Pressure Washer and Jetter Nozzle Calculator (amazingmachinery.com)
On the bottom calculator....
Add up all your nozzles on the gun, so for a xx03 and a xx02 type in 5
Type the pressure you are getting at the GUN not the pump unless you know for sure you are not getting friction loss. Typically a 3/8 50' hose has zero loss under 5 gpm.
This will calculate your total GPM at the gun.
For a single nozzle
Type in a 3 for a xx03 nozzle, 2 for a xx02 nozzle, etc. Then type in your pressure and it will calculate it for you.
This is a excellent way to see what you are getting exactly at the gun if you are between the numbers on the typical nozzle chart.
I hope this helps!