How it works
- Volume: round tubs are π × radius² × water depth; square and rectangular tubs are length × width × depth. Measure inside, and to the water line rather than the rim.
- Moulded seats: acrylic spas have seats and footwells moulded into the shell, which take up a large share of the box. For those, the maker's stated volume is more accurate than the geometry.
- Weight: a litre of water weighs 1 kg, so a US gallon weighs 8.34 lb. Add the people and the empty tub from its spec sheet.
- Heat: warming water takes 4.186 kJ per kg per °C, which is 1.16 kWh per 1,000 litres for every degree.
How much does a hot tub weigh?
Mostly water. A plain round tub 1.8 m (6 ft) across, filled 0.9 m (3 ft) deep, holds about 2,290 litres (605 US gallons): 2.3 tonnes of water before anyone gets in. Four adults add another 300 kg or so, and the tub itself comes on top. Wood-fired tubs with an internal stove hold a little less, because the stove takes up space.
Is my deck strong enough?
The calculator spreads the full weight over the tub's footprint and compares it with two common design figures: 40 lb per square foot for decks in the US residential code (IRC Table R301.5) and up to 2.0 kN/m² for floors in European homes (EN 1991-1-1). A filled tub usually exceeds both by a wide margin, and those figures are meant for people and furniture, not for a heavy load that stays all year. A deck or an upper floor needs a structural check before a tub goes on it. On the ground, the maker's instructions usually ask for a level concrete slab or a compacted gravel base.
Filling and heating
A garden hose fills a large tub in an hour or two: measure your hose flow with a bucket for the real figure. Heating a fresh fill takes the most energy of any session; the hot tub running cost calculator adds the heat losses for your climate and cover, and the wood-fired hot tub guide covers stoves and firewood. For cold plunges, the same volume feeds the chiller calculator.