Hot Tub Pipe: Sizes, Types and Pressure Ratings Explained

Last updated: September 2026

You may think that pipe is just pipe. When it comes to hot tub pipe, there are a few things you really need to know, and getting them wrong means parts that don’t fit, joints that leak, or jets that never have the power they should. This guide explains everything I’d want you to know before you order a single length.

Imperial vs metric hot tub pipe

The very first thing to know is that hot tub pipe is imperial, and specifically American imperial. The jets, pumps, filters, heaters and spa packs used on DIY builds are almost all designed around US pipe sizes, so that’s what you need to buy, even in the UK or Europe.

The question I get more than any other about pipe is “can I use the metric equivalent?” No. It won’t fit. 50mm pipe is not the same as 1.5in pipe, and 63mm pipe is not the same as 2in pipe. They look close enough on paper, but a solvent-weld joint relies on a tight fit between pipe and socket, so even a millimetre or two is enough to ruin it.

How the sizes are actually measured

How the sizes work. It’s easy to assume the imperial size is the external dimension of the pipe. It isn’t, and it confuses a lot of people. US pipe uses “nominal” sizes. For each size the outside diameter is fixed, but it isn’t the number on the label. A 2in pipe is actually 2.375in (60.3mm) on the outside. Thicker-walled pipe of the same size has the same outside diameter and a smaller bore. Metric plastic pipe is the other way round: the number is the outside diameter, so 63mm pipe is 63mm across the outside. That’s why the two never quite line up.

US size	Actual outside diameter	Nearest metric pipe	Difference	Typical use on a DIY tub
1in	1.315in (33.4mm)	32mm	1.4mm	Inner pipe of a gunite jet
1.5in	1.900in (48.3mm)	50mm	1.7mm	Air lines
2in	2.375in (60.3mm)	63mm	2.7mm	Water lines
2.5in	2.875in (73.0mm)	75mm	2.0mm	Gunite jet sleeves through the wall, long runs
3in	3.500in (88.9mm)	90mm	1.1mm	Long runs to a distant control room

Here’s a drawing I did showing the same thing: there’s no true metric equivalent of hot tub pipe.

US imperial is the standard

You’d think imperial is imperial. It isn’t always. In my experience, some UK and European imperial pipe doesn’t match the US equivalent. The one that caught me out was 2.5in, where there’s just over 2mm in it, and that’s enough that it simply won’t fit a US fitting. So make sure you’re buying pipe made to US sizes, and if in doubt, ask your supplier before you order. It’s one reason I stock 2in flexible pipe and 1.5in flexible pipe made for hot tubs.

What size pipe does a DIY hot tub need?

For a block, ICF or concrete tub with gunite jets, these are the sizes you’ll be buying:

• 2in: water lines. Suction from the drains and skimmer, and the manifold feeding the jets.
• 1.5in: air lines from the blower (or venturi) to the jet bodies.
• 2.5in: gunite pipe, the sleeve that goes from the jet body through the wall.
• 1in: inner gunite pipe, which runs inside the 2.5in sleeve and connects to the jet holder.

The way the gunite parts go together is explained step by step in How to install gunite jet bodies.

Those are starting points, not rules. The pipe has to be big enough for the flow going through it. Pool and spa standards cap water speed at 8 ft/s (2.4m/s) in return lines and 6 ft/s (1.8m/s) on the suction side of filtration piping. In practice, a 2in pipe carries about 63 GPM at 6 ft/s and about 84 GPM at 8 ft/s. So a big jet pump pulling 120 GPM through a single 2in suction line is asking for trouble. Split the suction across the drains and skimmer, or go up a size. The full velocity table, and how to work out your flow from the number of jets, is in my DIY hot tub plumbing guide.

Pipe size matters even more on long runs. If your control room is a way from the tub, going up to 2.5in or 3in on the long legs can save a lot of jet power. There’s a worked example in Hot tub control room position.

Can I convert metric pipe to imperial?

Yes. There are adaptor sockets and bushes that go between the two. But I’d only do it where there’s a good reason. The typical example is a long run between the control room and the tub. You can’t easily get imperial pipe locally, but you’ve found, say, 75mm pipe that would do the job. In that case it’s fine to convert at each end.

