you do not have to and could not calculate capacity of a cylinder as it not only depends on size but also heating coil arrangements inside some have the minimum requires coil size and some have large coils to increase heat recovery time
most people are governed by space so say you now have a standard 900mm by 450 cylinder (36"#18") with standard coil you would be looking at about 115 lts,you could increse that to say a 450mm keeping same width by 1200mm increasing hieght by 300mm giving about 160 lts,about 45lts more,actual capacity is indicated on cylinder
however you could replace your cylinder with the same size or smaller ,that has a quicker recovery time,which means by the time some one has had a bath or shower the water is being reheated in correct temperature in time for next user
tony..tsp heating, July 2007
