When people send me their first sketch of a DIY hot tub, they’ve usually thought hard about the length and width and hardly at all about the depth. Yet hot tub depth is what decides whether the tub is comfortable: whether the water comes up to your shoulders, whether you float off the seat, and whether you can get in and out without a scramble. In this post I’ll go through the numbers I design to for water depth, seat height, the footwell and the steps, in feet and inches and in metres.
My own tub, 1.1m deep, waterline at 90cm.
The short version
- Water depth: I design to about 90cm (35.5in) from the floor to the centre of the skimmer. Most comfortable built-in tubs land somewhere around 3ft to 3ft 6in (0.9–1.07m).
- Seat height: about 45cm (17.75in) from the floor, which leaves roughly 45cm (17.75in) of water over the seat.
- Seat depth: 50–55cm (19.75–21.75in) front to back.
- Footwell: whatever is left in the middle once the benches are in. On small tubs that’s less than you think.
- Steps: even risers, deep treads, non-slip, and ideally something to hold on to.
How deep should a hot tub be?
There’s a sweet spot. Too shallow and the water only reaches your waist when you sit, so it never feels like a proper soak and your shoulders get cold in winter. Too deep and the seats end up so far below the surface that shorter bathers float off them, and anyone standing in the footwell is up to their chest before they’ve sat down.
The number I use as my starting point is 90cm (35.5in) of water from the floor to the centre of the skimmer. The skimmer sets the water level, so that’s the figure that matters, not the height of the walls. Add the gap from the waterline to the top of the coping and the internal shell ends up a little deeper than that.
Most shop-bought acrylic spas are in the same rough range, around 3ft (90cm) at the deepest point, because that’s the depth that works for a seated adult.
Is there a maximum hot tub depth?
For a private garden tub, there usually isn’t a hard legal limit on depth, but there are rules worth knowing about. In the US, public spa codes commonly cap spa water depth at about 4ft (1.22m) from the waterline, and some also limit how far below the waterline a seat can be (Nevada, for example, sets 2ft or 61cm for public spas). Residential rules vary, so check with your local building department. In the UK, a domestic hot tub isn’t normally covered by depth rules, but the electrics are, and that’s a job for a qualified electrician under Building Regulations.
Depth also isn’t the same as temperature, but while we’re on safety: most safety bodies recommend a maximum of 104°F (40°C) for hot tub water, whatever its size or depth.
Seat height and hot tub depth go together
You can’t pick the water depth on its own, because what you actually feel is the water over the seat. With 90cm (35.5in) of water and a seat 45cm (17.75in) up from the floor, you’ve got roughly 45cm (17.75in) of water over the seat. For most adults that puts the water somewhere between mid-chest and the shoulders, depending on how you sit, and you can always slide forward to sink lower.
These are the figures I design to:
| Dimension | Metric | Imperial |
|---|---|---|
| Water depth (floor to centre of skimmer) | 90cm | about 35.5in |
| Seat height (from the floor) | 45cm | about 17.75in |
| Seat depth (front to back) | 50–55cm | about 19.75–21.75in |
| Water over the seat | about 45cm | about 17.75in |
They’re a starting point, not a law. A few things change them:
- Tall household? Drop a seat by a few centimetres or deepen the water slightly so the water still reaches the shoulders.
- Mixed heights? Vary the seat heights around the tub. A slightly higher seat in one corner suits children and shorter adults, and a lower one suits the tallest person in the house.
- Jets. Set the seat height first, then position the back jets from it.
On a block-built tub, the seat height is tied to your block courses, which is one reason I draw my plans around real block sizes. I explain why in imperial or metric plans.
The footwell: how much legroom you really get
The footwell is the floor area in the middle, between the benches. It’s where your feet go, where you stand when you get in, and where the main drains sit. It’s whatever is left once the benches are in, which can catch you out on a small tub.
Here’s what you’re left with if you run a 52.5cm (about 20.5in) bench along two opposite walls:
| Internal size | Footwell width between opposite benches |
|---|---|
| 6ft x 6ft (1.83m x 1.83m) | about 2ft 7in (78cm) |
| 7ft x 7ft (2.13m x 2.13m) | about 3ft 7in (108cm) |
| 8ft x 8ft (2.44m x 2.44m) | about 4ft 7in (139cm) |
| 10ft x 10ft (3.05m x 3.05m) | about 6ft 7in (200cm) |
On a 6ft x 6ft tub with benches on all sides, knees will meet in the middle. That’s cosy for two, less so for four. By 8ft x 8ft there’s room to stretch your legs out without kicking whoever is opposite, which is a big part of why it’s the most popular size I sell. If you want a small tub that doesn’t feel cramped, consider benches on only two or three sides, or a corner seat arrangement, rather than a bench all the way round.
The footwell floor is also where your main drains go. Always fit two drains with compliant anti-entrapment covers, so a single blocked drain can never hold someone down. I go through drains, suctions and returns in what makes a good DIY hot tub design.
