It depends. Is 12 ft the radius, diameter, or circumference? The volume can be found by multiplying the area of one side (top or bottom) times the height. So if 12 ft is our diameter for example, our area would be: Pi x r2 So 3.14 x 62 = 113.04 Now we find the volume: 113.04 x 3 = 339.12 ft3