Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate a round hot tub or spa's water volume in gallons from its diameter and depth.
Hot tub and spa chemical dosing (bromine, pH balancers) is specified per gallon of water, and most residential spas are round or oval, so this calculator uses the same circular-volume math as a round pool, sized and defaulted for spa-scale dimensions instead.
Enter your hot tub's diameter and average water depth in feet -- check your owner's manual first, since many manufacturers publish an exact rated volume that's more precise than a geometric estimate (seating, jets, and equipment displace some water volume a pure geometric calculation doesn't account for).
Volume (cu ft) = π × Radius² × Avg Depth
Gallons = Volume (cu ft) × 7.48
This geometric estimate doesn't subtract volume displaced by seats, jets, and internal equipment -- always check your manufacturer's published rated capacity if precise chemical dosing matters, since it's typically measured directly rather than calculated.
Measure the inside diameter across the widest part of the water surface, not the outside of the cabinet. The cabinet, skirt, and shell rim add several inches on every side that are not water. For depth, measure from the water line down to the floor of the footwell. Because hot tubs have seats, the average depth you should use is lower than the footwell depth, since a large part of the footprint is a raised bench.
The calculator treats the tub as a plain cylinder, so the answer is an upper estimate. Seats, molded footwells, and the bodies of bathers all take up space. A practical approach is to enter an average depth somewhat less than the footwell depth to reflect the seats.
Hypothetical example: a round tub 6.5 ft across inside, with a footwell 3.3 ft deep and seating that fills roughly a third of the floor at a height 1.5 ft from the bottom. A rough average depth is 3.3 minus (1/3 x 1.5) = 2.8 ft. Volume = 3.14 x 3.25 x 3.25 x 2.8 = about 92.9 cu ft, or roughly 695 gallons. Using the full footwell depth would have shown about 819 gallons.
Sanitizer, pH adjusters, and water-balance products are dosed by volume, and small tubs react quickly to overdosing. A tub holding 700 gallons has less buffer than a 15,000 gallon pool, so an error of 100 gallons changes the concentration noticeably. Draining and refilling is also easier to plan when you know the volume, including how long the hose will take and how much water you are using.
The best cross-check is to drain and refill through a hose with a flow meter, or fill it in buckets of known size and count them. A tub's owner manual or spec sheet often lists capacity, and if you find that number, prefer it for dosing and use this calculator for rough planning or when the tub's model is unknown.
Seats, jets, filters, and other internal equipment displace water volume that a simple geometric calculation doesn't account for -- manufacturer-rated capacity is measured directly and is more accurate for dosing purposes.
Yes -- a rectangular spa uses length x width x depth instead of the circular formula here; see the Pool Volume Calculator's rectangular mode for that math.
Average the long and short inside dimensions and use that as the diameter for a rough estimate. For dosing, the manufacturer's rated capacity is better.
Enter an average that reflects both. The footwell alone overstates the volume because seats fill part of the floor area.
It varies with body size, and the effect is small relative to total volume for most tubs, but it is another reason estimates run slightly high.
Yes as a starting point, but follow the product label and prefer the manufacturer's stated capacity if it is available.