How Much Paint Do I Need? Here’s an Easy Way to Calculate It

If you want to give your space a facelift, painting your rooms is an easy, DIY-friendly way to totally transform your home. But once you have that perfect shade picked out, you’ll probably run into the question that most of us have asked ourselves at this point: “How much paint do I need?” With a little bit of math, you can figure out exactly what you need and avoid having to dispose of leftover paint.

How Many Gallons of Paint Do I Need per Square Foot?

When painting a room, the last thing you want is to unexpectedly run out of paint or find yourself spending a lot more on paint than you planned. Before starting any large painting project, calculate how much paint you’ll need so you can be prepared from the start.

Be mindful of factors that can influence how much paint you’ll need, such as the material of the surface you’re painting, how many coats it’ll take to get the look you want, and how to be sure you have a little left over for touch-ups down the line.

How Much Primer Will I Need?

Primer actually covers less area than paint does, at about 200 to 300 square feet per gallon. For a small room of 400 square feet, you’d need 2 gallons of primer. A good rule of thumb is to divide the total square footage of what you’re painting by 200 to know how much primer you need to buy.

For example, if you’re priming a 144-square-foot room, you’ll divide 144 by 200. This leaves you with .72 gallons, meaning you'd be safe buying just 1 gallon of primer.

How to Calculate How Much Paint You Need

Divide the total square footage by the approximate coverage of your chosen paint (usually 400 square feet per gallon, but your container will have an exact number). This math will tell you how many gallons of paint you’ll need for one coat.

Multiply this number by two to estimate how much paint per square foot you’ll need in order to give the walls two coats, which pros usually recommend. Round up uneven numbers and purchase slightly more paint for waste allowance, but not so much as to have to throw the paint away. Doing so will keep you from making multiple trips to the home improvement store for more paint and save you some paint for future touch-ups.

To calculate the square footage of solid walls: 

• Measure the length of all walls from corner to corner.

• Measure the room’s height from floor to ceiling, minus any trim, baseboards, and crown molding. 

• Multiply the walls’ lengths by the height—the number you get is the room’s square footage. 

• For example, say you have a room with a solid wall that measures 10-by-10 feet (100 square feet) and a second solid wall of 12-by-10 feet (120 square feet). The two solid walls together would be 220 square feet.

• A gallon of paint covers about 400 square feet, so if you’re just doing one coat, one can of paint should be plenty for these two walls. If you’re painting the whole room (440 square feet), you may be able to stretch one gallon across the whole room, but it’s better to be safe than sorry and buy more than you need, especially if you’ll need to do multiple coats.

• To avoid ending up with a lot of excess paint, see if the paint of your choice comes in smaller cans. A gallon can plus a pint-sized can should work for one coat in our example room with a bit left over for touch-ups.

Calculating Walls With Doors and Windows

To calculate the square footage of a wall with a door or a window:

• Measure the length and height of a wall to find its total square footage.

• Measure the door/window area, including the window frame and glass, and leave out any molding. Typically, doors are 21 square feet, and most windows are 15 square feet on average. 

• Subtract the area of each door/window from the total wall area.

• As an example, if you have a 12-by-10-foot wall (120 square feet) with one 4-by-6-foot window (24 square feet), you should subtract 24 from 120, leaving you with 96 square feet of wall to paint. If you’re just doing one coat, you’ll use about a quarter of a gallon (one quart) for this wall.

Calculating Ceilings 

Whether you’re using the same color or a different color than the walls when painting the ceiling, here’s the process you should follow to measure the square footage:

• Measure the length and width of the room’s ceiling.

• Multiply the measurements to find the square footage. 

• If you’re using the same color, treat the ceiling as a fifth wall. If you’re using a different color, estimate the square footage and multiply by two to account for a second coat.

• Using the example of our 10-by-12 room, the surface area of the four walls adds up to 440 square feet while the ceiling is an additional 120, giving you 560 square feet to cover. Since a gallon of paint covers about 400 square feet, you’ll need a little less than a gallon and a half for one coat.

Calculating Rooms With Trim or Molding

If you’re painting your trim or molding a different color than the walls:

• Measure the length and width of each trim, crown molding, window, and door frame separately from the rest of the room.

• Multiply the length by the width to get the square footage of the area you’ll be painting.

• For example, if our 10-by-12 room has baseboards that are 5 inches tall, that’s a bit over 18 square feet to cover. Unless you plan on doing multiple coats or using the trim paint for another project, you should opt for a much smaller can of paint, if possible. Even a pint should be plenty. 

DIY vs. Hiring a Pro 

Painting is one area in home projects where you can save quite a bit of money by choosing to make it a DIY project versus hiring a professional. The cost of hiring a professional averages $1,100 for a standard-sized room. However, if you choose to paint the room yourself, the average cost drops to around $150 for materials.

While painting a room yourself can save a significant amount of money, it does take considerable time. A local interior painter has more tools for accurate prepping and painting and has the experience to work at a more efficient pace, so you’ll have to decide which route is better for your schedule and wallet.