Figure out, or in other words, estimate how much you need by measuring and calculating coverage. What does this mean? Well, start by measuring the dimensions of the surface area to be painted, and then multiply the height by the width, or in other words, find the surface area. Do not try and subtract out windows and doors, having a little extra for touch up and waste is good. However, if you do not want any extra at all, then perform the same calculation for areas that will not be painted, such as doors and windows. Then subtract non-paint areas from the overall surface area to get an estimate of the square footage you will be painting.

Your next step is to pick the paint you want. Every paint has different coverage, so in some paints you might need two gallons of paint, and in another paint you may only need one and half gallons for the same room. So, once you choose your paint, look at the label to determine the spread rate of the paint, or the number of square feet (m2) of surface each coat of paint will cover. For many paints, this will be 400 square feet per gallon (11m2 per L). Then, do some simple math. Divide the total surface area, which you figured before you went to the store, by this number to determine the number of gallons/liters you will need for each coat.

Next, determine how many coats of paint you are going to need. Many factors-including the type of paint, the color you are applying, the method of application, and the surface- will determine the number of coats you should apply.

For example, with new drywall more than one coat of paint is recommended. Or, if covering a dark color with a lighter one, you might need two coats of paint. Not to mention certain surfaces, like exterior masonry, are very porous and absorb a great deal of paint. In such cases, a second coat is often needed to give the paint a uniform appearance. And, the type of tools you use will determine how much paint you will use. For example, using a brush will put on a heavier, better-hiding coat than roller application.

So, figure out how many coats you want, but remember, it is always wise to apply two coats of paint when doing exterior painting, and it is always better to apply to thinner coats then one thick coat. Two-coat coverage does a better job of standing up to harsh weather and environmental conditions, and can extend the life of your paintjob.

Last but not least, when it comes to paint estimating it is better to buy more paint than you think you will need than to run out before the job is done. Purchasing all your paint at once will help ensure can-to-can color consistency, and save time on trips to the store. If you have leftover paint, you can always use it for touch-ups, which will be nice down the road. And, if you are really worried about it, you should know that unused and unopened paint can usually be returned for credit, unless it is a custom color. So, unless you are using custom colors, you won't be out any money if you over estimate. And, if you are using custom colors, overestimating is far better than underestimating as it can be difficult to match paint precisely and you might end up with slightly different colors in paint if you have to go back and get more to finish your job.