Hot Tub Calculator
How many gallons a hot tub or spa actually holds, from its shape, size and fill depth — the number chemical dosing and water care start from.
The gallon count behind every dose
Every number in hot tub care — sanitizer doses, shock amounts, water-care kits sized by the tub — starts from how much water the tub actually holds, and the nameplate figure is often the shell's maximum rather than your fill. The calculator works it out from the water's own measurements: the inside length and width (or the diameter of a round tub), and the fill depth — the water depth below the rim, not the wall height, which is always a few inches more.
Gallons = surface area × fill depth × 7.48052 · round tubs: area = π/4 × diameter²
Example: a square tub 7 × 7 ft filled 30 in deep holds 122.5 cu ft of water — 916 gallons (3,469 liters). That is the figure to dose chemicals against, not the bigger number on the brochure.
Seats, steps and loungers displace water — in a molded acrylic spa they can take 10–15% off the box volume this calculator computes, which is why two tubs with the same outside size hold different amounts. Treat the result as the top of the range and the manufacturer's stated capacity as the other reference; dosing to the computed figure is safe because spa chemicals are dosed in ranges, not drops.
Frequently asked questions
How deep is a hot tub usually filled?
To a few inches below the rim and above the highest jet — in practice a water depth of about 30–36 in in most residential spas, with the exact line set by the skimmer or the maker's fill mark. Measure your own fill depth for this calculator rather than guessing from the shell height.
Why does my tub's stated capacity differ from the calculation?
Manufacturers often quote the shell filled to the brim, or a nominal size class. Seats and contours displace water, and nobody fills to the rim — bathers displace more. The measured-inside, actual-fill-depth figure from this calculator is the one chemical dosing should follow.
How much does a full hot tub weigh?
Water weighs 8.34 lb per gallon, so a 900-gallon tub holds about 7,500 lb of water alone — before the shell and the people in it. That is why spas sit on a proper concrete pad or engineered deck, never on a raised wooden deck that was not built for the load.
How often should hot tub water be changed?
A common guide is every 3–4 months for a home spa, sooner with heavy use — the water accumulates dissolved solids that no filter removes. Knowing your true gallon count from this calculator is also what the drain-and-refill chemical reset is dosed against.
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