Once you've conquered the often Herculean challenge of choosing a paint color, you must then figure out how much paint to buy. It's a tricky calculation with a number of variables, ranging from your painting technique to the composition and condition of your walls. Buy too much paint, and you've not only wasted $30, but you've also got to store the surplus somewhere on your already crowded shelves. Buy too little, and on the day you finally work up the energy to paint, you're delayed by needing to make a second trip to the local home center. Neither outcome is desirable, but fortunately you can avoid both with proper planning.

The major paint manufacturers each provide an online calculator aimed at helping consumers decide how much paint they need. For a ballpark figure, visit:

• Benjamin Moore

• Sherwin-Williams

• Behr

As handy as they are, online calculators sacrifice precision for convenience. Though more tedious, handling the calculations yourself enables you to purchase exactly the right amount of paint-no more, no less. The math isn't difficult to do, and all you really need, besides a pencil and sheet of paper, is a tape measure.

**THE SCOPE OF THE PROJECT**

You first need to determine which surfaces you want to paint. Think it through: Are you going to paint the ceiling? What about the baseboards? Once you know exactly which surfaces you're going to paint, figuring out the amount of paint to buy is a simple matter of calculating the square footage of those surfaces. You'll also need to account for the fact that a satisfactory paint job usually requires at least two coats, particularly if you're painting a lighter color over a darker one.

**MEASURING SOLID WALLS**

Doors and windows tend to complicate things; solid walls are the easiest surfaces to deal with in terms of paint project planning. For each solid wall, simply multiply the width by the height to get the total surface area. For example, a solid wall that measures 12 feet by 10 feet would have an area of 120 square feet. If a second solid wall totals 100 square feet, the two solid walls together would be 220 square feet. Be sure to omit the trim-baseboards, crown molding, and so on-from your measurements.

**MEASURING AROUND WINDOWS**

To calculate the square footage to be painted on a windowed wall, first measure the wall to find its total area, then subtract the area of each window-just the window frame and the glass; leave out any molding. So for a 12-by-10-foot wall with one 4-by-6-foot window, you'd subtract 24 (the area of the window) from 120 (the total area of the wall), which would leave you with 96 square feet to be painted (120 - 24 = 96).

**MEASURING AROUND DOORS**

Follow a similar procedure to determine the surface to be painted on any wall with a doorway. First, measure the length and width of the wall and multiply those two measurements together to get the wall's square footage. Next, calculate the area of the door panel only; for now, ignore the case molding. So for purposes of explanation, if a 12-by-10-foot wall has one door that measures 3 feet wide by 6 feet tall (or 18 square feet), then you'd subtract 18 from 120, leaving 102 square feet to be painted (120 - 18 = 102).

**PUTTING IT ALL TOGETHER**

Once you've measured every wall and subtracted the area of any windows and doors, you know the total wall surface area to be painted. Let's say that, according to your calculations, you need enough paint to cover 500 square feet. How many gallons do you need to buy to get the job done?

Manufacturers typically say that one gallon of paint covers 250 to 400 square feet. That's a pretty wide range, largely due to the fact that different surfaces take paint differently. If you are painting a smooth surface, chances are you can stretch a gallon to cover 400 square feet. If the surface is rough, textured, or previously unpainted-or if you're making a dramatic color change-that gallon may cover only 250 square feet.

Let's assume that, based on the condition of the walls in your home, a gallon can cover 325 square feet. You've determined that there's 500 square feet of wall surface to cover. Those walls will require two coats, so you'll ultimately be covering 1,000 square feet. At 325 square feet per gallon, you'll need a little over 3 gallons (1,000 ÷ 325 = 3.08 gallons, to be precise).

In general, it's rarely a mistake to round up and purchase slightly more paint than the math indicates you'll need. Not only may your surfaces drink up a little more paint than you anticipated, but any extra paint will also be helpful for future touch-ups. But as rounding up to 4 gallons from 3.08 may leave you with more leftover paint than you really want, in this instance I'd suggest buying 3 gallons and 1 quart.

**ALLOWING FOR CEILINGS AND TRIM**

If you're painting the ceiling, chances are you're planning to use a color other than the one you've chosen for the walls. If that's the case, simply measure the length and width of the ceiling, and multiply the two measurements together to find the square footage. If the ceiling encompasses an area of, say, 100 square feet, you know that you need enough paint in the second color to cover at least that area.

As it happens, a quart of paint typically covers about 100 square feet, so if you're planning on just one coat, you may be able to get away with that smaller container size. But if you're doing two coats, you're going to need at least two quarts. The store salesperson is likely to remind you that two quarts usually cost the same as one gallon, so you might as well spring for the larger size, particularly if you're planning to use the ceiling color elsewhere in your home.

The same advice applies to trim-assuming that you are going to paint it something other than the color you've chosen for the walls, measure trim separately from the rest of the room. Once you know how much surface area the trim covers, calculate how much paint you will need in order to give the trim two coats.