I have a calculator that will do construction stuff like how much paint
to use for the number of square feet.
14 ft long X 2 = 28ft
7 ft wide X 2 = 14ft
for a total of 42 ft
42 ft Times 7 ft tall = 294 Sq Ft = .84 Gallon
The one thing about painting
over a dark undercoat is that the paint on top may not be as bright as painting
over a white undercoat. You might want to think about a white primer over the dark before applying the final coat.
Because this is spray and not brush/roller, the calculator may not be correct so get your total sq ft and ask the guy where you purchase the paint.