The capacity of different radiators varies greatly. Many radiators have their capacity printed on the filling cap or close it. It is always best to check individual manufacturer's specifications.
Radiators are generally used to heat buildings or cool fluids used in other processes, such as cooling the engine of an automobile. A radiator system connected to a boiler in a heating system may have a large capacity of several gallons. A typical automobile radiator and cooling system has a capacity of 2 to 3 gallons. Different systems using radiators require different solutions to be added to water. Common additives are antifreeze, as well as rust and mineral inhibitors.