[ANDI: add a note on the footwell size you see customers regret, or a photo of a well-proportioned footwell]Steps: getting in and out safely
A deep tub needs a safe way in. Steps inside the tub are the usual answer on a sunken build, and on most of my designs the bench itself becomes the last step down. So the route in is typically: coping, one or two steps, the seat, then the footwell floor.
What makes steps work:
- Even risers. Every rise the same height. Uneven steps are what trip people, especially when they can’t see them clearly through moving water.
- A comfortable rise. For a DIY build I’d keep each rise somewhere around 7–10in (18–25cm). Outside the tub, residential deck steps under the US pool and spa code are limited to 7.5in (191mm) risers with treads of at least 10in (254mm).
- Deep treads. At least 10in (25cm), so a whole wet foot fits on the step.
- Grip. Textured or non-slip tiles on the treads, and a contrasting edge tile or step light so the edge is visible at night.
- Something to hold. A grab rail or handrail costs very little compared with the build,.
Steps inside the tub take up seating space, so place them early. For deck steps, see my hot tub steps guide.
Sit-on edges for raised tubs
If your tub is raised or semi-sunken, think about the height of the wall above the ground. A coping at roughly seat height, about 18–20in (45–50cm), lets people sit on the edge and swing their legs over, which many find easier than stepping down into the water.
Loungers change the depth
A lounger is a different shape of seat altogether. Rather than sitting upright, you’re lying back with your legs out in front, so the water depth over your body depends on the angle of the backrest and how high the leg section sits.
That’s why I’d never freehand a lounger. My The Kyle plans are built around a dual lounger, with the lounger profile drawn out so the depth works for a reclining adult. A lounger also takes up the space of about two seated people, so allow for it when you choose the overall size.
How deep do you need to dig?
The depth you dig isn’t the water depth. Under the floor of the tub you need a compacted sub-base, insulation if you’re using it, and a reinforced slab. Above it, render, waterproofing, adhesive and tile all add a little. Then there’s the gap from the waterline to the top of the coping. All of that has to be added to the 90cm (35.5in) of water.
The exact build-up depends on your ground, your slab design and your wall system, and slab thickness and reinforcement are structural decisions for your drawings or a structural engineer. My in-ground hot tub guide goes through the layers from the bottom of the hole up. And remember that depth means weight: water is about 8.34lb per US gallon (1kg per litre), so a deeper tub is a heavier one. If you’re building on a deck, a roof or anywhere raised, have a structural engineer check the loads.
Depth, volume and running costs
Every extra inch of depth is more water to heat and treat. On an 8ft x 8ft (2.44m x 2.44m) tub, each extra inch of water depth adds about 40 US gallons (each extra 2.5cm adds about 150 litres). Make the tub 6in (15cm) deeper than it needs to be and you’re heating around 240 US gallons (about 900 litres) more water every time.
So don’t go deeper “just in case”. Spend the money on insulation and a good cover instead.
Hot tub depth vs cold plunge and plunge pool depth
If you’re building a cold plunge or a plunge pool alongside, the rules are different. A cold plunge usually has no seats, because people stand or lie down in it, so it tends to be deeper: typically 0.9–1.5m (3–5ft), enough to get your shoulders under. I cover that in how to build a DIY cold plunge pool. A plunge pool is deeper again, and needs its own design rather than a scaled-up hot tub.
Getting the numbers right before you dig
Depth, seat height, footwell and steps all depend on each other, and they all have to be fixed before the slab goes in. That’s what proper plans are for. My hot tub plans come in 21 sizes, imperial and metric, with the water depth, seat heights, steps and plumbing all drawn in CAD. The 8ft x 8ft plans are the most popular. For an unusual shape, a lounger or a plunge pool alongside, my Tub & Pool Designer Pro architectural CAD and plumbing design service draws it around your site.
Want the full drawings?
Every set of my hot tub plans gives you the water depth, seat heights, steps and footwell already worked out, with a full plumbing layout and shopping list. More than 1,500 customers have used my plans and designs.
FAQs
How deep is a typical hot tub?
Most comfortable hot tubs have around 3ft to 3ft 6in (0.9–1.07m) of water in the footwell. I design to about 90cm (35.5in) from the floor to the centre of the skimmer.
What is a good hot tub seat height?
About 45cm (17.75in) from the floor works for most adults, leaving roughly 45cm (17.75in) of water over the seat. Varying the heights a little around the tub suits people of different sizes.
Can a hot tub be too deep?
Yes. If the water over the seat is too deep, shorter bathers float off the bench and children can’t sit safely. It also adds water to heat. US public spa codes commonly cap spa depth at about 4ft (1.22m).
How high should steps into a hot tub be?
Keep every rise the same, ideally around 7–10in (18–25cm), with treads at least 10in (25cm) deep, non-slip tiles and a handrail or grab rail.
Got a question about your own build? Drop it in the comments below. I read them all.
Cheers, Andi