We estimate that you get about 20-25 square feet of coverage for 1 coat with 1 spray can, so for a 50 sqft garage door, then you would be about 4 cans. Keep in mind, you may nee more than 2 coats. The color of the spray and the color you are covering will all affect the number of coats needed. The spray paint works best with multiple thin coats. It is also very difficult to use spray cans to paint a large surface area such as a garage door. It dries very fast and the overspray affects the area you've already painted. It would be very difficult to achieve a good finish.