Pick any set of hot tub dimensions and you’ll find two ways to write them down: 8ft x 8ft or 2.44m x 2.44m. On paper they’re the same tub. On site they aren’t quite, because the blocks, pipe, tiles and timber you buy locally come in their own sizes, and a plan drawn in the “wrong” system can leave you cutting every corner block and hunting for adaptor fittings.
This is the question I get from almost every builder outside the US: should I buy the imperial plans or the metric ones? Here’s how I’d decide, and what actually changes between the two.
The short version
- Choose the system your local suppliers use. In the US that’s imperial. In the UK, Europe and most of the world, the structure is usually metric.
- Blocks set the rhythm. Good hot tub dimensions line up with whole (or half) blocks, so you aren’t cutting at every corner.
- Pipe doesn’t convert. 1.5in and 50mm pipe are different sizes and won’t glue together without an adaptor.
- Internal size is what you sit in. Render, adhesive and tile all take a little off the walls, so plan for the finished size, not the block size.
- Gallons aren’t all the same. A US gallon is 3.79 litres and a UK gallon is 4.55 litres, which matters for heaters and chemicals.
Why the units matter more than you’d think
A hot tub is a box built from standard parts. If your drawings and your materials share the same “module”, everything falls into place: walls finish on a whole block, courses land at the right seat height, and the parts list matches what’s on the shelf.
When they don’t match, you get a slow drip of small problems: half-blocks at every corner, a seat a course too high or low, tile cuts down one wall, and fittings that don’t quite fit. I covered the wider planning side in what makes the best DIY hot tub plans. This post is just about getting the numbers to suit your materials.
For example pipe – 2″ pipe is not the same as 65mm pipe, likewise, 50mm is not 1.5″ pipe – they are totally different sizes!
Hot tub dimensions and the materials you can buy
Here’s where the two systems differ in practice, material by material.
Concrete blocks
US concrete masonry units (CMU) are sold as 8in x 8in x 16in. That’s the nominal size: the block itself is 7⅝in x 7⅝in x 15⅝in, and the ⅜in mortar joint makes up the difference. So every course is 8in high, and wall lengths work best in multiples of 8in (half a block).
A standard UK dense concrete block is 440mm x 215mm, and with a 10mm joint it works to a 450mm x 225mm grid. Courses are 225mm high, and lengths work best in multiples of 225mm.
You can see the problem. A tub drawn around 8in blocks won’t land neatly on 225mm blocks, and the other way round. The difference is small per block (about 3mm), but it adds up over a wall and, more importantly, the course heights decide where your seats and steps end up. If you’re still choosing a wall type, I compare block, ICF and poured concrete in hot tub wall construction options.
ICF blocks
Insulated concrete forms are brand-specific. Many are around 4ft (1.2m) long, and heights and core widths vary between makers. Whatever your plans say, check the block you can actually buy and set the tub out to suit it. The concrete core matters more than the outside size, so if you switch brands, check the core width with whoever is signing off the structure.
Pipe and fittings (the big one)
This is where people get caught. US PVC pipe is sized in inches: Schedule 40 1.5in pipe is 1.900in (48.3mm) on the outside, and 2in pipe is 2.375in (60.3mm). Metric pressure pipe is sized by its outside diameter: 50mm and 63mm.
They look close, but they aren’t interchangeable. A 50mm pipe won’t glue into a 1.5in fitting, and a 63mm pipe won’t go into a 2in one. You’ll need adaptor sockets wherever the two meet.
Most hot tub equipment (pumps, spa packs, heaters and many jet bodies) is made to inch sizes, even in the UK. Pump unions, for example, are named after the inch pipe they fit, which I explain in hot tub pump union sizes explained. So a UK build often ends up with a metric structure and inch plumbing. That’s fine, as long as you decide on it at the start and buy all your pipe and fittings in one system.
Always use imperial pipe on your DIY hot tubs!
Tiles, render and finishes
Mosaic tiles usually come on sheets of about 12in (300mm) square, though a “12in” sheet is often closer to 305mm. Large-format porcelain coping and paving are normally metric. If your internal dimensions are close to a whole number of tile sheets, you’ll have fewer thin cuts in the corners, and a tidier finish.
Timber, plywood and rebar
Formwork and step framing use whatever is local: 2x4s (actually 1½in x 3½in) and 4ft x 8ft plywood in the US, and 47mm x 100mm timber and 2440mm x 1220mm sheets in the UK. Reinforcing bar differs too. A US #4 bar is ½in (12.7mm), while UK builders usually use 10mm or 12mm bar. Rebar size and spacing is a structural decision, so follow your drawings or your engineer, not a conversion table.
Don’t just convert the numbers
It’s tempting to take an imperial plan and run every figure through a calculator. That’s a “soft” conversion, and it gives you dimensions like 2,438mm that don’t suit any metric block, tile or sheet. A proper metric plan is a “hard” conversion: the tub is redrawn around metric materials, so the numbers are round and buildable.
