Okay.  That’s a 3.5 gallon tub of RedGard.  Per Google, 3.5 gallons is 808 cubic inches.  Let’s assume that between the rollers, the paintbrushes, the paint tray, the sides of the container, etcetera that a full third of the RedGard has ended up wasted in some way or another.

Therefore there are 808 x .66 = 533.28 cubic inches of RedGard on the walls.

The two side walls are roughly 32 inches wide by 80 high; the back wall is 58 inches long by 80 high.  That’s 32 x 80 x 2 = 5120 square inches on the sides and 58 x 80 = 4640 square inches on the back for a total of 9760 square inches.

Divide the RedGard’s volume by the wall’s area to determine the thickness: 533.28/9760 = .0546 inches thick.

One mil is 1/1000 of an inch, therefore the coating of RedGard is 54.6 mils thick.

The coating is supposed to be 30 mils thick.

We good.