Lonne, You can measure the tank, length X height X width equals total square inches. Then divide by 231 will give you "gross gallons" The tank has built in draft, so measure each end and then average the difference. Example 60 X 20 X 10=12,000 divided by 231 equals 51.94 gross gallons. Subtract 10% for tank thickness and air space equals 46.75 approximate capacity. Fred