SettingsSettings for this calculationUS
The inside diameter of the spa.
Most residential hot tubs range from about 6-8 ft (1.8-2.4 m) in diameter.
The typical water fill depth.
Most hot tubs fill to around 3 ft (0.9 m) deep, though seating ledges reduce the true water volume slightly below a simple cylinder estimate.
Estimated spa volume
864 gallons
This treats the spa as a simple cylinder — built-in seats, jets, and equipment displace some water, so actual capacity is typically 5-15% less than this raw geometric estimate. Check your manufacturer's specified capacity for precise chemical dosing.
- Volume in cubic feet
- 115.45 ft³
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Hot Tub & Spa Volume Calculator: 864 gallons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Round vessel volume (gallons) = pi x radius² x depth (all in ft) x 7.4805 US gallons per cubic ft, the exact 1,728 ÷ 231 (a US gallon is 231 cubic inches)
Inputs used
- Spa Diameter
- 7 ft
- Average Water Depth
- 3 ft
Intermediate steps
- Volume in cubic feet
- 115.45 ft³
Confidence note: This treats the spa as a simple cylinder — built-in seats, jets, and equipment displace some water, so actual capacity is typically 5-15% less than this raw geometric estimate. Check your manufacturer's specified capacity for precise chemical dosing.
What this calculation does not cover
- This models the shell envelope, not the water body. The full plan area is taken at the depth you enter, so seat pans, loungers, moulded armrests, corner steps, the filter and skimmer bay and the taper toward the footwell are never deducted — on a seated tub that moulding removes a large share of the envelope. Treat the answer as a ceiling and meter the next fill before dosing against it.
- Nothing outside the shell is counted. Pipework, the heater manifold, filter housing, jet plumbing and any surge or spillover tank hold circulating water that no shell geometry can see.
- Round and octagonal shells only. A rectangular, oval, kidney or lounger-dominated tub is not a cylinder; an octagon measured across the flats holds more than the circle drawn inside it, and a shell that narrows toward the floor holds less than a straight-sided one.
- This is a volume, not a dose. It carries no sanitiser demand, bather load, stabiliser build-up or drain-and-refill interval, and the gallons it reports are US gallons of 3.79 litres — a product label printed per imperial gallon, which is 4.55 litres, is a further 20% out before the arithmetic even starts.
- It does not replace a metered capacity where one is expected. A tub in a rental, holiday let, gym or hotel is a commercial spa pool under a managed regime — HSG282 in the UK, the Model Aquatic Health Code in the US — and those regimes are run against a documented actual capacity, not a tape-measure estimate.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.1.0
Regulatory standards & verification citations1
- Round vessel volume (gallons) = pi x radius² x depth (all in ft) x 7.4805 US gallons per cubic ft, the exact 1,728 ÷ 231 (a US gallon is 231 cubic inches)
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