What Size Heater Do I Need for My Pool or Hot Tub? (Free Calculator)

Last updated: September 2026

What size heater do you need for your hot tub, plunge pool or swimming pool? It’s one of the most common questions I get, and there’s no single answer, because it comes down to two things only you can decide: how quickly you want to heat the water, and what temperature you want to hold it at. Don’t just copy what your friend has or what a forum says. Think about how you’ll use it.

The good news is that the maths is simple. In this guide I’ll show you how it works, give you some worked examples and typical sizes, and you can use my calculator further down to run the numbers for your own tub or pool.

What do you need to decide first?

Before you look at any heater, answer these:

• How much water? A typical shop-bought 2m × 2m (6.5ft × 6.5ft) hot tub holds very roughly 1,000–1,500 litres (about 260–400 US gallons), and a DIY tub often holds much more. A plunge pool can be 5,000–15,000 litres, and a garden pool 30,000 litres or more. If you don’t know, length × width × average water depth in metres × 1,000 gives litres.
• How fast do you want it hot? Do you keep it hot all the time, or fire it up at the weekend? Occasional use from cold needs a much bigger heater than holding temperature.
• What temperature? A hot tub sits at 38–40°C (100–104°F). Don’t go above 40°C (104°F), the maximum the US Consumer Product Safety Commission recommends. A pool is more like 26–29°C (79–84°F).
• What can your supply take? A UK 13A plug-in tub is limited to about 2kW. A hard-wired UK tub usually runs 3kW, and a US 240V pack 4–5.5kW. More than that needs a heat pump, gas or a much bigger electrical supply.

How the maths works

To heat water you have to put a fixed amount of energy into it, whatever heater you use. It takes about 1.16kWh to raise 1,000 litres by 1°C. In US units, it takes 1 BTU to raise 1 pound of water by 1°F, and a US gallon of water weighs about 8.34 pounds.

Metric example: a hot tub

• A 1,500 litre tub, filled from the mains at 10°C, heated to 38°C, is a 28°C rise.
• Heat needed: 1,500 × 28 × 0.00116 ≈ 49kWh.
• On a 3kW spa pack heater: 49 ÷ 3 ≈ 16 hours, best case.
• On a heat pump putting out 14kW: 49 ÷ 14 ≈ 3.5 hours, best case.

US example: a pool

• A 20,000 gallon pool weighs 20,000 × 8.34 = 166,800 pounds.
• Raising it 10°F needs 1,668,000 BTU.
• A gas heater rated at 200,000 BTU/h input and 85% efficient puts about 170,000 BTU/h into the water, so that’s roughly 9.8 hours. If you forget the efficiency and divide by the full 200,000, you get 8.3 hours, which is too optimistic.

That’s the whole calculation. The calculator below just does it for you.

Use the heater’s real output

This is where most sizing goes wrong. The number on the box isn’t always the heat that ends up in the water.

Heater type	How it’s rated	What to use in the sums
Electric element	kW at a stated voltage	The kW, but only at the right voltage. A 5.5kW Balboa M7 heater gives 1.375kW at 120V.
Gas or propane heater	BTU/h input (fuel burned)	Input × efficiency. The US DOE says some new models reach 89–95% efficiency. Older units are lower.
Air source heat pump	Output in warm air (often 26°C / 80°F)	The cooler-air figure: 15°C in Europe, or the AHRI 50/63/80 point in the USA.
Heat exchanger	Output at a stated boiler and pool temperature	Much less at hot tub temperatures. See heating a hot tub from your boiler.

Heat pumps are the big one. The US Department of Energy notes that the federal test for pool heat pumps uses 80°F air and 80°F water, and that they work efficiently only while the air stays above about 45–50°F (7–10°C). For example, the Comfortline 20kW is rated at 20kW with 26°C air but 14kW with 15°C air. The Hydro Royal HR 110 is about 99,000 BTU/h at the AHRI 80°F point but about 66,000 BTU/h at 50°F. I’ve gone through those ratings in my Hydro Royal review and Comfortline review.

Typical heater sizes

Here’s what different heaters do on different volumes of water, best case, using the real-world outputs above.

Water volume	3kW element	14kW (heat pump at 15°C)	50kW (≈170,000 BTU/h gas output)
1,500 litres (hot tub)	1.7°C / 3.1°F per hour	8.0°C / 14.4°F per hour	28.7°C / 51.6°F per hour
2,800 litres (my DIY tub)	0.9°C / 1.7°F per hour	4.3°C / 7.7°F per hour	15.4°C / 27.6°F per hour
10,000 litres (plunge pool)	0.26°C / 0.5°F per hour	1.2°C / 2.2°F per hour	4.3°C / 7.7°F per hour
50,000 litres (garden pool)	0.05°C / 0.09°F per hour	0.24°C / 0.43°F per hour	0.86°C / 1.5°F per hour

That table explains a lot. A spa pack element is fine for holding a small tub at temperature, but it’s hopeless for a plunge pool. On my own 2,800 litre tub, the 3kW element managed about half a degree an hour in practice, well below the best case above, because the tub isn’t insulated. My 21kW heat pump (now sold as the 20kW) took that to about 4°C an hour in 15°C air. That story is in How I cut my hot tub running costs by 66%.

