How Much Paint Do I Need for a Room?

Buying paint is easy to get wrong. Grab too little and you are back at the store mid-project, hoping the new can matches. Grab too much and you have leftover gallons drying out in the garage. The good news is that figuring out how much paint you need comes down to a little measuring and one simple formula. Here is how to do it.

Measure Your Wall Area

Paint coverage is based on square footage, so your first job is to find the total area of the walls you plan to paint. The quickest way is to measure the perimeter of the room (the distance all the way around the floor) and multiply it by the wall height.

The formula is:

Wall area = perimeter × height

To find the perimeter, add up the length of every wall. For a rectangular room, that is two times the length plus two times the width. Then multiply by how tall the walls are, floor to ceiling. Measure in the units you are comfortable with, just stay consistent.

Subtract Doors and Windows

You are not painting the door or the glass, so those areas come out of your total. This step matters most in rooms with lots of openings, where doors and windows can easily account for a good chunk of a wall.

Use these rough standards, or measure your own for accuracy:

A standard interior door is about 21 square feet (roughly 3 ft wide by 7 ft tall).
An average window is about 15 square feet, though this varies a lot.

Add up the openings and subtract that number from your wall area. If you only have one door and one small window, you can skip this and let the extra paint be your safety margin.

Know Your Coverage and Why You Need Two Coats

Every can of paint lists its coverage, usually somewhere around 350 to 400 square feet per gallon (about 10 to 12 square meters per liter). Check the label, since thicker paints and rough surfaces cover less.

Here is the catch: that number is for a single coat on a smooth, similar-colored surface. In real life you almost always want two coats. A second coat evens out the color, hides the old wall underneath, and gives a more durable finish. So take your one-coat number and double it. If you are painting over a drastic color change or bare drywall, budget for a coat of primer too.

A Worked Example

Say you have a bedroom that measures 12 ft by 10 ft with 8 ft ceilings, plus one door and one window.

Perimeter: 12 + 10 + 12 + 10 = 44 ft.
Wall area: 44 ft × 8 ft = 352 sq ft.
Subtract openings: 352 − 21 (door) − 15 (window) = 316 sq ft.
One coat: 316 ÷ 350 = about 0.9 gallons.
Two coats: 0.9 × 2 = about 1.8 gallons.

So you would buy two gallons for this room, which leaves a little extra for touch-ups.

Always Round Up

Paint is sold in whole cans, and running short is the worst-case scenario. A fresh can opened later may not match perfectly, and a partial third coat will show. Whenever your math lands between sizes, round up to the next full gallon or quart. Leftover paint stores well and is handy for repairs down the road.

Rather not do the arithmetic by hand? Our room paint and flooring calculator lets you enter your room dimensions in 3D, automatically accounts for doors and windows, and tells you how many gallons to buy for one or two coats. It takes the guesswork out so you can get straight to painting.