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Pool Volume Calculator

Volume of a hot tub or spa

Calculate hot tub or spa water volume in gallons, accounting for seats and the actual fill line. Free calculator.

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The formula

π × radius² × Average Water Depth × 7.48 = US gallons

Worked example

A 7 ft round spa filled to about 2.5 ft of water above the seats holds roughly π × 3.5² × 2.5 × 7.48 ≈ 720 gallons. Spas hold far less than their shell depth suggests because of seats and the fill line.

This calculator uses the precise cubic-foot-to-gallon value (about 7.48 US gallons per cubic foot) for your hot tub or spa and lets you switch between US gallons, imperial gallons, litres, and cubic metres.

Why hot tubs hold less than you'd expect

A spa's shell might be three feet deep, but you never fill it that full and the seats, footwell ledges, and loungers take up a large share of the interior. Real water volume is usually far less than shell-depth math suggests.

Measuring spa volume accurately

Rule of thumb

Many 6–7 ft portable spas hold 300–500 gallons despite a shell that looks like it should hold far more. When in doubt, fill with a metered hose to confirm — it is the only way to be exact, and it matters because spa chemicals are dosed per gallon in a small volume where errors concentrate fast.

How to calculate hot tub or spa volume step by step

You will need these measurements:

  1. Measure the inside dimensions at the waterline. Measure the inside diameter (or inside length and width) at the water surface — not the outer cabinet, which includes the shell and insulation.
  2. Measure the seat depth and the footwell depth. Note the water depth down to the seat shelf, and the additional depth of the open footwell below the seats. A spa is full depth only over the footwell.
  3. Blend the depths into an effective average. The water is deep over the footwell and shallow over the seats, so the calculator uses an effective mean depth that weights the footwell area against the seating shelf — this is what keeps a spa from being overstated like a plain cylinder.
  4. Multiply by area and convert. Volume in cubic feet = surface area × effective mean depth, then × 7.48 for US gallons. Dose spa chemicals from this figure, confirmed with a test kit.
Worked example

A 7 ft round spa with about 3 ft to the seats and a footwell roughly a foot deeper holds far less than a 7 ft × 3 ft cylinder would suggest — commonly around 400 gallons once the seats are accounted for. Enter your spa's measurements above for its own figure.

Why spa volume isn't just diameter times depth

A hot tub or spa holds less water than its outer dimensions suggest, because the molded seats and the footwell displace and reshape the water. Treating a spa like a simple cylinder overstates the volume, sometimes by a third or more. The realistic approach is to account for the seat line: water sits at full depth only over the open footwell, and at a shallower depth above the seat shelf that rings the tub. This calculator uses an effective mean depth that blends the deep footwell with the shallower seating area, rather than assuming the whole tub is as deep as the footwell. Measure the inside diameter or inside length and width at the waterline, the depth down to the seat, and the extra depth of the footwell below the seats.

Questions

Common answered

Most home spas hold roughly 300 to 500 gallons, far less than their outer size implies, because seats and the footwell take up space. Enter your measurements above for an estimate that accounts for the seat line.
The molded seats and benches displace water and reduce the average depth — only the footwell is full depth. Calculating a spa as a plain cylinder overstates the volume, often substantially.
Measure the inside width or diameter at the waterline, the depth from the water down to the seat, and the extra depth of the footwell below the seats. The seat line is what makes the estimate realistic.
Spa chemicals are dosed by volume, and because spas are small and hot, an overstated volume leads to over-dosing. A realistic gallon figure keeps your water balanced — confirm with a test kit before adding chemicals.
A swim spa is closer to a small rectangular pool with a more uniform depth, so the rectangle calculator often fits it better than the seat-line spa model used here.