How Much Paint Do You Need?

Calculate the number of cans to avoid buying too much or too little

Lisa Teague Design Studios, original photo on Houzz

My husband and I are avid DIYers, and whenever we start a new project we joke about the number of trips to the home-improvement store it’s going to take to complete the task. A “one-trip job” is considered a smashing success. The unfortunately more common “five-trip job” tends to feel slightly less triumphant.

 would argue that running out of paint midproject is especially frustrating. Dashing out to the store is the last thing you want to have to do while deep in the throes of painting. But you also don’t want to overbuy either, because custom-tinted paint is not returnable. 

Fortunately underbuying or overbuying is easily avoidable. It just requires a bit of measuring and some basic math before you head out to purchase the paint. Here’s how you can answer the question, “How much paint do I need?”

Depending on the manufacturer, a gallon of paint will cover 300 to 400 square feet. Check with your paint vendor, or on the paint manufacturer’s website, to find the estimated coverage.

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Once you know the coverage rate of your chosen paint, you need to calculate the square footage of the walls or ceilings to be painted. To get this figure, simply multiply the width by the height (for walls) or the length by the width (for ceilings). You then need to subtract the area of anything within the plane that is not being painted, such as windows and doors.

Sophie Loubaton / Photographe, original photo on Houzz

For example, let’s say you want to paint an accent wall that measures 25 feet wide by 10 feet high, which has a door opening within it that measures 3 feet wide by 7 feet high.

1. Multiply the width and height of the wall: 
25 x 10 = 250 square feet

2. Multiply the width and height of the door: 
3 x 7 = 21 square feet

3. Subtract the door opening from the area of the wall:
250 square feet – 21 square feet = 229 square feet

4. Divide this figure by the estimated square footage coverage amount of your paint. In this example, let’s say the coverage is 350 square feet per gallon:
229 ÷ 350 = 0.65

5. Round up to the nearest whole number to get the number of gallons needed. In this example, you’d round up to 1 gallon, to apply one coat of paint. 

Note: If you need to apply two coats, you simply double the square footage to be painted and then divide by the coverage:
458 ÷ 350 = 1.31
(Here, you’d round up to 2, so 2 gallons of paint would be needed to apply two coats.)

Alden Miller Interiors, original photo on Houzz

Painting multiple walls in a room? Here are the steps to calculate the square footage of a sample room that measures 12 feet by 20 feet with 8-foot ceilings and door openings that measure 6 feet wide by 7 feet high:

1. Add together the width of each wall to be painted:
12 + 20 + 12 + 20 = 64 feet

2. Multiply this sum by the height of the walls:
64 x 8 = 512 square feet

3. Multiply the width and height of the door:
6 x 7 = 42 square feet

4. Subtract the door openings from the area of the wall:
512 square feet – 42 square feet = 470 square feet

5. Divide this figure by the estimated square footage coverage of our paint. Again, in this example, we are going with 350 square feet per gallon:
470 ÷ 350 = 1.34

6. Round up to the nearest whole number, in this case 2. You would need 2 gallons to paint one coat. 

Note: Again, for two coats of paint, you double the square footage to be painted:
470 + 470 = 940

Then divide by the coverage per gallon:
940 ÷ 350 = 2.69
(Round up to 3. You would need 3 gallons of paint to apply two coats.)

Ask a Professional Painter Before Buying Materials

Sherwin-Williams, original photo on Houzz

Math not your strong suit? No sweat. Most paint manufacturers offer a calculator on their website that does the math for you. You still need to measure your space and plug in the square footage, but the calculator does all the rest, based on the paint you plan to use.

Of course, there are a few other factors to take into consideration when calculating how much paint you need. Darker colors, especially red hues, require more coats than lighter hues. If you are going with a deeper, darker color, talk to your paint vendor about prepping with a tinted primer before applying paint.

Also, take into account the surface you are painting. Rougher surfaces as well as more absorbent surfaces will require more paint than smooth surfaces. And if you are painting a lighter color over an existing dark hue, you will likely need extra coats. A tinted primer might also be useful here, depending on the existing and new hues.

The quality of your paint will also affect the amount you need. Higher quality paint tends to offer better coverage, requiring fewer coats, whereas a lower-cost paint may be thinner and therefore require more coats for proper coverage.

Finally, a few tips I’ve learned the hard way: There can be subtle differences in hue and sheen between two different cans of the same paint brand, finish and color. So try to stick to one single can of paint for the top coat of each wall or ceiling. You likely won’t notice a slight hue or sheen difference from wall to wall, but within the same plane it could be apparent. 

Also, it’s a good idea to keep a record of the paint brand, colors and sheens you used, so if you need to touch anything up down the road you know exactly what you need. 

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LOOKA DECO, original photo on Houzz

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