Clayton Kerr Thompson

I’m just starting out building my own forge and was curious how you calculated the volume to be 600 cu.in. If 2” of kaowool lined the entire inside of the tank, wouldn’t it be 8” diameter, so 4” radius and 20” length? It comes out to 1004.8cu.in. for me. Am I missing something? I ask because I have a 12”x25” air compressor tank I’m planning to use, but was planning on cutting down to 12”x20” due to the volume. Thanks in advance!