Calculations:

41' x 16' x 3.5' x 7.46 gal/cft = 17,128 gallons
(36'-16') x 16' x 6' (average) x 7.46 = 14,323 gallons

Total = 31,415 gallons.


Keep in mind that pool stores benefit, in pounds of chemicals sold, by overestimating pool volume! Also, pool salesman win points, on the 'bigger is better' scale, by doing the same. But the errors make it harder to manage your pool well.