Cinder Block Hot Tub: A DIYer’s Guide to Building One

Last updated: September 2026

Can you build a hot tub from cinder blocks? Absolutely. It’s how I built my own DIY hot tub, and I’d never laid a block before I started. For me, block is still one of the most straightforward ways to build a square or rectangular tub. The methods you’ll see on YouTube, like shotcrete and gunite, are skilled trades (a bit like plastering) and not something most of us can do ourselves. Laying blocks is.

This guide covers the main points you need to think about, from sizing and the base to the walls, plumbing, insulation and waterproofing.

What is a “cinder block”, anyway?

In the USA, most people say cinder block when they mean any grey concrete block. Strictly, a cinder block was made with fly ash or cinders as the aggregate. The blocks you’ll buy today are usually concrete masonry units (CMU). In the UK you’d call them concrete blocks, or breeze blocks. For a hot tub, the name doesn’t matter much. What matters is:

Use	USA	UK
Structural outer walls	8in hollow CMU (nominal 8 x 8 x 16in, actually 3/8in smaller)	215mm hollow concrete blocks
Seat fronts, steps and outer skins	4in solid or hollow block	100mm dense concrete block
Avoid for the shell	Lightweight aerated (aircrete) blocks. They’re solid, so there are no cores for rebar, and they aren’t meant for a structure that holds water.

Planning your cinder block hot tub

Without a plan, you’re going to fail. This isn’t something you can go into blindly. Sit down and work out exactly what goes where and how you’re going to build it.

How big?

Size is decided by the space in your garden or backyard and by how many people you want in the tub. To give you a rough idea, a 6ft x 6ft (1.8m x 1.8m) tub suits about four people comfortably, and a 10ft x 10ft (3.05m x 3.05m) will happily take ten.

The size then sets how much water it holds, and that decides your heater, your pump and your filter. As a guide, a 50 sq ft cartridge filter is enough for most block-built hot tubs. See cartridge or sand filter? for more.

Why volume matters for heating

It takes about 1.16kWh to raise 1,000 litres (264 US gallons) of water by 1°C. My tub holds around 2,800 litres, so a 3kW heater raises it by roughly 1°C an hour in ideal conditions, with the cover on.

When I first sketched my tub, the volume was so big it was going to need around 12kW of heat to be practical. That would have cost me an absolute fortune to run. So I changed the design, made the seats bigger and brought the volume down to a level where a standard 3kW heater would do. In the USA, the smallest standard spa-pack heater tends to be 5.5kW. If you want a big footprint without a huge heating bill, bigger seats are the trick. They take up volume without making the tub feel smaller. There’s more in Hot tub heaters explained, and an air source heat pump can cut the running costs a lot.

The layout: walls and seats

Decide where the walls, seats and steps go. Will you have seating all the way round? Is it a traditional square or rectangle, or are you going for curves? Remember that curves in block are a different level of skill to straight lines. After a few YouTube videos I was laying blocks well enough, but I’m pretty sure I still couldn’t lay a curved wall. I just don’t have a professional bricklayer’s skills. If you want curves, look at ICF or poured concrete instead (see DIY hot tub construction methods).

Think carefully about seat height and depth. You want your shoulders under the water when you’re sitting down.

The base for your block tub

Once the area is clear (and you’ve checked for buried services with LinesearchbeforeUdig in the UK, or 811 in the USA), you need a solid base. A simple pad poured into a timber frame is fine for a shop-bought tub sitting on top. A built tub holds a few tonnes of water, though, so this is what I’d do:

1. 100–150mm (4–6in) of crushed rock (Type 1 or MOT in the UK), properly compacted with a plate compactor.
2. Sand blinding and a 1200-gauge polythene DPM, lapped, taped and turned up the sides.
3. Rigid insulation board if the tub is in-ground (more below).
4. Rebar or mesh on spacers, so it ends up in the middle of the slab. My rebar guide suggests around 12in (300mm) centres for most tubs. Leave starter bars sticking up where the walls go.
5. The main drain, a hydrostatic relief valve and any sleeves go in before you pour. See Installing a main drain and How to install a hydrostatic relief valve.
6. Ready-mix C25/C30 concrete, about 150mm (6in) thick, poured in one go and levelled carefully.

My in-ground hot tub build guide goes through all of this step by step, including how deep to dig.

