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One of the easiest ways to completely transform a room is to paint it. You can estimate just how much paint you’ll need to finish the job by taking some measurements of the space and inputting those numbers into an equation. Write that final number down and take it with you to the store when you go to purchase your paint. Say goodbye to leftover paint cans that take up valuable storage space in your garage.

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## How to Calculate How Much Paint You Need

On average, one gallon of paint will cover 350 square feet. A standard door is about 20 square feet, while a window is usually about 15 square feet. Because those areas won’t need to be painted, you can count them out.

### 1. Find the Area of Windows and Doors

To get specific, you can calculate the exact areas of your windows and doors instead of going by these estimated numbers. Measure the height and width and multiply them together to get the area of the window or door.

### 2. Find the Area of Each Wall

Find the area of each wall that needs painting by multiplying its height by its width. Round each number to the nearest foot to make calculations easier.

### 3. Add the Areas and Take Sloping Ceilings into Account

Next, add the area of each wall (minus windows and doors) together to get the total surface area of the space to be painted. Include the area of the ceiling if you’re planning on painting that too. If you have a sloping ceiling, calculate the square footage of the triangular wall space by multiplying the length of the wall at the base of the triangle by its height, then divide that number by two.

### 4. Divide Sum by 350

Divide the sum of all of your wall areas by 350 to get an estimate of how many gallons of paint you’ll need.

### Other Calculations to Consider

If you’re planning on doing more than one coat of paint, multiply this number by the number of coats. Typically, two coats are enough, but it depends on the quality and color of the paint you use. Painting a lighter color over a darker one, for example, will require more coats to completely cover up the old paint. If you’re refreshing your white paint with a wash of the same color, you probably only need one coat.

The standard bedroom wall is approximately 8-by-12 feet. Assuming there are four walls with one standard door and one standard window, the total wall area would be 349 square feet. You will need about one gallon of paint to paint one coat over this room.

If you’re painting a small area, you may need less than a gallon of paint. Most paint comes in quarts, which will cover about 90 square feet.

## Paint Cost Per Gallon

Paint costs vary depending on each product’s quality and finish. Lower-quality paints with flat finishes cost as low as $10 per gallon, while higher-end glossy finish paints run between $20 and $40 per gallon. Paints from designer brands often cost upwards of $100.

## Best Type of Paint Per Room

Flat finish paints work best in living rooms, bedrooms and on ceilings, while semi-gloss and gloss finish paints work well on trim and doors. Matte, eggshell and satin finish paints also work well for painting walls in interior rooms.

## How Much Paint You Need for Trim

Painting trim a different color than the walls can really make a room pop. Consider painting baseboards, windows, doors and crown molding in a different finish or color.

You can calculate how much paint you’ll need to paint a room’s trim by finding the area of each piece of trim—do this by multiplying its length by its width. Finding the area of trim seems tedious, but most trim has the same width so you may not need to recalculate each item’s width, just its length.

Add all of these areas together to get the total area of trim you’ll need to paint. Then, divide that number by 350 to get an estimate of the number of gallons of paint you’ll need to paint your trim.

In most cases, you won’t even need an entire gallon of paint to paint one room’s trim. Consider purchasing paint in quarts instead.

## How Much Primers You Need

Primer helps paint adhere properly to walls, especially unpainted surfaces like stucco and new drywall. It also ensures that your paint looks even, so applying primer before paint is an important step to make sure your paint job looks perfect.

You may need more gallons of primer than you do paint because primer covers 200 square feet per gallon, compared to the typically 350 square feet per gallon that paint covers.

Once you have calculated the total area of all the walls that need painting in your room, divide that number by 200 to estimate the number of gallons of primer you’ll need.

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