90 gallon tank:
exterior dims: 48"W x 18"D x 24"H
interior dims: 47 x 17 x 23.5
Water Level 3" from top: 47 x 17 x 20.5 = 16,379.5 cu in (allow for drilled overflow plus 1" above hole and 1" below)
divided by 231 cu in = 70.9 gallons capacity
And of course, this does not account for any "stuff" inside the tank...