help with tank size please

hi i got a tank the same as this in the picture (note not my tank only for a ref )hXXp://s5.postimg.org/qrib28a53/example.jpg



now when i got the tank i got told it was 200l

but i worked it out and its coming in at 216l

but the advert from the the tank on pic it says 220l



the tank is from the end to end 60cm

square height is the same any help with this would be great



tia



john