Stock tank sizes are the first thing to settle if you’re turning a farm trough into a hot tub or a plunge pool. Get the tank right and everything else (the heater, the jets, the base, the cover) falls into place. Get it wrong and you end up with a tub that’s cramped, slow to heat or far heavier than the spot you put it on can take.
Here are the common sizes and shapes, how much water each really holds in use, how many people fit, and which tank I’d pick for which job.
The short version
- 8ft (2.4m) round is the sweet spot for a stock tank hot tub. It seats four or five adults and still heats in a reasonable time.
- 6ft (1.8m) round suits two or three people and is the cheapest to heat.
- 10ft (3m) round is better thought of as a small plunge or dipping pool than a hot tub.
- Almost all tanks are about 2ft (60cm) tall, so whatever the diameter, you sit on the floor in roughly 20in (50cm) of water.
- 12ft tanks are hard to find. If you want something that big, a built pool is usually the better answer.
Common stock tank sizes at a glance
Sizes are nominal and vary between makers and batches (big tanks nest for shipping, so diameters can differ by a few inches). These are typical figures from the main US makers’ published specs in October 2026. Always check the actual tank before you plan the plumbing.
| Tank | Typical inside size | Brim-full capacity | In use (filled to ~20in / 50cm) | As a hot tub |
|---|---|---|---|---|
| 4ft round | 44–47in x 24in (1.1–1.2m x 60cm) | ~165 US gal (625 L) | ~130 US gal (500 L) | Cold plunge for one |
| 6ft round | 65–69in x 23.5in (1.7m x 60cm) | ~389 US gal (1,470 L) | ~300–330 US gal (1,150–1,250 L) | 2–3 adults |
| 8ft round | 89–96in x 23.5in (2.3–2.4m x 60cm) | ~700 US gal (2,650 L) | ~600 US gal (2,270 L) | 4–5 adults |
| 10ft round | 113–120in x 23.5in (2.9–3m x 60cm) | ~1,100 US gal (4,200 L) | ~915 US gal (3,460 L) | Small plunge pool |
| 300 gal poly (e.g. structural foam) | about 69in x 63in x 25in (1.75m x 1.6m x 64cm) | 300 US gal (1,135 L) | ~240 US gal (900 L) | Cold plunge, or a tight two-person soak |
The “in use” column matters more than the label. You won’t fill a hot tub to the brim, because bodies displace water and the jets throw it about, so I work on about 20in (50cm) of water in a 2ft tank. That’s the figure to use for heater sizing, chemical doses and weight.
Round, oval or poly: which stock tank shape?
Round galvanised tanks
This is the classic stock tank hot tub. Round tanks come in the widest range of diameters, they’re strong (the round shape resists the water pressure evenly), and everyone sits facing in, which is how a hot tub should feel. If you’re building a hot tub, start here.
The downside is rust. Warm, sanitised water attacks the zinc coating over time, which I cover in detail in how to build the ultimate DIY stock tank hot tub.
Oval (round-end) tanks
Oval tanks are long and narrow, typically around 2–3ft (60–90cm) wide. They’re great against a fence or along the edge of a deck, and they make a decent cold plunge, because you can lie back with your legs out. As a hot tub they’re less sociable: two people can sit at each end, but nobody’s really facing anyone.
Poly (plastic) tanks
Poly and structural foam tanks won’t rust, and they’re lighter to handle. The catch is temperature. A tank designed for drinking water for cattle isn’t necessarily rated for water at 100–104°F (38–40°C) all year round, and soft plastics can bulge or deform when they’re warm and full. Check the maker’s rating for hot water before you buy a poly tank for a hot tub. For a cold plunge, poly is a very good choice.
How many people fit in each stock tank size?
Because there are no seats, people sit on the floor with their backs against the wall. A rough rule is that each adult needs about 2–2.5ft (60–75cm) of wall to lean on, and the legs need somewhere to go in the middle. That’s why the numbers don’t scale as neatly as you might expect:
- 6ft round: two people comfortably, three if you’re friendly. Legs start to tangle in the middle.
- 8ft round: four adults comfortably, five at a push. This is where it starts to feel like a proper hot tub.
- 10ft round: six or more, but it’s so wide that it behaves more like a pool.
If depth is the bigger question for you, my guide to how deep a hot tub should be explains why a 2ft tank feels so different from a built tub with seats.
Stock tank sizes for a hot tub vs a plunge pool
For a hot tub
Go for an 8ft round if you’ve got the space and the budget, or a 6ft round if it’s mainly for two. Smaller volumes heat faster and cost less to keep warm, and a hot tub is about the soak, not the swim. On my stock tank pool heater figures, raising an 8ft tank by 10°C (18°F) takes about 26kWh before any heat is lost, while a 6ft tank needs roughly half that.
For a plunge pool or cold plunge
A 4ft round, 300 gal poly or oval tank is plenty for a cold plunge for one or two. You want enough depth to get your shoulders under, which means sitting or kneeling in a 2ft tank. For a summer dipping pool for the family, a 10ft round is the usual choice. It still won’t be deep, so think of it as somewhere to cool off and float rather than swim.