What you can’t do (technically you can, but it’s a real mess) is plumb the whole tub in metric and adapt every fitting. The adaptors alone cost a fortune, and every extra joint is another place for a leak. The other time you’ll need an adaptor is a metric pool part such as a skimmer. I used a Waterway bush on my Certikin skimmer, which was on 63mm pipe.

What pressure rating do I need?

A hot tub is a pressurised system, so the pipe must be pressure rated. Standard grey or white waste pipe from the DIY shed is not the same thing. Pressure pipe has its rating printed along the side, so there’s no excuse for getting it wrong.

• UK: Class C (9 bar, about 130 PSI) is the minimum I’d use. Class E (15 bar, about 218 PSI) is also common on hot tub builds. Don’t use anything rated below 9 bar.
• USA: Schedule 40 PVC is the standard rigid pressure pipe for pools and spas.

Hot water reduces the rating

This is something I didn’t cover the first time. PVC gets weaker as it warms up. Pipe ratings are quoted at about 23°C (73°F), and Westlake Pipe’s derating table puts PVC at 62% of its rated pressure at 38°C (100°F) and 50% at 43°C (110°F), with 60°C (140°F) as the maximum operating temperature. At a hot tub’s 40°C (104°F), that’s roughly half the printed rating.

That sounds alarming, but a hot tub doesn’t run at high pressure. In Waterway’s hydraulic design example, the jets and pipework together need about 22 PSI. Half of a 9 bar rating is still several times that. The lesson is simply not to go below pressure-rated pipe, and to keep PVC away from anything that runs hotter than the tub. If you’re fitting a gas heater, follow its manual for the pipe on the outlet side.

What about Schedule 40 and Schedule 80?

The schedule number describes the wall thickness. The higher the number, the thicker the wall. The same schedule on different pipe sizes gives a different actual thickness. And because the outside diameter stays the same, Schedule 40 and Schedule 80 pipe of the same size fit the same fittings.

Schedule 40 or 80? In the USA, Schedule 40 is simply the normal rigid pipe for spa plumbing, buried or not. Schedule 80 (usually grey) is heavier-walled pipe for industrial and high-pressure work. It’s overkill for a DIY hot tub. Where pipe is buried under a driveway or anything heavy, bed it properly in sand or fine gravel and protect it, rather than relying on the wall thickness alone.

Rigid or flexible pipe?

When I built my own hot tub, I went for semi-flexible pipe because I thought it would be more forgiving. It is, in the loosest sense of the word, as it’s still pretty rigid. But I’ve since found that the “forgiving” part is exactly what makes it more likely to leak.

Because there’s a bit of give in it, you can force a joint. And as I’ve said many times on this blog and on my YouTube channel, if you force a joint, it’ll leak. Mine did. My worst one was an awkward joint at a funny angle in the corner of the control room, which was the first leak when I filled up.

So wherever possible, I now recommend rigid pipe. It makes you cut accurately and make a good joint, and your workmanship benefits from that in the long run. Flexible pipe still has its place:

• Use it for gentle curves and awkward connections, not to avoid fittings. Use proper 90° and 45° fittings for changes of direction.
• Glue it into smooth sockets. My flexible pipe is designed for socket joints and smooth barbs, and it won’t seal well on ribbed barbs.
• Use a cement rated for flexible PVC. For example, Weld-On’s 747 Pool ‘R Spa is made for rigid and flexible PVC.

Support your pipework

Pick up a 2m (6ft) length of empty pipe and there’s hardly any weight in it. Fill it with water, cap both ends and try again. A 2in pipe holds around 2 litres per metre (roughly 1.5lb per foot), and it adds up quickly on a long run.

If you don’t support your pipes, that weight pulls on the joints, and over time they work loose and leak. It only takes a couple of minutes to support them properly:

• In the ground: lay pipes on a firm, even bed of sand or fine gravel, with no stones pressing on them, and backfill carefully.
• On the tub: tie and pin runs to the blockwork or ICF so nothing hangs off a joint.
• In the control room: support the pipes either side of the pump, filter and heater so the kit isn’t carrying the weight. Unions make it easy to swap parts later. See Hot tub pump union sizes explained.

