To find out how much pressure you have behind your tap is very easy but it does require a piece of equipment you can attached to a tap with a pressure gauge on fortunately we sell the perfect thing here https://shop.ejwoollard.co.uk/pressure-test-kit
With this tool you simply attached it to your outside tap with either the Snap Lock fitting or a Geka fitting, once securely attached to your tap you should close off the silver lever valve on the outlet and then turn your tap open as much as you can then all you need to do it read the pressure gauge! And there you have it, now you should know how much pressure you have behind your tap and what you can run on your irrigation system.
And here’s a tip from the professionals at E.J.Woollard make sure once you’re finished with measuring your pressure that you turn the tap off and then open the silver lever outlet before you remove it from the tap as a it does hold a surprising amount of pressure and water in the kit!