What about a 12 foot stock tank?
People search for 12ft tanks a lot, but they’re rare. The mainstream makers’ round ranges top out at around 10–11ft (3–3.3m) diameter, and still only 2ft deep. A 12ft tank, if you can find one, would hold well over 1,500 US gallons (5,700 litres) and weigh more than 6 tonnes full, yet you’d still be sitting in 20in of water.
At that point I’d look at building something instead. A 12ft x 8ft cocktail pool gives you a similar footprint with proper depth, seats and steps, and it lasts for decades. My cocktail pool cost guide shows what that really costs.
Weight: check your base before you buy
Water weighs about 8.3lb per US gallon (1kg per litre), so even a small stock tank is heavy:
- 6ft round at 20in of water: about 2,700lb (1.2 tonnes) of water.
- 8ft round: about 5,000lb (2.3 tonnes), before you add four or five people.
- 10ft round: about 7,600lb (3.5 tonnes).
Spread over the base, 20in (50cm) of water puts roughly 105lb per sq ft (about 500kg per m²) on the ground, plus people. A level, compacted gravel pad or a concrete slab is the usual answer on the ground. See how to build a concrete slab or base for your hot tub. If you want to put a tank on a deck, a balcony or a roof, have a structural engineer check it first. Local building regulations apply.
Matching the equipment to the tank size
Bigger tanks need more heat, more filtration and more jets. People get caught out buying a 10ft tank and then finding a small plug-in heater barely moves the temperature.
- Heater: for hot tub temperatures, a 3–4kW spa pack heater on a dedicated circuit is the realistic minimum for a 6ft or 8ft tank. A heat pump helps a lot with running costs on the bigger sizes.
- Pump and filter: size them so the whole volume turns over several times a day. A larger tank needs a bigger filter area or longer filter cycles.
- Jets: more diameter means more wall to cover.
- Suction: always two suction drains, whatever the size, with covers that meet the relevant standard (ANSI/APSP/ICC-16 in the USA).
- Electrics: it’s a metal tank full of water, so it needs a qualified electrician, RCD/GFCI protection and the correct bonding. Local regulations apply. See my guide to hot tub electrics.
My complete stock tank hot tub kits come with 4, 8 or 16 jets, or as heating and filtration only, with a Balboa spa pack, pump, filter and all the fittings matched to each other. You add the tank and the pipe.
What a stock tank costs in 2026
Prices move around and vary a lot by region, so treat these as a guide. In October 2026, US farm stores were listing an 8ft x 2ft galvanised round tank at about $550–$660 (roughly £410–£500), an 8ft round poly tank at about $500 (£375) and a 300 gallon structural foam tank at about $300–$390 (£225–£295). In the UK, big round galvanised troughs are harder to find off the shelf, so check agricultural suppliers and allow for delivery.
The tank is the cheap part. The heater, pump, filter, insulation, cover and electrics are where most of the money goes. My cheapest hot tub to build post compares a stock tank with the other budget options.
My pick
For most people, I’d buy an 8ft round galvanised tank, set it on a level slab, insulate the sides, fit a proper spa pack with filtration and put a good insulated cover on it. If it’s just for two, or your electrics are limited, a 6ft tank is cheaper to run. If you want a cold plunge, go small and consider poly.
And if you find yourself wanting more depth, seats or something that lasts decades, that’s the point to look at a built tub instead. My CAD-drawn hot tub plans come in 21 sizes, including the best-selling 8ft x 8ft plans, which have a similar footprint to an 8ft stock tank but around twice the depth.
Ready to build your stock tank hot tub?
My complete stock tank kits include the spa pack, pump, filter, jets, fittings and a full DIY guide, so the equipment is matched to your tank from day one. If you’d rather build something deeper and permanent, my CAD-drawn hot tub plans come in 21 sizes, and I’ve helped 1,500+ customers with their projects.
FAQs
What size stock tank is best for a hot tub?
An 8ft (2.4m) round tank is the best all-rounder, seating four or five adults. A 6ft (1.8m) round tank suits two or three people and costs less to heat.
How many gallons is an 8ft stock tank?
About 700 US gallons (2,650 litres) brim full. As a hot tub filled to around 20in (50cm), it holds roughly 600 US gallons (2,270 litres).
How deep are stock tanks?
Most round and oval stock tanks are about 2ft (60cm) tall, whatever the diameter. Filled for use, that’s about 20in (50cm) of water, so you sit on the floor rather than on a seat.
Can you get a 12 foot stock tank?
They’re rare. Most makers’ round tanks stop at about 10–11ft. For something that size, a built cocktail or plunge pool usually gives you a better result for the money.
Is a poly stock tank OK for a hot tub?
Only if the maker rates it for water at 100–104°F (38–40°C). Poly tanks are ideal for cold plunges because they don’t rust, but some soften or bulge when they’re hot and full.
Which stock tank size did you go for? Tell me in the comments below.
Cheers,
Andi