How long a hot tub takes to heat
Most people fill a tub in the morning and expect to be in it by lunch. Then they watch it creep up two degrees an hour and start wondering what is wrong. Usually nothing is.
The short answer
A typical 400 gallon tub with a 4 kW heater gains roughly 2 to 3 °F an hour. From cold tap water at about 60 °F to a soak at 100 °F, that is most of a day.
Smaller plug-in tubs are slower still. A 1.5 kW heater in a 500 gallon tub is under 1 °F an hour, so a fresh fill is genuinely an overnight job.
Why it is slower than the sum says
The obvious maths is energy needed divided by heater size. That would be right if the tub were a perfect flask. It is not.
From the moment you switch on, heat escapes through the shell, the plumbing and the cover. And the hotter the water gets, the faster it escapes — loss scales with the gap between water and air. So the heater is fighting a target that fights back harder the closer you get.
The first few degrees come quickly. The last few crawl.
What actually changes the time
The cover. Put it on while heating. This is free and it is the single biggest factor. An open tub on a cold day can take twice as long.
The air temperature. The same tub and heater will be dramatically faster in July than in January, because the gap the heat is escaping across is smaller.
Where you start. Filling from a hot tap instead of cold can remove hours. Worth doing if your supply allows it.
Wind. An exposed tub loses heat far faster than a sheltered one. A fence or a screen does more than people expect.
When it will never get there
Every tub has a ceiling: the temperature at which heat lost equals heat added. Below it you are still gaining. At it, you have stopped, and no amount of extra time changes that.
A 1.5 kW heater on a large uncovered tub at freezing can sit at 85 °F all day and all night. Nothing is broken. The heater has simply met its match.
If you are stuck well below target, more time is not the fix. The cover, a windbreak, or a bigger heater is.
Is it cheaper to leave it on?
Almost always, yes. Holding a well-covered tub at temperature costs less than letting it drop and reheating, and you do not spend a day waiting.
Switching off only makes sense for a long absence — a holiday, say — and even then a good cover narrows the gap. The running cost calculator will price both for your tub.