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Filling a 1,000 litre hot tub from a garden hose costs roughly the price of a cubic metre of mains water, which in most of Europe sits between 1 and 3 euros. That is the whole bill. Water is not what makes a hot tub expensive to run.
The arithmetic changes for a real swimming pool holding 18,000 litres, or when there is no mains connection and the water has to be delivered by tanker. Here is what each method actually costs, how often the water genuinely needs changing, and where the money really goes over a year.
What it costs to fill a hot tub
You need two numbers: how many litres the tub holds, and what you pay per cubic metre. The first is on the spec sheet, the second is on your water bill.
Look up the volumetric rate on your bill rather than a national average: tariffs vary by region, by supplier and by consumption band, and a household average can be off by a factor of three. The line you want is the charge per cubic metre, not the total.
With that figure, the calculation for two real models from our range.
| Model | Capacity | Cost to fill |
|---|---|---|
| Hot tub BL-837, 3 seats | 720 litres | 0.72 × your rate per m³ |
| Hot tub BL-832, 7 seats | 1,340 litres | 1.34 × your rate per m³ |
| Average above-ground pool | 18,000 litres | 18 × your rate per m³ |
Here is the part that surprises people: filling a hot tub costs less than a cup of coffee almost anywhere. The confusion comes from treating a hot tub like a swimming pool, when the pool holds 15 to 25 times more water.
The real cost is not the water. It is keeping it warm, and I come back to that below.
No mains connection? What tanker delivery costs
Everything changes when there is no mains supply, on rural land or an unserviced plot. Here water is bought by the load.
A tanker typically carries 4,000 to 5,000 litres, and the price is driven almost entirely by the distance from the extraction point rather than by the water itself. Get two or three quotes locally: the spread between suppliers in the same area is usually wider than people expect.
For a swimming pool the maths is simple, four loads for 18,000 litres. For a hot tub it is different, and worth knowing before you call: one load covers 700 or 1,300 litres several times over, but you still pay for the whole load. That is when the cost per litre goes through the roof, and when a storage tank starts to make sense if you will repeat the operation.
Three things to ask when booking, because they change the price and nobody volunteers them: whether the water is certified potable, how far the truck will stand from the discharge point (beyond a certain hose length a pump is needed), and whether discharge time is included or billed by the hour.
Rainwater and wells: free, but not immediately
These are the two alternatives everyone considers, and both work on one condition: the water is treated before it reaches the tub.
Rainwater is not clean water. It washes the roof and carries metals, particulates and debris with it, and its acidic pH attacks acrylic walls, pipework and the metal parts of the pump over time. Filling straight from a downpipe is the fastest way to cause damage that costs more than the water you saved.
To make it usable you need a first-flush diverter, a filter, and pH correction. That is a real upfront investment, and only after it is in place is the water genuinely free.
A well has the opposite problem: the water is often too hard, or rich in iron and manganese, which stain acrylic yellow or rust-coloured. Before connecting one, a chemical analysis costs very little and tells you whether a softener is needed.
The hardness reference, which applies to any source: below about 80 ppm (roughly 4.5 °dH) the water is soft and needs no intervention, between 120 and 180 ppm a softener is advisable, and above 180 ppm it becomes necessary. Past 250 ppm limescale builds up quickly on the heater element and the jets.
How often the water needs changing
For a hot tub under 2,000 litres, a complete change every 4 to 6 weeks. That is the figure in our water treatment guidance, and it assumes normal family use.
The reason is the ratio between bathers and volume. In an 18,000 litre pool, four people are an enormous dilution; in the 720 litres of a hot tub, those same four people concentrate sweat, cosmetics and residues into a volume twenty-five times smaller. It is not about visible dirt: it is chemistry.
The interval shortens if the tub is outdoors without a cover, if you use it daily, or if more than two people are in it at a time. It lengthens with an ozonator, which reduces the load on the chemicals but does not remove the need for a change.
Between changes the water has to be maintained, and here the numbers are precise:
- pH between 7.0 and 7.6, checked with a test kit. Above that range the sanitiser loses effectiveness, below it metal parts corrode
- Bromine tablets: replaced weekly, 20 gram format
- Filter cartridge: rinsed every 30 days with a hose, never in a dishwasher. Replace it when the pleats stay dark after washing
- Algaecide: 100 ml per 10 cubic metres, weekly
Full dosing and the correct sequence are in the guidance linked above. The rule that covers most of it: correct the pH first, then sanitise, because a sanitiser added to water outside its range works poorly and you waste it.
