Hot Tub Volume Calculator: How Many Litres Does It Hold?

Almost every hot tub job starts with one number: how many litres of water are in it. Chemical doses, heating times and running costs all scale with it, and the "4 to 6 person" label on the box tells you little about it. There are two honest ways to find out at home: measure the inside and do the geometry, or time how long your hose takes to fill a bucket and then the tub. The calculator below does both, and gives the answer in litres, UK gallons, US gallons and the weight of the water.

How the volume is worked out

Water volume is the area of the water surface multiplied by the depth. For a round tub the area is π × the radius squared, for a square or rectangle it is length × width, and for a regular octagon measured flat side to flat side it is about 0.828 × the width squared. Every 1,000 cubic centimetres is one litre. So a round inflatable 140 cm across inside, filled to 50 cm, holds π × 70 × 70 × 50 = 769,690 cubic centimetres, or about 770 litres.

The hose method skips the geometry. If a 10 litre bucket fills in 40 seconds, the hose delivers 15 litres a minute, and a tub that took 60 minutes to fill holds about 900 litres. It works for any shape, which makes it the better choice for hard-shell tubs. The catch is that water pressure varies, so time the bucket just before or after you fill the tub, not on a different day.

Why hard-shell tubs come out high

A hard-shell spa is not an open box. The seats, loungers and footwell are moulded into the shell and all take up space that would otherwise hold water, so the measured outline overstates the real volume, often by a wide margin. The calculator still shows the outline figure for a hard-shell tub, labelled as the most it could hold, but for anything that matters, such as chemical dosing, use the volume on the manufacturer's spec sheet or the hose method instead.

What to do with the number

• Dosing chemicals. Every chlorine, bromine and pH product dose is given per volume of water. Put your litres into the hot tub chlorine calculator to get a dose in grams.
• Heating and running costs. More water takes more energy to heat from cold. The hot tub running cost calculator turns that into pounds per day and year.
• Weight on the base. A litre of water weighs about a kilo, so the water alone in a 1,000 litre tub weighs about a tonne before you add the tub and the people in it. Check that decking or a balcony is rated for the filled weight given in the manual before you set a tub on it.
• Overflow. Each person who gets in raises the water level, by up to about a litre for every kilo they weigh (less in practice, as heads and shoulders stay above the water), so fill to the line in the manual, not to the brim.

If you would rather someone else did the filling and the chemistry, our guide to private spa hire across the UK lists places to book a hot tub by the hour.