Calculating the water flow from a tap is a fairly simple process and requires a container and timer.
Open the tap full blast, place the container underneath the flow and time the amount of time it takes, in seconds, to fill the container.
Example: 5 litre bucket takes 15 seconds to fill
5 litres --- 15 seconds = 5 / 15 * 60 = 20 litres per minute average flow rate

Alternately you can take a certain amount of time and then measure the amount of water in the container.
Example: 5-litre container placed under the water flow (full blast) for 10 seconds remove the container and then measure the amount of water in the bucket. If in 10 seconds you have 3.4 litres of water enter 3.4 under container size and 10 seconds in the time it takes to fill the container.
3.4 litres ---10 seconds = 3.4 / 10 * 60 = 20.4 litres per minute average flow rate

To get a more accurate result all other water sources in the house should be turned off.