Building the block walls

Rebar in the cores

If you’re using hollow blocks for the outer walls, drop vertical rebar into the cores. #3 bar, which is 3/8in (about 10mm) thick, is a common size for pool walls, but your plans or engineer set the size and spacing. Tie the bars to the starter bars from the slab so the walls and floor work as one structure. Then fill the cores with concrete. Concrete block walls reinforced like this are much better at resisting sideways forces, which is exactly what water pushing out (or soil pushing in) does.

Setting out and laying

• Get the first course square and level. Measure both diagonals. If they match, you’re square. Mine ended up about 2cm out because I didn’t put proper guide lines in at the start.
• Use string lines and check level on every block, not just at the corners.
• Get the mortar right. Slightly wetter gives you more working time if you’re slow like I was. Too wet and heavy blocks won’t sit properly.
• Put the plumbing through before you fill the cores. It’s far easier to set pipes and tie rebar around them before the concrete goes in.

Cinder block hot tub plumbing

The plumbing for a block tub is no different from any other concrete tub. The key decisions are:

• The control room. This is where the heater, pump, blower and filter live. Decide at the start whether it’s above or below the water level, because that dictates the type of pump you need. See Hot tub control room position.
• Distance. The further the control room is from the tub, the more your flow drops, so the pipe sizes need to be calculated properly.
• Number of jets. If you’ve followed my YouTube channel or this blog, you’ll know I say around 16 jets is the most you can run on a single pump in most cases. If you want more, you need a multiple-pump design. See How many pumps do you need?
• The blower, if you’re having one, and where it goes.

To fit the jets, you core holes through the blocks for the gunite jet bodies and their pipework. Pressure test everything before it’s buried. There’s much more in my DIY hot tub plumbing guide.

Insulation

Insulating a block tub is key. If you don’t, it’s going to cost you a lot to run, and I speak from experience because I didn’t insulate mine. Foil-backed PIR boards (Celotex-type) about 4in (10cm) thick, fitted between the plumbing, are excellent where they stay dry. Anything in direct contact with wet ground or backfill is better done in extruded polystyrene (XPS), which is made for below-ground and under-slab use.

For an in-ground tub, insulate behind the walls around the plumbing, then backfill. If your tub is above ground or partly above, build a second skin of block around the outside to hide the plumbing and insulation, then finish it with render, tiles or cladding. The full detail is in DIY hot tub insulation.

Waterproofing

I used to think a render with a waterproofing additive, then tiles and grout, would keep the water in. Additives help, but tiles and cement grout aren’t waterproof, so here’s what I’d do now:

1. Waterproof render: a scratch coat and a float coat, to give a flat, sound surface.
2. A pool-rated membrane in two coats. That’s Ardex S 7 Plus in the UK and Europe, or Laticrete Hydro Ban in the USA. Follow the data sheet for primers and cure times.
3. A flood test before you tile. Fill it, mark the level and leave it.
4. Tiles on a flexible, pool-grade adhesive. See Waterproofing, grout and tile adhesive.

Common mistakes with block tubs

• Building on a thin, unreinforced pad instead of a proper slab.
• Leaving out the rebar in the cores, or not tying it into the slab.
• A first course that isn’t square or level. You’ll fight it all the way up.
• Seats that are too high, so your shoulders are out of the water.
• No insulation, and then wondering why the heater never stops.
• Tiling straight onto render without a membrane or a flood test.

Planning your own build?

Getting the design right from the start is key, especially the plumbing layout and how you want the tub to work. Everything is possible, but it has to be planned and designed in. Making big changes later is always difficult.

My DIY hot tub plans are designed for exactly this kind of block or ICF build. You get CAD drawings, plumbing, a shopping list and a video course, in sizes from 6ft x 6ft up. If you want something bespoke, I offer a custom design consultancy, and there’s The Ultimate Hot Tub Control Room Guide for the equipment side.

FAQs

Can you build a hot tub out of cinder blocks?

Yes. Use hollow blocks with rebar in grouted cores on a reinforced slab, then render, apply a pool-rated membrane and tile. It’s how I built mine.

Are cinder blocks waterproof?

No. Concrete block is porous. The block walls are the structure, and a pool-rated waterproofing membrane is what keeps the water in.

Do I need to fill the blocks with concrete?

For the structural walls, yes. Fill the cores that have rebar in them, at the very least. Your plans set exactly which cores get bars.

How much water does a block hot tub hold?

It depends on the size and seat layout. Mine holds around 2,800 litres (about 740 US gallons). Larger seats reduce the volume and the heating bill.

Can I build a round hot tub with blocks?

You can, but curves in block need real bricklaying skill. For curves, ICF or poured concrete is usually easier for a DIYer.

Building with blocks? Tell me about your project 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