Hot tub volume calculator

Measure your hot tub, pick its type, and get an estimate of how many gallons or litres of water it holds, the number every chemical dose starts from.

Inputs

Tub shape

Portable spas: measure the outside of the cabinet. Everything else: measure the inside.

Ground to the top of the cabinet.

Hot tub volume

353gal

≈ 1,335 L. An estimate: your maker’s spec sheet, or timing a fill against a bucket, is more exact.

Litres
1,335 L
Water weight
2,936 lb
Share of the cabinet box
32%
Formula
King Technology
Your spa cabinet from above
84 in84 in

A square or rectangle tub, 84 in × 84 in with a cabinet height of 36 in, holds about 353 gal (1,335 L) by King Technology’s spa formula, 32% of the cabinet box it fits in.

How it works

Three formulas, one for each kind of tub and what you can measure:

Portable acrylic spa (outside of the cabinet):   gallons = L × W × H (inches) × 2.4 ÷ 1728
Wooden, soft or in-ground spa (inside):          gallons = L × W × depth (inches) × 4.8 ÷ 1728
Tub without seats (inside, water depth):         gallons = surface area × water depth (in³) ÷ 231
  • King Technology’s formulas (the first two) start from the box your tub fits in and keep the share of it that is usually water: about 32% of a portable spa’s cabinet and 64% of a wooden or in-ground tub’s inside. For round and multi-sided tubs they use diameter × diameter, so the factor covers the round shape too.
  • Epic Hot Tubs’ formula (the third) is plain geometry: length × width for a square tub, π × radius² for a round one and 0.785 × length × width for an oval, times the average water depth. There are 231 cubic inches in a US gallon.

Worked example

A portable acrylic spa 84 × 84 in (7 × 7 ft) and 36 in tall:

  • The cabinet box: 84 × 84 × 36 = 254,016 cubic inches, or 1,100 gallons of space.
  • King Technology: 254,016 × 2.4 ÷ 1728 = 352.8 gallons (1,335 litres), 32% of the box. The water alone weighs about 2,940 lb.

A round inflatable tub 72 in across with water 36 in deep, by Epic’s formula: 3.1416 × 36² × 36 ÷ 231 = 635 gallons. If the same tub were a wooden one measured on the inside, King’s formula would give 72 × 72 × 36 × 4.8 ÷ 1728 = 518 gallons, since it allows for a bench.

Tips from the tub

  • Measure to the fill line. A tub is usually filled a few inches below the rim; if you measure the water depth, measure to where the water actually sits.
  • Refill time is a good time to check. When you next drain and refill, time the fill against a bucket and you’ll have a real number.
  • A few percent doesn’t matter. Chemical doses are small and you’ll test and adjust anyway. A 10% volume error is a 10% dosing error, though, so don’t guess wildly.

FAQ

How many gallons does a hot tub hold?

Most hold roughly 300 to 700 gallons, according to Epic Hot Tubs. A 7 × 7 ft portable spa 36 in tall comes out at about 350 gallons by King Technology's formula; a big 8 × 8 ft, 40 in model at about 510.

Why is the portable spa formula so much less than length × width × height?

Because you measure the outside of the cabinet. Much of that box is insulation, the shell, the pumps and heater, molded seats and the air gap under the lid. King Technology's factor keeps about 32% of the box as water; for wooden and in-ground tubs, measured on the inside, it keeps about 64%.

Which type should I pick?

Portable acrylic spa if it sits in a cabinet with molded seats, and measure the cabinet. Wooden, soft or in-ground spa if you can measure the inside, rim to floor. Tub without seats for inflatables and plain soaking tubs, where the inside and the water depth are all there is.

My maker says a different number. Which is right?

The maker's. Formulas can only estimate how much room the seats and equipment take. Makers also differ on whether they quote the full tub or the water to the normal fill line. If you have no spec sheet, the fill-time method is the next best.

Sources

  1. Calculating Spa Volume, King Technology. Portable acrylic spas, outside of the cabinet in inches, length × width × depth × 2.4 ÷ 1728 = gallons (round, diameter × diameter); in-ground, soft and wooden tubs use 4.8. Fill time is more accurate, e.g. 1800 s to fill the spa ÷ 6 s per gallon = 300 gallons.
  2. Hot Tub Volume Calculator, Epic Hot Tubs. Round, π × radius² × depth ÷ 231 (72 in across, 36 in deep ≈ 634 gallons); rectangular, length × width × depth ÷ 231 (84 × 72 × 36 in ≈ 943); oval, 0.785 × length × width × depth ÷ 231 (≈ 740). Use the average depth if the floor varies.

Formula and sources last checked September 30, 2026. How we test formulas.

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