Hot Tub Pump Union Sizes Explained (With a Collar Size Guide)

Last updated: September 2026

A hot tub pump union is basically a screw nut that attaches to the front (suction) and the top or side discharge of the pump, so you can connect it to your plumbing.

The point is that you can isolate the pump with gate valves and take it off for servicing or replacement without cutting pipe. Let’s look at the sizes and how to work out which one you need.

How do we know the size of the union?

This matters when you’re changing your pump. Models change or go out of production, so you may have to fit a different pump, and you need the union size to make sure it connects to your plumbing.

The industry doesn’t make this easy. The size of a union isn’t the physical size you can measure. The size you need is set by the pipe it connects to, not by the collar.

That sounds confusing, so I made a diagram. It shows the physical collar sizes and the union sizes that go with them. For example, if your collar measures 3.1 inches, you need a 2 inch union.

It’s confusing, yes, but you’ll get there. The diagram lists the three most popular union sizes and their collar sizes.

Typical union sizes

Union size	Usually found on
1.5 inch	Smaller circulation pumps and some small jet pumps
2 inch	Most standard two-speed and jet pumps
2.5 inch	Larger jet pumps and bigger flow systems

Treat the table as a guide and check your own pump, since brands vary. Use the diagram above to match the collar you measure to the union size.

Tips when changing a pump

• Fit gate valves either side of the pump so you can isolate it. See DIY hot tub plumbing.
• Replace the union O-rings every time. They are cheap and stop slow leaks.
• If the new pump has a different union size, use reducers or new tail pieces. My circulation pump guide gives an example.
• Match frame size and wet end at the same time. See the wet end guide.

Deciding whether to replace at all? Read repair or replace a hot tub pump, and for sizing a brand-new pump see how to size a hot tub pump.

Things aren’t easy in the hot tub industry, but that’s maybe part of the fun. I hope this helps you pick the right union. If you need unions, tail pieces or O-rings, they’re on parts4tubs.co.uk (UK) and parts4tubs.com (USA).

FAQs

What size union does my hot tub pump need?

It depends on the pipe size that connects to it. The common sizes are 1.5, 2 and 2.5 inch. Use the collar size chart above to match yours.

Why does a 2 inch union measure more than 2 inches?

Because union sizes are named after the pipe they fit, not the physical collar. A collar of about 3.1 inches is a 2 inch union.

Can I use a different union size on a new pump?

Yes, but you will need reducers or new tail pieces, and you should check the pipe size does not restrict flow.

Do I need to replace the O-rings when I take a pump off?

It is a good idea. They are cheap and stop slow leaks from the unions.

Are pump unions the same in the UK and USA?

The sizes are described in inches on both sides, but always check the fittings match your pipe (imperial or metric).

Got a question about your own setup? Drop it in the comments below. I read them all.

Cheers,
Andi

You can also find me on my socials;

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

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