Planning to paint a room this weekend? Are you wondering just how much paint to buy? When you "paint by numbers" you avoid purchasing blunders. Here's how: How much paint you need depends on three things: room size, surface condition and spreading rate. To estimate how much paint you need, measure the room's perimeter and multiply by wall height. Then deduct 20 square feet for each door and 14 square feet for each window. Divide this number by the spreading rate on the paint can label (usually 300 to 400 feet per gallon). To calculate trim paint (on average), buy one-fourth of what's needed for the walls. And that's the On The House tip for today.