Hot tub sizes and gallons chart

Compact hot tubs (under 7 ft)

7 ft hot tubs

Large hot tubs (7 ft 7 in and up)

The pattern: gallons per square foot

Divide each tub’s gallons by its outside footprint and the numbers bunch together: 15 of the 18 tubs above hold between 6 and 8 gallons per square foot, and the full range is 5.9 to 9.0. Deeper shells tend to sit higher, and small tubs, where the cabinet walls are a bigger share of the footprint, tend to sit lower.

rough gallons ≈ outside length (ft) × width (ft) × 7

That gives a 7 × 7 ft tub about 343 gallons and an 8 × 8 ft tub about 448 — close to the makers’ figures above, but only a ballpark. Even among the 7 ft class here, published volumes run from 330 to 410 gallons. Seat count is a poor guide too: Hot Spring lists its 7-seat Grandee and 5-seat Envoy at the same 455 gallons.

If you know your tub’s size but not its model, the worked pages estimate it from the shell: how many gallons in a 6x6 hot tub, how many gallons in a 7x7 hot tub and how many gallons in an 8x8 hot tub. For your own tub, the hot tub gallons calculator takes inside dimensions and water depth.

Fill line vs maximum fill

Some makers list two volumes. Bullfrog’s A7L holds 368 gallons at the normal fill line and 508 gallons at maximum fill; the A8L holds 457 and 620. The gap — 38% and 36% — is the water between the operating level and the brim.

Use the fill-line figure for chemicals and for draining: it is the water you actually treat and the level the tub runs at. The maximum figure matters for things like floor loading, where the maker’s full-weight spec is the number to check.

Inflatable hot tubs

Inflatables are the exception to the per-square-foot rule. Their walls are thick air chambers, so the outside size overstates the water badly; measured on the inside, the SaluSpa holds about 12 gallons per square foot, because there are no seats displacing water. Use the inside diameter and the water depth, or simply the maker’s figure.

Swim spas

A swim spa is a long hot tub with a current to swim against. Bullfrog’s S150 measures 94 × 152 × 52 in and holds 1,190 gallons to its normal fill line — about 12 gallons per square foot, because it is deep and mostly open water. Dose it by its own volume like a small pool, and follow the maker’s manual for sanitizer and temperature.

Why the gallons matter for chemistry

A hot tub is a few hundred gallons of hot water shared by several people, so a small mistake in dose is a big change in ppm. Chlorine for a 1 ppm rise, worked from the label strength:

The worked pages take it from there: how much chlorine to start a new hot tub, how much bleach to put in a hot tub and how to start a hot tub on bromine. For balance, see how much muriatic acid for a hot tub and how much baking soda for a hot tub. The target levels are in the pool chemistry levels chart.