What Is a Spool? The Spa-Pool Hybrid Explained (Sizes, Costs and DIY)

Last updated: September 2026

What is a spool? It’s one of my favourite things to design. Take a small pool, add the heating and jets from a hot tub, and you get a “spool”: spa plus pool. Warm it up in winter for a soak, let it cool in summer for a dip, and in a typical back garden it fits where a full-size pool never would.

In this guide I’ll explain what a spool is (and isn’t), how it compares with a pool, a hot tub and a plunge pool, what size it should be, what it costs, how to keep the water right, and whether you can build one yourself. Spoiler: you can.

What is a spool?

A spool is a small pool that borrows features from a hot tub. It’s heated, it usually has jets (and sometimes an air blower), and it has benches or loungers built in. Because you control the temperature, you can use it both ways: a warm soak in winter and a cooler dip in summer. Essentially, it’s a super-sized hot tub, often around 12ft by 12ft or a bit longer.

You’ll hear the names used interchangeably, so here’s how I think of them:

• Cocktail pool: a small pool for relaxing with a drink rather than swimming lengths. It may or may not be heated.
• Spool: a cocktail pool with hot tub features, meaning heating and jets.
• Plunge pool: a small, slightly deeper pool for cooling off, usually unheated or gently heated, and often without jets.

Spool vs pool vs hot tub vs plunge pool

	Hot tub	Spool / cocktail pool	Plunge pool	Swimming pool
Typical size	7–10ft (2–3m) square	10–16ft (3–5m) long	10–20ft (3–6m) long	Much larger, with room to swim
Depth	~3ft 3in (1m), with seats	3½–4½ft (1.1–1.4m), with benches	4–5ft (1.2–1.5m)	Varies, often with a deep end
Water temperature	37–40°C (98–104°F)	Anything from pool to hot tub temperature	Unheated to about 30°C (86°F)	Usually unheated or warm
Jets	Yes	Yes (that’s what makes it a spool)	Usually no, but a swim jet is an option	Rarely
Used for	Hot soaking	Socialising, soaking and cooling off	Cooling off and wading	Swimming and playing

If you’re still choosing, Hot tub vs plunge pool: which should you build in 2026? goes through the trade-offs, and Plunge pool: everything you need to know covers the cooler option.

What size should a spool be?

There’s no official definition, but most spools and cocktail pools are around 10–16ft (3–5m) long, 6–10ft (2–3m) wide and 3½–4½ft (1.1–1.4m) deep. There’s no deep end. As a rough guide:

• A couple or a small family: around 10ft x 7ft (3m x 2.1m) is plenty for a bench and a lounger.
• Entertaining a group: 12ft x 8ft to 16ft x 9ft (3.7m x 2.4m to 4.9m x 2.7m) gives room for benches along two sides, loungers and steps.

Bigger isn’t always better. Every extra foot is more water to heat, a bigger cover and a bigger bill. Size it for how many people will really use it most of the time, not the one party a year.

Pros and cons of a spool

Pros	Cons
Use it all year: hot in winter, cool in summer	Far more water to heat than a hot tub, so higher running costs
Fits a garden that can’t take a full pool	Too short for swimming lengths (unless you add a swim jet)
Much cheaper to build and run than a full pool	A cover is bigger, heavier and often needs to be made to measure
Benches and loungers make it very sociable	Deep enough to need proper safety barriers

To put the heating in perspective: a 12ft x 8ft spool 4ft deep holds about 11,000 litres (2,900 US gallons). Raising that by 10°C (18°F) takes about 128kWh of heat. A 3kW spa heater would take over 40 hours, even with no heat loss. That’s why I recommend an air source heat pump for a spool. See how to choose the right air source heat pump for your hot tub or pool.

Can I build my own spool?

Absolutely. If you can build an in-ground hot tub, you can build a spool. It’s the same method, just bigger: dig, lay a reinforced slab, build ICF or core-filled block walls with rebar, fit the drains, jets and skimmer, then render, waterproof and tile. What you’re probably not going to do is gunite or shotcrete. That’s a job for a specialist crew.

Where people go wrong is treating it structurally like a big hot tub. That 11,000-litre spool is around 11 tonnes of water, about four times what my own tub holds. So the slab, rebar and walls need to be done properly, and on a deep build or a tricky site it’s worth getting an engineer to check the design.

I’ve written the full step-by-step in Build the ultimate DIY cocktail pool or spool. The groundwork stage is covered in detail in my DIY in-ground hot tub build guide.

Design considerations

Benches, loungers and steps

These are what make a spool feel different from a plunge pool. A bench along one or two sides at seated depth, a step-down entry and an in-water lounger are the classic features. See how to build a concrete lounger.

Jets

Decide on the jets at the design stage, because the jet bodies and pipework go in during the build. Keep the jet circuit separate from the filtration and heating circuit. As a rough guide from our builds, up to about 16 jets can run on a single 4HP pump if the control room is close. See why pool and hot tub plumbing should remain separate and how to correctly size a hot tub pump.

Heating

An air source heat pump is the sensible main heat source. It’s the cheapest to run, and many models can cool as well. Gas heats faster but costs more to run. A spa pack heater is fine as a top-up but too slow on its own. If you run it at hot tub temperatures, the CPSC says hot tub water should never exceed 104°F (40°C).

