Paint Calculator

Paint Calculator: Buy the Right Amount

Walk into any paint store without a number in hand and you are either going home with too much paint or making a second trip halfway through the job. A paint calculator gives you that number before you leave the house. Enter your room dimensions, subtract your doors and windows, set your coats, and you get a gallon count you can actually shop from.

Here is what the results mean and what you need to know before you use one.

Why Room Dimensions Are Only Part of the Equation

Most people measure length, width, and ceiling height and call it done. That gets you total wall area, but it is not what you are actually painting. Every door and standard window you skip over is roughly 20 square feet of surface that does not need paint. Leave those out and your estimate runs high — sometimes by a full gallon on a room with several openings.

The calculator above handles that automatically. Enter the number of doors and windows and it deducts them from your paintable square footage before calculating gallons. The result is a tighter, more accurate number.

The Coverage Gap Nobody Talks About

Paint cans say 400 square feet per gallon. That number is a lab result, not a job site result.

Real-world coverage lands closer to 350 square feet per gallon once you factor in surface texture, roller nap, lap marks, and the simple fact that no one applies paint at perfectly uniform thickness across an entire room. The calculator above uses 350 square feet per gallon for exactly this reason. It is a more honest number and it keeps you from running short.

How Coats Change Everything

One coat of paint is rarely enough. Two coats is the standard for most interior painting jobs, and it shows up in the math in a significant way. Double the coats, double the paint needed.

Where people get caught is assuming one coat will cover when it will not. If you are painting over bare drywall, fresh primer, or making a dramatic color change — going from dark to light or light to dark — two coats is the minimum. In some cases, particularly with deep reds or yellows, three coats is realistic. Set your coat count honestly in the calculator and your gallon estimate will reflect it.

When to Add an Extra Coat

Beyond the standard two-coat job, a few specific situations call for more coverage.

Bare drywall absorbs paint aggressively on the first coat. What looks like full coverage when wet will dry patchy. A third coat or a dedicated primer pass is the fix. The same applies when you are making a significant color shift. The old color bleeds through until the new one builds enough film thickness to block it completely. Adding a coat in the calculator before you shop is cheaper than buying an extra gallon at the last minute.

Textured walls are another situation where coverage estimates run short. Knockdown, orange peel, and popcorn finishes all have more surface area than a flat wall of the same dimensions. If your walls have significant texture, bumping up your coat count by one is a reasonable adjustment.

Buying Gallons vs. Quarts

Once you have your gallon count, there is one more decision before you head to the register. Paint comes in quarts and gallons. A quart is a quarter gallon and costs significantly more per unit than a full gallon.

If your calculator result lands at something like 1.3 gallons, resist the temptation to buy one gallon and one quart. Buy two gallons. Leftover paint stores well in a cool dry space and is valuable for touch-ups down the road. Running out and needing a quart at the end of the job is one of the more frustrating ways to spend an afternoon.

FAQ

How do I calculate how much paint I need for a room? Measure the length and width of the room to get the perimeter, multiply by ceiling height to get total wall area, then subtract roughly 20 square feet per door and 15 square feet per window. Divide the remaining square footage by 350 (a realistic coverage rate) and multiply by the number of coats. The calculator above does this for you automatically.

How much does one gallon of paint cover? Most paint manufacturers list 400 square feet per gallon on the label. In real-world conditions with standard rollers and typical wall surfaces, 350 square feet per gallon is a more accurate estimate. The calculator uses 350 to give you a more reliable result.

Do I need two coats of paint? For most interior painting jobs, yes. Two coats produces a more even finish and better color saturation. One coat may look acceptable when wet but often dries with thin or patchy areas, especially near edges and corners.

How much extra paint should I buy for a color change? If you are making a significant color shift, add at least one extra coat to your calculation. Going from a very dark wall to a light color — or the reverse — may require three coats to fully cover the previous color. Setting your coat count to three before calculating will give you an accurate gallon estimate.

Why does my paint can say 400 sq ft per gallon but the calculator uses 350? The coverage rate on the label is measured under controlled conditions with a smooth surface and consistent application. Real painting involves textured walls, lap lines, and uneven application. Using 350 square feet per gallon accounts for those real-world variables and helps ensure you have enough paint to finish the job.

Should I paint the ceiling with the same calculator? The calculator above is designed for walls. Ceiling paint is typically calculated separately by multiplying room length by width to get ceiling square footage. Ceilings usually take one to two coats depending on color and surface condition.

What happens if I run out of paint mid-job? Running out is more than an inconvenience. If your paint was custom mixed, a new batch may not match exactly, even from the same store. The difference becomes visible when the two batches dry side by side. Buying enough to finish the job in one purchase — and keeping the lid sealed on the remainder — avoids the problem entirely.

How do I account for doors and windows in my paint estimate? The calculator lets you enter the number of doors and windows and deducts their square footage automatically. A standard interior door is approximately 20 square feet and a standard window is approximately 15 square feet. Removing these from your total gives a more accurate wall coverage estimate.

Use the paint calculator at the top of this page to get your gallon count, then download the free supplies checklist so you have everything you need before the first roller hits the wall.

When you finish your painting project, you might want to visit our Windows and Interior Calculators for Blinds, Curtains and more.