How much water does a fire engine use in a normal fire?

Depends upon the amount of fire, amount of fuel for the fire, type of fuel, configuration of fuel (open pallet stacks, tire heaps, twelve-story apartment, 200 acres of dead pine trees), whether there are sprinklers, how fast the water can be applied to the fire, among other things. Some types of fires cannot be safely put out using any amount of water, such as ammonium nitrate or jet fuel, and different chemicals and techniques would be used.

For a compartmented structure fire (e.g., a house), the rule of thumb for the US National Fire Academy "Estimated Fire Flow" is based upon the floor area and the amount of fire involvement, assuming 10-foot ceilings. Length x width x percent involvement / 3 = gallons per minute.

For instance, a 40 x 30 single-story structure would have 1,200 sq ft, requiring a minimum fire flow of 400 gallons per minute when fully involved. If there is less water than that, the fire cannot be extinguished until some of the fuel has been exhausted (i.e., the building collapses). If it takes 30 minutes to extinguish the fire, there will have been about 12,000 gallons used.

These estimates vary by flame intensity, ventilation and other tactical elements that might restrict interior firefighting operations.