So, you want to repaint your kitchen, bathroom or bedroom — but you're a DIY beginner. You've chosen your color and you're ready to buy the paint, but how do you know how much you'll need to finish the job? 

Naturally, you want to purchase enough paint so that you can avoid making multiple trips to the store. But more importantly, you don't want to over-buy. Paint can be pretty pricey, so knowing exactly how much you need for your space means you won't be wasting money.

Save money and ensure that you won't have a bunch of leftover paint with these tips from "Trading Spaces" designer John Gidding.

According to John, there are three main elements that will help you determine how much paint you'll need.

1. THE ROOM'S SQUARE FOOTAGE

"The general rule of thumb is for one gallon of paint, it will cover about 350 square feet," John says.

You will need slightly more paint if the walls are patchy, unpainted or unfinished (i.e. drywall), because they will absorb more of the paint. We'll come back to this!

So, the first step is to measure the length of each wall as well as the height of the room from the floor to the ceiling.

Then you can determine the square footage of the room by adding together the lengths of all the walls and multiplying that number by the height of the room. The number you get is the room's square footage.

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2. THE TEXTURE OF THE WALL

"One of the most important things when you're trying to figure out how much paint you need is the texture of the wall," the designer says. "The porosity of the surface — how much water is going to soak in as you paint."

Like we said above, how porous the surface is, what condition it's in and the type of paint you use can all affect what's called the "spreading rate" of the paint. Essentially, it's how well one coat of paint will actually cover the wall.

Porous surfaces have lower spread rates, so you'll find that it's harder to cleanly cover drywall with a single coat. If you're starting with rough drywall, you should probably plan on using more paint. 

If the wall is already primed and painted, you won't need as much paint, according to John, because you should be able to cover it in one coat. You might need to touch up some spots, but you shouldn't have to do an entire second coat.

RELATED: What Colors Go Together In a Room?

3. THE CURRENT COLOR OF THE WALL

The third big factor to keep in mind is the color differential. If you're painting walls that are already a dark color, you should plan to apply two coats of paint.

If you're going from a dark color like blue to a light one like yellow, just double the amount to account for the second coat, the designer suggests.

Watch John and Rach demonstrate color differential in the video above.