A few conversions are still handy for comparing sizes and ordering:
| Imperial | Metric |
|---|---|
| 1in | 25.4mm |
| 7ft | 2.13m |
| 8ft | 2.44m |
| 1 cubic foot | 28.3 litres (7.48 US gallons) |
| 1 cubic yard (concrete) | 0.765 cubic metres |
| 1 US gallon | 3.79 litres |
| 1 UK (imperial) gallon | 4.55 litres |
Two traps worth knowing about. Ready-mix concrete is ordered in cubic yards in the US and cubic metres in the UK, and a cubic metre is about 30% more than a cubic yard, so get the unit right on the phone. And if a heater, chemical dose or water treatment guide quotes “gallons”, check whether it means US or UK gallons. The difference is about 20%.
Internal vs external: which hot tub dimensions matter?
Every tub has two sets of numbers, and it’s worth being clear which one you’re talking about:
- External size. The outside of the walls. This sets the size of the hole, the base slab and the gap you need around it for insulation and backfill. My in-ground hot tub guide goes through the whole build-up from the bottom of the hole.
- Internal (finished) size. What you actually sit in, after render, adhesive and tile. This sets the seating, legroom and water volume.
- Depth, seat height and footwell. These set comfort. On a block build they’re tied to your course height, which is another reason to choose plans that match your blocks.
Each layer on the inside of the wall takes a little off the internal size, on both sides. Allow for it when you set out, or your finished tub ends up a little smaller than you expected.
Working out water volume
Volume is the one calculation you’ll use again and again: for heater sizing, chemical dosing and filling time. The formula is the same in either system:
- Imperial: length x width x water depth (all in feet) x 7.48 = US gallons.
- Metric: length x width x water depth (all in metres) x 1,000 = litres.
For example, a tub of 7ft x 7ft inside with 3ft of water is 147 cubic feet, or about 1,100 US gallons (4,160 litres). Seats, steps and the footwell take up some of that, so the real figure will be lower.
So which should you choose?
My rule is simple: buy the plans in the system your suppliers use for the structure.
- In the US, imperial. Your blocks, rebar, lumber and pipe are all sized in inches, and so is your equipment.
- In the UK and Europe, metric for the structure, so it works with local blocks, timber and tiles. Then decide on your pipe system at the start and stick to it.
- Anywhere else, look at what your builders’ merchant actually sells. If blocks are sold in millimetres, go metric.
- If you’re using a contractor or applying for a permit, use the system the contractor and the building authority work in.
My hot tub plans come in imperial (feet and inches) and metric, in sizes from 6ft x 6ft up to 12ft x 11ft, including metric versions such as 2.13m and 2.44m. If you’re not sure where to start, the 8ft x 8ft hot tub plans are the most popular size, and the 2.44m version is the metric equivalent.
Common mistakes to avoid
- Mixing systems mid-build. Buying half the pipe in inches and half in millimetres is the classic one. Decide once.
- Soft-converting an imperial plan. You end up with awkward dimensions and lots of cuts. Get plans drawn for your materials.
- Forgetting the finishes. Render, adhesive and tile all come off the internal size.
- Confusing gallons. US and UK gallons differ by about 20%.
- Ordering concrete in the wrong unit. Check whether your supplier quotes cubic yards or cubic metres.
- Changing the structure without advice. Wall thickness, rebar and base slab are structural. If you change them, or if your site has poor ground or a slope, get a structural engineer to check it and follow your local building regulations.
Want the full drawings in your units?
My CAD-drawn hot tub plans come in 21 sizes, in imperial or metric, with dimensioned drawings, plumbing diagrams and a full shopping list, so the numbers suit the materials you can actually buy. I’ve helped 1,500+ customers plan their builds. If your space doesn’t fit a standard size, I can draw a bespoke design for you.
See the hot tub plans | Get a custom CAD and plumbing design
FAQs
What are standard hot tub dimensions?
There isn’t one standard, but most DIY in-ground tubs fall between about 6ft x 6ft (1.83m) and 12ft x 11ft (3.66m x 3.35m). For four to six people, around 7ft to 8ft square (2.13m to 2.44m) is a popular choice.
Can I build from imperial plans in the UK?
You can, but expect cut blocks and adaptor fittings, because UK blocks, timber and tiles are metric. It’s usually easier to build from metric plans and keep the plumbing in one system throughout.
Is 50mm pipe the same as 1.5in pipe?
No. 1.5in Schedule 40 pipe is 48.3mm on the outside, and metric pipe is 50mm. They won’t glue together directly, so use an adaptor socket where they meet.
How do I work out how many litres or gallons my hot tub holds?
Multiply length x width x water depth. In feet, multiply the result by 7.48 for US gallons. In metres, multiply by 1,000 for litres. Then allow for seats and steps, which reduce the real volume.
Building in metric or imperial? Tell me which you went for, and why, in the comments below.
Cheers,
Andi