Tank size = (Length x Width x Height) / 231
So in your case ( 30 x 18 x 24) / 231 = 56g
The dimensions you give are not what the standard size would be for a 55G. Is this a Hex tank or a corner tank you have? If so then you have to do a little more figuring to get the size in gallons because the dimensions are not the same all the way through. (If that makes sense?)