Hot water pipe sizes

Hi, can someone help me out with pipe sizing. I'm planning a new bathroom fitting and just sizing things up and making some checks before I start taking things apart and ordering new things.

There's a single hot water feed coming into the bathroom via what I thought was a 22mm pipe. This runs straight back to the hot water tank (with a couple of T-joints branching off to the kitchen and utility room downstairs along the way).

I've measured the outside pipe diameter using a set of calipers and it's not quite 22mm though. It's not 20mm either, it's like 21.something. I had an offcut of the same pipe lying around, measured that and it's the same and a 22mm compression olive I had is loose around it. Also have a brand new piece of 22mm pipe and according to my calipers (admittedly cheap ones!) it's bang on 22mm, and the olive isn't as loose.

Is it an imperial size? It's definitely larger the 3/4" (that would be about 19mm?). Any help appreciated, thanks.

It will be three quarter inch

Old gauge pipe 3/4.
You can buy a fitting which converts to metric. A 3/4 inch to 22mm socket

Thanks. Are you able to get Speedfit or compression fittings to do the job?

Or use a 3/4" olive which will replace the standard 22mm one in a 22mm compression fitting. I've got some that came from B&Q (pack of 3), admittedly a couple of years ago and B&Q have a habit of ceasing to stock anything useful! I wouldn't recommend following the method suggested by a few on youtube of bodging a 22mm olive with PTFE tape or similar - you might regret it when it pops whilst you're on holiday for three weeks.

OD of 3/4" copper is supposed to be 21.43mm.

Philmac do a universal transition coupling. Depends how much room you have because the fitting itself is quite bulky, but if under a bath or floor you will be ok.

Thanks. I found a JG Speedfit that looks neat. Used JG before and like them. Hopefully will solve my problem!