A 10 litre (liter in the US) tank, has an empty volume of 10 litres, at 1 bar. Multiply that by the current tank pressure, and you get the amount of gas in the tank. eg if the tank is a 200 bar tank, there is 2000L of air in the tank, 230 bar tank, 2300L, etc etc.
An aluminium (aluminum in the US) 80, supposedly contains 80 cubic feet of air. A cubic foot is approximately 28.32L, so an 80cf tank contains about 2265L of air. But thanks to marketers, an aluminum 80 will contain 77.4, 77.7, 78.2, or 80cf of air, depending on the manufacturer and the model of tank.