If you learn to calculate paint correctly, you'll have enough to finish the job with just a little left over for touch-ups and repairs. Calculate it incorrectly, and either you'll have to make a return trip to the store for more paint, or you'll be stuck with lots of extra paint (such a waste). All you need is a tape measure and a calculator to figure out the right amount of paint you'll need for your deck or patio painting project.
Measure the deck with a tape measure. Write down the length and width of each section of deck.
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Multiply a section's length by its width to get the square footage. If there are multiple sections, figure the square footage of each section and add these figures together.
Determine how much paint you will need to cover the total square footage. You need one gallon of paint for every 300 to 400 square feet. If bare wood is being covered, it will soak up more paint and you'll get closer to 300 square feet of coverage per gallon. If you're covering a previously painted deck, you will get closer to 400 feet per gallon. To make it simple, assume you will need one gallon of paint for every 350 square feet of deck.
Double the amount of paint so you have enough for a second coat, since most decks require two coats of paint. For instance, if the square footage of your deck is 700 feet, purchase 4 gallons--2 gallons for the first coat and 2 for the second.
Round up the number of gallons you will need. If you calculate that you need just over 4 gallons, don't purchase 4 gallons and 1 quart. Instead, purchase 5 gallons so you have enough left over for touch-ups.