What size rectangular tank will I need to hold 841.5 gallons of water. The water level is 3'-0" from the bottom of tank.

One american gallon is 231 cubic inches so 841.5 gallons is 231 841.5 = 194386.5 cubic inches. The height of the water is 3 feet which is 36 inches so the surface area of the water in the tank must be ^194386.5/₃₆ = 5399.625 square inches, which is 37.5 square feet. Thus the length and width of the tank in feet are any two numbers which have a product of 37.5. For example a square tank would need to have length and width 6.12 feet or it might be 5 feet wide and 7.5 feet long since 5 7.5 = 37.5.

Penny