Firefighters need to know the flow rates of fire hydrants to know which fire hydrants to connect to to have enough water supply for the size of fire they are fighting.
Larger fires need more water supply to fight off the fire while smaller fires can be fought and put out using lower flow rate fire hydrants.
Fire hydrants are connected to the drinking water supply.
The fire hydrants are connected to the same water supply pipes that you get your water from for your home.
A fire hydrant puts out 499 gallons of water per minute and a fire trucks pump increases that pressure.
And some fire hydrants can put out as much as 1,000 GPM to as much as 1,500 GPM.
A red fire hydrant signifies that it is known to flow up to 499 gallons per minute (gpm).
All fire hydrants are red when they are first installed, and then the fire department will test the flow of a new hydrant to determine how much water it is capable of producing.
Fire hydrants don't ever run out of water since fire hydrants are connected to the public water supply.
As long as the public water supply such as the water tower and the well or water source the water is coming from has plenty of water then the fire hydrant will never run out of water.
Fire departments also keep a tanker truck full of water to be ready to begin fighting a fire and in case there's no water supply locally.
But when a fire truck is connected to a fire hydrant connected to the water supply the fire department has plenty of water to fight the fire.
However there are also some fire hydrants known as dry fire hydrants which are just a pipe that is buried in the ground and pulls water from a lake or other reservoir.
So if that lake or reservoir runs out of water then the fire hydrant would run out of water.
But water from fire hydrants that are in cities and towns have plenty of water under pressure being sent through them and the fire trucks pump increases the water pressure to help fight the fire.