rite-serkular cylinder (base=serkel) diameter=41inch, hite=12 inch=1 foot
volume=hite*base area=hite*pi*r*r
=12inch*20.5*20.5*3.141592654...20.5^2=420.25; 12*420.25=5043
times pi...volume=15843.051...kubik inches
1 ft=12 inch, so 1 kubik foot=1728 kubik inch
15843 /1728=9.1684 kubik ft
1 kube foot=7.480681 gallon, so 9.1684*7.48068=68.585896 gallons
or bout 68.6 gallons