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put a (major axis) in cell B1
put b (minor axis) in cell B2
put L (lenght of tank) in cell B3
and put variable height in cell
B6 to ....
put this formula to Microsoft excel in cell C6
=(B$1/2*B$2/2*ACOS(1-B6/B$2*2)-B$1/2*(B$2/2-B6)*SQRT(1-(1-B6/B$2*2)^2))*B$3
result will given in cell C6