How to Calculate Amount of Paint to Paint a Room

Measure the length of each wall.

Wall length helps you figure out total square footage. Grab a tape measure and place it flush along one end of the wall. Check that the tape measure is firmly in place, and then pull it in a straight, taut line lengthwise across the wall. To be safe, do a couple of measurements to make sure your calculations are totally accurate. Then, measure the length of all the other walls in your room. It may help to ask a friend to hold the measuring tape in place while you pull it across the wall.

Calculate the height of each wall.

The total height is the other part of your square footage equation. Anchor your tape measure at the bottom edge of the wall and slowly pull the tape measure upward, keeping it flush against the wall as you extend it. Jot down the measurement once the tape reaches the edge of the ceiling. Then, check the height of all the other walls to make sure your measurements are exact. You may need to use a stepstool or ladder to measure the entire height of the wall. Don’t be afraid to ask a friend for help if you need it! All of your height measurements might be the same, depending on the size and shape of your room. Don’t include the trim in your measurements—you’ll be calculating that separately.

Multiply the total length by the height for each wall.

Length times height gives you the total surface area of your walls. Find the area of each individual wall, jotting down the area for each one. There’s a chance that your walls are all different sizes, so doing individual calculations helps you get more accurate results. For example, if a wall is 15 ft (4.6 m) and 10 ft (3.0 m) tall, the total surface area would be 150 sq ft (13.94 m).

Add all of the square footages together.

This gives you a rough estimate of how much surface area you need to paint. Plug all of the surface areas you just calculated into a calculator and see what the total surface area comes out to. This isn’t the exact amount of surface you have to paint—it’s just a starting point!

Measure the area of each door and window.

Multiplying length by width gets you this number. Use your tape measure to figure out the length and height of all the windows in your room, as well as the length and height of any doors. Then, multiply the measurements for each individual structure together to get the total surface area of each structure. If all of your windows and doors are the same size, you only need to measure them once—but you still need to multiply by the total number of structures that you have. For example, if you have 3 windows that have a total surface area of 17 sq ft (1.6 m), you’d multiply that by 3 (giving you a total of 51 sq ft (4.7 m)).

Subtract each door and window from the total.

This gives you the exact amount of surface area that you need to paint. Let’s say your walls’ total surface area is 576 sq ft (53.5 m), and there are 2 windows that are each 15 sq ft (1.4 m) and a door that’s 20 sq ft (1.9 m). Subtract the 50 sq ft (4.6 m) of doors and windows from the original 576 sq ft (53.5 m), which gives you a total of 526 sq ft (48.9 m) to paint.

Double the total square footage if you plan on painting 2 coats.

Your square footage calculation only counts for 1 coat of paint. Chances are, you’re probably giving your walls at least 2 coats of paint for a polished, long-lasting look. For each extra coat of paint, add the total surface area to your final calculation. If your total wall surface area is 650 sq ft (60 m) and you plan on adding 3 coats of paint, you’d need enough paint for 1,950 sq ft (181 m) of wall space. You may need extra coats of paint if you’re repainting dark walls with light-colored paint.

Divide the total square footage by 350 to 400.

This helps you figure out how much paint you need. A single gallon of paint can cover about 350 to 400 sq ft (33 to 37 m) of wall space. So, dividing your total square footage by 350 to 400 gives you an accurate idea of how many gallons of paint you’ll need for your project. Paint Calculation Formula: Surface Area / 350 = gallons of paint needed or Surface Area / 400 = gallons of paint needed If you’d rather err on the side of caution, divide your total square footage by 350 rather than 400. That way, you’ll have a little extra paint on hand if the project ends up being bigger than you expect. If you have 1,600 sq ft (150 m) of wall space to paint, you’d need 4 to 4.75 US gal (15.1 to 18.0 L) to paint your space. If your walls are textured or completely unpainted, divide your total square footage by 250 sq ft (23 m) instead. These surfaces tend to use more paint than regular, smooth walls.

Round up the nearest quart.

A little extra paint is good, but you don’t want too much leftover. Chances are, you won’t get an exact number of gallons or liters; instead, it’ll be a decimal. Rather than rounding up to the next whole number (gallon), round up to the next quarter (quart) instead. If you need 4.44 US gal (16.8 L) of paint to cover your wall space, you’d round it up to 4.5 US gal (17 L) instead.

Calculate the total amount of paint for trim.

Multiplying the width and length gets you the total square footage. Grab your tape measure and take down the exact width and length for each individual section of trim. Multiply the length and width of each individual section together, and then add up all the sections to get your total surface area. Then, divide the surface area by 350 to 400 to get the amount of paint you need in gallons. Let’s say the sections of trim are 30 sq ft (2.8 m), 40 sq ft (3.7 m), 60 sq ft (5.6 m), and 25 sq ft (2.3 m). The total square footage would 155 sq ft (14.4 m); divided by 400, you’d get 0.38 US gal (1.4 L). To be safe, you’d round up to 0.5, giving you a total of 0.5 US gal (1.9 L).