My rules of thumb

• Small shop-bought hot tub, kept hot: the spa pack’s 2–3kW element (or 4–5.5kW in the USA) will cope. A heat pump is about cutting costs, not speed.
• DIY hot tub for 4–6 people: I’d look at a heat pump of at least 9kW. For bigger tubs, 12kW or more. There’s little point going beyond about 24kW on a hot tub.
• Plunge pools and pool-and-spa combos: think in tens of kW, which means a big heat pump or gas. See How to choose the right air source heat pump for your hot tub or pool.
• Occasional use from cold: gas heats fastest, whatever the weather.

Is a bigger heater a waste of energy?

No, and this is a common misunderstanding. Raising 1,500 litres by 28°C takes the same 49kWh of heat whether it arrives over 16 hours or 3. A bigger heater just delivers it faster. If anything, a faster heat-up loses a little less along the way, because the water spends less time hot.

What does change your running costs is the type of heater, because each one costs a different amount per kWh of heat. At the UK Ofgem price cap for October to December 2026 (26.32p per kWh for electricity), an element costs about 26p for every kWh it puts into the water. That 49kWh heat-up is about £13. At the EIA’s July 2026 US average of 18.31¢ per kWh, it’s about $9. A heat pump delivers the same heat for a third to a quarter of that in mild weather. I’ve compared every option in What is the best way to heat a DIY hot tub?

The real downsides of oversizing are the purchase price, the bigger electrical or gas supply, and flow. Big heat pumps need a lot of water through them. The Comfortline 20kW wants 8–10m³ an hour, which a small circulation pump won’t deliver. Gas heaters that are far too big for a small tub can cycle on and off constantly.

Pool and hot tub heater calculator

Enter your water volume, start and target temperatures, and pick a heater or type in your own. Optionally, add how many hours you’d like it to take, and it’ll tell you the heater output you need.

This is a “lab calculation”: it ignores heat loss. Good insulation, a good cover, the air temperature and wind all affect real-world heating, so expect it to take longer than the figure shown, especially with the cover off or in cold weather.

Heating up vs holding temperature

The calculator tells you how long it takes to heat the water up. Once it’s hot, the heater only has to replace what the tub loses through the surface, the walls and the pipework. That’s much harder to put a number on, because it depends so much on your cover, insulation, the weather and how long the cover is off. For pools, the US DOE says evaporation is by far the largest source of energy loss, and that a cover can save 50–70% of heating costs. The same principle applies to a hot tub, which is why a good cover is the first thing I’d sort.

Two practical points:

• Size for your coldest months. A heat pump that’s perfect in May may struggle in January, when its output drops and the tub loses heat faster.
• If you keep a tub hot all the time, holding it a few degrees lower between soaks and turning it up before you use it is usually the cheapest way to run it. I cover that in hot tub heating and energy efficiency.

Common sizing mistakes

• Using a heat pump’s headline figure. Use the 15°C (or AHRI 50°F) output.
• Treating a gas heater’s BTU rating as output. It’s input, so multiply by the efficiency.
• Forgetting the voltage. A 240V element on 120V gives a quarter of its rated output.
• Ignoring flow. Heat pumps and gas heaters need a minimum flow, and a small circulation pump may not provide it. See How to correctly size a hot tub pump.
• Relying on a spa pack element for a big DIY tub or plunge pool. It will get there, eventually, at the highest cost per kWh.
• Skipping the cover and insulation. A bigger heater can’t make up for heat pouring out of the top of the tub. See DIY hot tub insulation and the ultimate guide to hot tub covers.

For more on how the heaters themselves work, see Hot tub heaters explained, the ultimate guide to air source heat pumps for hot tubs and hot tub gas heaters.

Planning your own build?

Heating is far easier to get right on paper than to retrofit. My DIY hot tub plans and plunge pool guides include the plumbing layout and parts list, so you can plan the pump, heat pump or gas heater, and the electrics from day one. For something custom, such as a pool and spa combo, I can draw up an architectural CAD and 3D plumbing design with the heating circuit sized in.

For heaters, heat pumps and connection kits, have a look at parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

What size heater do I need for a hot tub?

For a small tub kept hot, the spa pack’s 2–3kW element (4–5.5kW on 240V in the USA) is enough. For a larger DIY tub, I’d add a heat pump of at least 9kW, or 12kW or more for bigger tubs.

How long does it take to heat a hot tub?

Divide the heat needed (litres × temperature rise × 0.00116 kWh) by the heater’s output in kW. A 1,500 litre tub heated from 10°C to 38°C on a 3kW heater takes about 16 hours, best case.

How many BTU do I need to heat a pool?

Multiply the US gallons by 8.34 and by the temperature rise in °F. A 20,000 gallon pool needs about 1.67 million BTU for a 10°F rise. Divide that by the heater’s real output (BTU input × efficiency) to get the hours.

Does a bigger heater cost more to run?

Not for the same type of heater. The water needs the same energy either way. It just heats faster. Running costs depend on the heater type and your energy prices.

How many kW is 100,000 BTU?

About 29.3kW, as 1kW is 3,412 BTU per hour. For a gas heater, that’s usually the input, so the heat into the water is lower.

Got a sizing question for your own tub or pool? Drop it in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi