Measure the tank’s internal length, width, and height if it is rectangular
Use the formula: Capacity = length × width × height
Measure the tank’s internal diameter and height if it is cylindrical
Use the formula: Capacity = π × radius² × height
Measure the tank’s internal radius and height if it is spherical
Use the formula: Capacity = 4/3 × π × radius³
Convert all measurements to the same unit before calculating
Calculate the volume in cubic units
Convert cubic units to liters, gallons, or other desired capacity units
For irregular tanks, divide the tank into simple shapes and add their volumes
Use the tank’s manufacturer specifications if available