I work in the paint department at Home Depot and come across a lot of customers who have this problem. Generally a quart of paint will cover between 80-100 square feet.
Now the thing that sucks for you is that if you buy another quart of paint, you will have spent as much money as if you had bought a gallon in the first place.
Maybe you could say you did it on purpose... for uh... a faux finish technique.... or something...