How to Calculate Paint for an Exterior Deck
The US Census Bureau shows that 85 percent of American homes have a deck, porch, patio or balcony. Exterior surfaces are especially prone to wear and tear caused by extreme outdoor temperatures. Weatherization of decks can reduce a home's curb appeal and depreciate property values. A fresh coat of paint may be all you need to spruce up deck areas and brighten your home's exterior. Calculating the right amount of paint needed will help you get the job done quickly and efficiently.

Use a steel tape measure to find the length and width of the deck. Round both numbers to the nearest foot. Write down the measurements.

Multiply the length by the width. This will give you the square footage of your deck. For example, if the length is 20 feet and the width is 30 feet, multiplying 20 X 30 amounts to 600 square feet of deck.

For oddshaped areas, divide the deck into rectangular sections, and measure each length and width separately. Multiply the length by the width for each section. For a triangular shape, measure the length and width as if it were rectangular. Multiply the length by the width, and then divide it in half. Add all sections together to get the total square footage.

Most deck paint jobs require at least two coats of paint for best results. Multiply your total square footage by the number of coats your deck will need. For example, if your square footage is 600 square feet and you need two coats of paint, your new number will be 1200 square feet.

Check the brand of paint you will use to get an estimate of how many square feet each gallon will cover. Most kinds of paint cover an average of 350 square feet. Divide your total square footage by this number. Round to the largest whole number. This number shows how many gallons of paint you will need to purchase. For example, if your total square footage for two coats of paint is 1200 square feet, dividing it by 350 will give you approximately 3.4. Round this number to 4. This means you will need to purchase four gallons of paint to cover a 600square foot deck with two coats of paint.

Having too much paint is better than not having enough. Try to get all your paint in one batch so that you will not have to try to match the color if you need more paint later. Add an extra gallon or two to ensure you have enough to cover the entire area. Extra paint is also handy for touchups and correcting mistakes.
Things You Will Need
 Steel tape measure
Tip
 If you plan to use a sprayer instead of a brush or roller to paint your deck, add at least two more gallons to your final total. Prime your deck first before painting to seal and waterproof the wood. This will also help paint adhere better and prevent air bubbles that can cause peeling.