Measure the tank’s dimensions
Identify the tank shape
Use the correct volume formula for the shape
Calculate the volume in cubic units
Convert cubic units to gallons
For inches, divide cubic inches by 231
For feet, multiply cubic feet by 7.48052
For liters, divide by 3.78541 to get gallons
For irregular tanks, use the fill-level method or manufacturer’s capacity chart
Subtract any unusable space if needed