When you’re getting ready to paint a room, it’s tempting to grab a few gallons and hope for the best. But to avoid waste, extra trips to the hardware store, and color inconsistencies, it’s worth taking a few minutes to calculate exactly how much paint you’ll need. With a simple method, you can figure out the right amount and ensure a smooth, even finish from start to end.

Start by Measuring Your Room

The first step is to measure your walls. You’ll need the perimeter of the room and the height of the walls to calculate the total wall area to be painted.

1. Measure the length of each wall and add them together to get the total perimeter.
2. Measure the height from floor to ceiling.
3. Multiply the perimeter by the height to find the total wall area in square feet.

Example: A room that’s 12 feet by 15 feet with an 8-foot ceiling has a perimeter of 54 feet. 54 ft × 8 ft = 432 square feet of wall area.

Subtract Doors and Windows

You usually won’t paint over doors and windows, so subtract their areas from your total. A standard door is about 20 square feet, and an average window is around 15 square feet.

If your room has one door and two windows, subtract 50 square feet from the total.

In the example above: 432 sq. ft. – 50 sq. ft. = 382 square feet to paint.

Know Your Paint’s Coverage

Every paint can lists its coverage rate—how many square feet it covers per gallon. Most interior paints cover between 300 and 400 square feet per gallon, depending on the surface and paint type.

If your paint covers 350 sq. ft. per gallon and you have 382 sq. ft. to paint: 382 ÷ 350 = about 1.1 gallons per coat.

Don’t Forget the Number of Coats

Most walls need two coats for a consistent, durable finish. Multiply your paint amount by two.

In the example: 1.1 gallons × 2 coats = 2.2 gallons total.

If you’re painting over a dark color or a porous surface, you may need an extra coat or a primer first. Ask your local paint store for advice if you’re unsure.

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Consider Surface and Paint Type

Different surfaces absorb paint differently. Smooth drywall uses less paint than textured plaster or brick. The rougher the surface, the more paint you’ll need.

• Smooth walls: about 350–400 sq. ft. per gallon
• Lightly textured walls: about 250–350 sq. ft. per gallon
• Heavily textured or rough surfaces: about 150–250 sq. ft. per gallon

Always check the manufacturer’s recommendations on the label—they can vary by brand and finish.

Add a Little Extra

It’s smart to buy about 10% more paint than your calculation suggests. This covers touch-ups, roller absorption, and small measurement errors. It’s better to have a little left over than to run out mid-project.

In the example: 2.2 gallons × 1.1 = about 2.4 gallons – round up to 2½ gallons.

Quick Recap – Step-by-Step

1. Measure the perimeter and height of your room.
2. Multiply to find total wall area.
3. Subtract doors and windows.
4. Divide by the paint’s coverage rate.
5. Multiply by the number of coats.
6. Add 10% for safety.

That’s your total paint requirement—no guesswork needed.

A Great Finish Starts with Good Planning

Knowing how much paint you need helps you buy the right amount in one trip, ensuring color consistency and saving time, money, and effort. With a little preparation and a simple calculation, you can paint your room efficiently—and enjoy the satisfaction of a perfectly finished space.