average depth | Gallons |
|---|---|---|
| 3.5 x 3.5 ft | 3 ft | 274.89 gallons |
| 7 x 7 ft | 3 ft | 1,099.56 gallons |
| 14 x 14 ft | 3 ft | 4,398.24 gallons |
How to measure your hot tub
Measure the tub’s outer footprint length and width in feet, then estimate the average water depth across the seating shelf and deeper foot-well area. Because molded seats, headrests, and equipment housings displace roughly 10% of the boxed volume this calculator returns, use your manufacturer’s listed actual fill capacity for precise chemical dosing whenever it’s available.
Frequently asked questions
How do I calculate my hot tub's volume in gallons?
Multiply length x width x average depth (in feet), then multiply by 7.48 gallons per cubic foot. This calculator does the conversion for you — enter your spa's footprint and average depth in inches, and it returns the total gallons.
Why does my hot tub hold less water than this calculator says?
This treats the tub as a simple rectangular box, but real spas have molded seats, foot wells, and equipment housings that displace water — typically reducing actual capacity by around 10% from the boxed-volume figure. Most manufacturers list an actual fill volume in the owner's manual that accounts for this; use that number for chemical dosing if you have it.
Why does chemical dosing need an accurate gallon count?
Sanitizer, pH adjusters, and shock treatments are all dosed per gallon of water — dosing based on a volume that's too high under-treats the water, while dosing for too low a volume over-treats it and can irritate skin or damage equipment. Because hot tubs are much smaller than pools, a 10% volume error matters far more here than in a large pool.
What's a typical average depth for a hot tub?
Most residential spas run 30-40 inches of water depth, factoring in that seating areas are often shallower than the deeper foot-well area. Check your manufacturer specs for exact depth if precision matters, since spa shapes vary more than standard pool shapes.
Does the 'cost' figure mean anything for a hot tub?
It estimates the cost of the water itself to fill the tub, using a default of $0.01/gallon — adjust to your local water rate. It doesn't include the cost of chemicals, heating the water, or the electricity to run jets and pumps.
Estimates only. Verify quantities with your supplier before purchasing.