The cover

Heat is lost mostly from the surface, so a spool without a good cover is expensive to run. The US Department of Energy says evaporation is by far the largest source of heat loss from a pool, and a cover can cut heating costs by 50–70%. Design the spool around a cover you can buy or have made. My guide to hot tub covers covers the options, including automated ones.

Finish

Tiles on a pool-rated membrane cost the most but last the longest, and they’re my preference. A liner is cheaper upfront but needs replacing over time. Render and pool paint is the budget option, but it needs repainting.

Safety

• Barriers: in most of the USA, a pool barrier at least 48in high with self-closing, self-latching gates is required (see the CPSC barrier guidelines). In the UK it isn’t law for a private pool, but RLSS UK recommends secure fencing at least 4ft high.
• Drains: fit two main drains, at least 1m (3ft) apart, with anti-entrapment covers. See Installing a main drain.
• Electrics: leave them to a qualified electrician. A spool is treated as a pool under BS 7671 Section 702 in the UK and NEC Article 680 in the USA.

Keeping the water clean

A spool needs the same water care as any pool or hot tub: filtration, a sanitiser and balanced water. The twist is temperature. When you run it hot, treat it like a hot tub. When it’s at pool temperature, pool levels are fine.

Parameter	Target	Notes
Free chlorine	At least 1ppm at pool temperature. At least 3ppm when run hot	CDC guidance for pools and hot tubs
Bromine (alternative)	4–6ppm	More stable in hot water, but sunlight breaks it down
pH	7.2–7.6	Within the CDC’s 7.0–7.8 range
Total alkalinity	80–120ppm	Buffers the pH
Calcium hardness	200–400ppm	The pool range, which protects grout and render in a tiled spool
Cyanuric acid	Only if you run it as an outdoor pool	The CDC advises against stabiliser in hot tubs

A good filter, sized for the volume, does most of the work. Clean it regularly and shock after heavy use. My guide to balancing hot tub chemicals explains each step, and Should I use bromine or chlorine? helps you choose a sanitiser.

How much does a spool cost?

Nobody publishes a price survey for “spools” specifically, so the closest comparison is a professionally built plunge pool:

• USA: Angi’s cost guide puts the average plunge pool at about $28,000, with most people paying $10,000–$40,000 and concrete pools $20,000–$75,000 (Angi figures, professionally installed).
• UK: Checkatrade puts a concrete in-ground plunge pool at £15,000–£25,000, plus £1,500–£4,500 labour, and £1,200–£2,600 for a heater (Checkatrade figures).

Add jets and a heat pump to turn it into a spool, and a pro build moves towards the top of those ranges.

DIY is a fraction of that. Based on the large hot tub projects we’ve worked on, my rough estimate in 2021 was that a DIY cocktail pool or spool should come in under $10,000, and possibly around $7,000 if you kept costs as low as possible. Prices for concrete, steel and equipment have gone up since then, so treat those figures as a floor, not a budget. The things that push the cost up are:

• The finish. Pool tiles cost more than a liner or render and paint.
• Jets and blowers, which need extra pumps, pipework and wall fittings.
• Heating. A heat pump costs more upfront but much less to run.
• What you hire in: digger, skips, concrete pump, bricklayer, electrician.
• The surroundings, such as decking, paving and fencing.

For running costs, my 2021 estimate was around $1,000 a year at the low end and up to $3,000 if you heat it a lot. The real figure depends on how often you heat it, your climate, your energy tariff, insulation and whether you use a cover.

Common mistakes

• Building it too big to heat affordably.
• Relying on a small spa heater for a pool-sized volume of water.
• Adding jets as an afterthought, once the walls are up.
• Skimping on the slab and rebar because “it’s only a big hot tub”.
• Forgetting the cover until the end, then finding nothing fits.
• No barrier or lockable cover around something deep enough to drown in.

Want it drawn up for you?

My plunge pool / cocktail pool plans work perfectly as a spool. They come in standard sizes from 10ft x 7ft and 12ft x 8ft up to 16ft x 9ft and 20ft x 10ft, with architectural drawings, plumbing layouts, steps and a lounger area, in imperial and metric.

Want something bespoke? Draw it in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design.

FAQs

What does “spool” stand for?

It’s a blend of “spa” and “pool”: a small pool with hot tub features like heating and jets.

Is a spool the same as a cocktail pool?

Nearly. A cocktail pool is a small pool for relaxing. Add heating and jets and most people call it a spool.

How big is a spool?

Usually around 10–16ft (3–5m) long, 6–10ft (2–3m) wide and 3½–4½ft (1.1–1.4m) deep.

Can you swim in a spool?

Not lengths. It’s too short. If you want to swim, add a counter current swim jet and make it at least 14ft (4m) long. See swim jet considerations.

How much does it cost to heat a spool?

It depends on the size, your climate, your tariff and the cover. A heat pump and a good cover are the two biggest savings.

Can I build a spool myself?

Yes. It’s built the same way as an in-ground hot tub, just bigger. Start with my step-by-step cocktail pool and spool guide.

Thinking about a spool? Ask me anything 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