What the water costs to maintain over a year
Putting the items together, for a 700 to 1,000 litre hot tub in normal family use.
- Water: 8 to 12 changes a year, each one a cubic metre or less. On any European tariff this lands in the low tens of euros annually
- Chemicals: a pack of Bromine tabs costs € 79 and lasts several months, the SimplyBlu algaecide € 68. With pH correctors, budget a few hundred euros a year: they are all in our cleaning and maintenance range
- Filter cartridge: € 145 each, one or two a year depending on use. The model depends on the tub, and the five formats are among the paper filters
- Electricity: the dominant item by a wide margin, for a well insulated and covered tub
The breakdown says one thing clearly: water is the smallest line of all. Anyone worrying about the cost of filling is looking at roughly one percent of the spend and ignoring the ninety percent, which is heating.
It is also the one you can actually act on. A thermal cover left in place between soaks holds the accumulated heat: an uncovered tub loses it continuously and can consume twice as much. Before you look at your energy tariff, look at whether the cover seals properly and the gasket is intact.
The mistakes that cost the most
There are four, and I see them repeat.
Topping up instead of changing. Adding water to compensate for evaporation renews nothing: salts and residues stay behind and concentrate, because the water evaporates and what is dissolved in it does not. Topping up maintains the level, it does not replace a change.
Draining without switching off. A pump running dry is damaged in seconds. Power off first, then drain.
Filling with hot water from the boiler. It looks like a way to save on heating and it is the opposite: domestic hot water is more aggressive on limescale, and a household boiler is far less efficient at this job than the tub's own heater.
Leaving it full and idle for weeks. If you are away for a month, either keep the filtration cycle and the treatment running, or drain it. Still, untreated water in a warm tub is the worst case of all. If the break is over winter, the correct procedure is winter closure, which drains the circuits completely.
Frequently asked questions
How much does it cost to fill a hot tub?
Multiply the tub's capacity in cubic metres by your water rate: for a hot tub that is well under one cubic metre, so the cost is negligible on any tariff. A swimming pool at 18,000 litres is 18 cubic metres, which is still modest but no longer trivial.
How much does a water tanker cost?
It depends almost entirely on distance from the extraction point, so get local quotes. A tanker carries 4,000 to 5,000 litres. Always ask whether the water is certified potable and whether discharge time is included.
How often should you change hot tub water?
Every 4 to 6 weeks for a tub under 2,000 litres in family use. Sooner if you use it daily or it sits outdoors without a cover.
Can you fill a hot tub with rainwater?
Yes, but only filtered and pH corrected. Rainwater is acidic and carries debris from the roof: used as it comes, it attacks the acrylic, the pipework and the pump. You need a first-flush diverter, a filter and correctors.
How much does hot tub maintenance cost per year?
A few hundred euros in chemicals and cartridges, plus electricity, which is the dominant item. Water accounts for a few tens of euros a year, which is almost nothing.
Can hot tub water be reused on the garden?
Only once several days have passed since the last treatment and the sanitiser has broken down. Freshly treated water with bromine or chlorine should not go on plants or into a soakaway.
What to do, in order
What matters, in sequence.
- Check the litres on your spec sheet and multiply by the rate on your bill: that is the real cost of a fill, and it is almost always negligible.
- Put the change in the calendar every 4 to 6 weeks, rather than guessing when the water looks dirty.
- Test the pH before every treatment and keep it between 7.0 and 7.6: outside that range the products work poorly and you waste them.
- Rinse the cartridge every 30 days and keep a spare, so you do not skip a cycle while one dries.
- Check the thermal cover: it moves the bill far more than any water tariff.
The most common mistake is focusing on the cost of filling, worth a few euros, while ignoring heating, worth hundreds. The second is topping up for months in the belief that it renews the water.
If you are still choosing a tub and want a realistic running cost, the number to ask for is not the purchase price: it is the declared consumption with the cover in use, at the temperature you will actually keep it.
Full filling and treatment procedures are in our first fill guidance. Who we are and the experience behind these guides is on our about us page, and for your specific case you can write to us through the contact page.
The catalogue figures in this guide (capacities, dimensions and prices) are read from the product pages and update with the catalogue: what you see is the current value. Water, energy and delivery costs vary widely by country, region and tariff, so your own bill and local quotes are the only exact figures. Dosages and intervals are general guidance and do not replace your model's manual or the instructions on the chemical products you use.
