how to calculate how much paint you need
To determine how much paint your house needs, measure the house's perimeter. Then multiply that figure by the height, excluding gable ends. Take the measurements with a steel tape measure, or reel out a ball of twine around the house and mark and measure the twine.
If you will use a different paint on your home's trim, subtract 21 square feet for every door and 15 square feet for each typical window. Divide the final figure by the square-foot coverage specified on the can of paint to determine the number of gallons you will need for one coat.