Have you ever had this happen to you? You've gotten almost the entire room painted and you're scraping the bottom of the paint can praying that you don't have to run back out to the store to buy more paint.
To keep that from happening, it's good to use this little paint calculation before getting the paint:
Multiply the height of each wall by it's width (don't subtract for floors and windows unless they take up more than half the wall). This will give you the wall area.
Do this for each wall and then total the figures up. For two coats, double the number.
Add a little bit more (approximately 5-10%) for touchups.
One gallon of paint equals to about 450 square feet.
If you're not a math person, they also have online paint calculators. Here are a couple I like:
Do you calculate the paint before buying or just guesstimate?