Quick start
- Enter the room length, width, and wall height in feet. Do not enter floor area unless you have already converted it into wall area another way.
- Enter the number of doors and windows. The calculator subtracts 20 sq ft per door and 15 sq ft per window from the wall estimate.
- Enter the number of coats and the paint coverage from the can or product page. Use the lower end of a coverage range for rough, patched, porous, or dark-to-light changes.
- Use extra percent when the surface is textured, patched, absorbent, or you want a safer buying estimate for roller and tray loss.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Estimate gallons for a bedroom, office, or living room.
- Adjust for one or two coats before buying paint.
- Subtract common doors and windows from wall area.
- Compare coverage values from different paint labels.
What this calculator is solving
The Paint Calculator estimates interior wall paint for a simple rectangular room. It starts with wall area, subtracts standard door and window openings, then applies coats, paint-label coverage, and extra percent before rounding up whole gallons.
Match each input label on the calculator to room length, room width, wall height, doors, windows, coats, coverage in sq ft per gallon, and extra percent.
The formula in plain language
In plain language: The calculator finds wall area as 2 x (length + width) x wall height, subtracts 20 square feet per door and 15 square feet per window, multiplies by coats and extra percent, then divides by square-foot coverage per gallon. The examples on the page are there so you can compare your inputs with a paint gallon example before copying the answer.
The calculator uses 2 x (length + width) x wall height for wall area, subtracts 20 sq ft per door and 15 sq ft per window, multiplies by coats and extra percent, then divides by paint-label coverage.
How to read the answer
Read the whole gallons to buy first, then check paintable wall area and gallons before rounding. The before-rounding number explains whether the result is barely over a gallon or safely into the next can.
- Gallons to buy rounds the calculated need up to whole gallons because most wall paint is sold by whole containers.
- Gallons before rounding shows the math result before the shopping-friendly round-up. A result of 1.898 means the default example rounds to 2 gallons.
- Paintable wall area shows the wall estimate after subtracting doors and windows but before coats, extra percent, and coverage are applied.
- Coverage used reminds you which square-feet-per-gallon assumption drove the result.
Common mistakes to avoid
Most bad paint estimates come from using floor square footage as wall area, forgetting a second coat, trusting a high coverage number on rough walls, or mixing ceilings and trim into the wall estimate.
- Do not use floor square footage as wall square footage.
- Do not forget that two coats roughly doubles the paintable area.
- Do not treat ceilings, trim, doors, cabinets, or primer as automatically included in this wall estimate.
- Check the actual product label because coverage varies by paint, surface, color, sheen, and primer.
- Do not use the highest advertised coverage number on rough texture, patched drywall, bare surfaces, or strong color changes unless you are comfortable buying more later.
Quick 12 x 10 room example
Say the room is 12 ft long, 10 ft wide, and 8 ft high. The wall area is 2 x (12 + 10) x 8, or 352 square feet before openings.
With 1 door and 2 windows, the calculator subtracts 50 square feet, leaving 302 paintable square feet. Two coats and 10% extra make 664.4 adjusted square feet. At 350 sq ft per gallon, that is 1.898 gallons before rounding, so the buying estimate is 2 gallons.
Coverage and coats
Coverage per gallon is not a universal constant. A smooth primed wall may get closer to the high end of the label range, while rough texture, fresh drywall, patches, or a dark color change can use more paint.
Coats multiply the wall area before coverage is applied. If you enter 2 coats, the calculator is assuming each paintable wall area gets painted twice, then it adds your extra percent.
Doors, windows, and what is left out
This calculator subtracts standard openings instead of asking for every door and window measurement. That keeps the estimate fast, but unusual openings, built-ins, half walls, closets, and wainscoting can move the real number.
Ceilings, trim, doors, cabinets, and primer are separate estimates. They often use different paint, finish, coverage, or prep assumptions, so putting everything into one wall number can make the answer look more exact than it is.
When paint math is not wallpaper math
Paint and wallpaper both start with wall area, but they stop being the same calculation after that. Paint uses gallons, coats, and coverage per gallon. Wallpaper uses roll coverage, pattern matching, trimming, and a waste percent before the roll count is rounded up.
If you are covering the wall with paper, vinyl wallcovering, or peel-and-stick wallpaper, switch to the Wallpaper Calculator so the estimate can handle roll coverage and waste instead of pretending gallons and rolls work the same way.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Paint Calculator
302 sq ft paintable, buy 2 gallons
491 sq ft paintable, buy 3 gallons
269 sq ft paintable, buy 1 gallon
366 sq ft paintable, buy 3 gallons
FAQ in plain language
When should I use the Paint Calculator?
Use it when your task matches one of these common needs: Estimate gallons for a bedroom, office, or living room. Adjust for one or two coats before buying paint. It works best when you already know room length, room width, wall height, doors, windows, coats, coverage in square feet per gallon, and extra percent.
What is the Paint Calculator doing with my inputs?
In plain language: The calculator finds wall area as 2 x (length + width) x wall height, subtracts 20 square feet per door and 15 square feet per window, multiplies by coats and extra percent, then divides by square-foot coverage per gallon. The examples on the page are there so you can compare your inputs with a paint gallon example before copying the answer.
What do the main Paint Calculator inputs mean?
Room length, width, and wall height: the rectangular room dimensions used to estimate wall square footage. Doors and windows: standard openings subtracted before coats and extra paint are added. The calculator uses 20 sq ft per door and 15 sq ft per window. Coats: how many full wall coats you plan to apply. Two coats roughly doubles the paintable area before coverage is applied. Coverage per gallon: the square feet one gallon covers for one coat according to the paint label or product page. Extra percent: extra paint for texture, roller and tray loss, touchups, small measurement errors, and a safer shopping estimate.
How should I read the Paint Calculator answer?
Read the headline estimate first, then check the material, waste, coverage, and unit lines. For project tools, the supporting lines are often the difference between a rough idea and a list you can actually shop from.
What should I double-check before trusting the answer?
This is an interior wall buying estimate. Paint coverage still depends on the product label, primer, surface texture, color change, application method, ceiling or trim work, and how much paint remains in the can or tray. Also check whether you are painting ceilings, trim, closets, textured walls, patched drywall, dark color changes, or primer because those can change the final order.
How many gallons of paint do I need for a 12 x 10 room?
With the default 8 foot walls, 1 door, 2 windows, 2 coats, 350 sq ft per gallon coverage, and 10% extra, the calculator estimates 302 paintable sq ft and 1.898 gallons before rounding. Buy about 2 gallons for that example.
Does the Paint Calculator include ceilings and trim?
No. This calculator estimates interior wall paint for a simple rectangular room. Estimate ceilings, baseboards, doors, cabinets, and trim separately because they use different areas, products, finishes, or application methods.
Related tools
- Wallpaper Calculator Figure out how many wallpaper rolls to buy, plus rough material cost when you add a roll price.
- Square Footage Calculator Calculate square feet, square yards, and square meters from length, width, and quantity.
- Drywall Calculator Estimate drywall sheet count from project area, sheet size, and waste percentage.
- Flooring Calculator Estimate flooring boxes, adjusted square feet, coverage ordered, and optional material cost.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Home & Projects Browse the full category for related tools that help with the same job.
- All free tools Search the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guides Find more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resources Start here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.