Measure the inside diameter of the drum in inches
Divide it by 2 to get the radius
Multiply the radius by itself (r squared)
Multiply that answer by 3.142 (pi)
Multiply that answer by the depth of the drum - again in inches.
The result is the capacity in cubic inches.
Divide this figure by 1728 (12x12x12) to get the capacity in Cubic Feet - it will probably be around 3.2 CuFt.
Good luck..., March 2015