How do you find the flow rate through pipe?

Either by determining the core diametre of the pipe and how long the pipe would be to hold a gallon of water , or simply by running a gallon of water through said pipe and timing it to fill a bucket.

Alternately find out the water pressure of the water delivery system and that will help you solve.

Is an equation but cannot remember but simplicity is best appraoch...work logically with the information you have re the pipe etc.