Abstract: A gauge for indicating available water supply is disclosed which may be used in combination with a water pressure gauge (compound gauge) attached to a fire truck pump for checking fire hydrant pressure. Compound gauges are standard on most fire engines. The gauge is positionable over such a water pressure gauge and indicates the additional number of fire hoses that may be connected to a fire truck pump as the water pressure in the hydrant drops when water starts to flow. The water delivered from a fire hydrant decreases exponentially with a linear drop in pressure and the more demand placed on a hydrant the more this will cause a corresponding drop in pressure. The apparatus of the present invention uses an indicator that converts a linear decrease of water pressure into an exponential decrease in the number of additional fire hoses that a given hydrant will supply.