How to work out the size of tap connectors

[craig1234]

This is a daft question I know but I have a tap (European) with connectors and I am struggling to work out what size the connectors are. Ideally I want to convert so that that I can connect to my 15mm isolator valves the ends of which are too large for the tap connectors.
The internal diameter of the connectors appears to be 15mm.

 

[Exedon]

If they are not 1/2 inch size of iso valves they are probably 3/8 inch .

 

[craig1234]

But how do I measure to confirm which size they are?

 

[Madrab]

In the UK, you only get 3 modern sizes - 3/8" - 1/2" - 3/4"

If the female end of the tap tail won't fit on a standard 15mm ISO as the threaded end is too large then it will be 3/8", just need to get a 15mm>3/8" tap tail adapter.