Making a joint that doesn’t leak

A well-made joint doesn’t leak. A bad one will. My full step-by-step is in How to make the perfect PVC solvent weld joint, but here’s the short version, in line with what the cement makers say:

1. Cut the pipe square, at 90°.
2. Deburr it and put a slight chamfer on the outside edge so it doesn’t scrape the cement off as it goes in.
3. Dry fit it, and mark the socket depth on the pipe.
4. Clean both surfaces. In the USA use a primer on PVC pressure joints. In the UK, use the cleaner that goes with your cement.
5. Apply the cement to the pipe and the socket and push it straight in to the full depth. (US instructions say with a quarter turn, some European ones say without twisting. Follow your cement’s instructions.)
6. Hold it for around 30 seconds and wipe off the excess.
7. Leave it to cure before you pressurise it.

How long to wait. I used to work on a joint taking 4 hours to reach full strength, but it really depends on the pipe size, temperature, humidity and pressure. Oatey’s cure chart gives 1.5–3in PVC joints 30 minutes at 16–38°C (60–100°F) for pressures up to 180 PSI, 45 minutes at 4–16°C (40–60°F), and 50% longer in damp weather. UK cements can be much more conservative: Henkel’s Tangit PVC-U data sheet says to wait 24 hours before a pressure test. Always go by the chart for the cement in your hand.

Pressure test before you bury anything

Once the joints have cured, cap the open ends and pressure test the system. Do it before you concrete, backfill or tile. Two things the cement makers are very clear about:

• Test with water, never compressed air. Both Oatey and Weld-On warn against testing solvent-welded plastic pipe with air or gas. If a joint lets go under air pressure, it can go with real force.
• Don’t over-test. Weld-On suggests a hydrostatic test at 150% of the system’s design pressure. Your plans should give you the figure.

Leave it under pressure and check the gauge holds. Finding a leak now costs you a fitting. Finding it after tiling costs you a weekend. If you do end up hunting one, see How to detect and fix a hot tub leak.

Common hot tub pipe mistakes

• Buying metric pipe because it “looks the same” as imperial.
• Using non-pressure waste pipe.
• Undersizing the suction side, so the pump starves and the jets feel weak.
• Forcing flexible pipe into a joint at an angle.
• Leaving long, water-filled runs unsupported.
• Pressurising joints before the cement has cured.
• Pressure testing with air.
• Mixing parts from different imperial standards without checking they fit.

Planning your own build?

Pipe sizes, lengths and the number of fittings all feed into how well your jets perform, and it’s much easier to get right on paper. My DIY hot tub plans include the full plumbing layout with pipe sizes, a parts list and the video course.

If you’ve got a custom design, the Tub & Pool Designer Pro plumbing design service works out the pipework for you. For pipe itself, I stock 2in, 1.5in and 1in flexible pipe in 25ft rolls. For pumps, spa packs and fittings, head to parts4tubs.co.uk in the UK or parts4tubs.com in the USA. If you’re thinking of borrowing kit from a swimming pool, read Can I use pool parts on a hot tub? first.

FAQs

What size pipe is used for hot tubs?

On a DIY gunite-jet tub, 2in for the water lines and 1.5in for the air lines, with 2.5in and 1in pipe for the gunite jet bodies. Check your flows, as big pumps or long runs may need 2.5in or 3in pipe.

Is 2in pipe the same as 63mm?

No. 2in pipe is 60.3mm on the outside, so it won’t fit 63mm fittings. Use an adaptor where you have to connect the two.

Can I use metric pipe for a hot tub in the UK?

Not for the tub itself. The jets and equipment are made for US imperial pipe. Converting a long run to metric and back at each end can work, but not the whole system.

Is Schedule 40 PVC OK for hot water?

Yes, at hot tub temperatures. PVC loses strength as it warms up, to about half its rating at 43°C (110°F), but a hot tub runs at low pressure, so there’s plenty of margin. PVC’s maximum operating temperature is 60°C (140°F).

Rigid or flexible PVC for a hot tub?

Both work. I prefer rigid because it forces accurate joints. Use flexible for awkward connections, never to force a joint.

How long should PVC cement cure before I pressure test?

It depends on the cement, pipe size and temperature. For 2in pipe at low pressure, US charts give around 30 minutes in mild weather and 45 minutes when it’s cool, with more time when it’s damp. Some UK cements say 24 hours. Check the chart for your cement.

Got a question about pipe for your own build? Drop it 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