Paint Coverage Calculator
Estimate how many gallons of paint you need for any room.
Reviewed by the ToolNestr Editorial Team — July 2026
How the paint coverage calculator works
The calculator estimates the total paintable surface area of any rectangular room and divides it by the typical coverage of a gallon of paint (350 sq ft). It accounts for the number of coats you plan to apply, subtracts the area of doors and windows, and adds 10% extra for waste, roller absorption, and touch-ups.
To get started, measure your room's length, width, and height in feet. Count the number of doors and standard-sized windows. Decide whether you are painting the ceiling and trim, then choose how many coats you need. Click "Calculate" and the tool shows your wall area, ceiling area, total adjusted area, and the exact gallons required per coat and for all coats combined.
Coverage rates and application guidance follow PDCA (Painting and Decorating Contractors of America) industry standards and typical manufacturer spread-rate labeling of 350-400 sq ft per gallon.
Measure the room
Use a tape measure to get the length, width and ceiling height in feet. Count doors and windows.
Enter your values
Type the dimensions, choose surfaces, pick the number of coats, and click Calculate.
Buy the right amount
Take the gallon total to the store — you will know exactly how many cans to buy without guessing.
The formula explained
The paint calculation follows a straightforward set of steps. Each one converts a room measurement into the number of square feet that paint must cover:
Wall area
Multiply the perimeter by the ceiling height: 2 × (L + W) × H. For a 12′ × 10′ room with 8′ ceilings, that is 2 × (12 + 10) × 8 = 352 sq ft.
Ceiling area
Length × width: L × W. The same 12′ × 10′ room has a ceiling of 12 × 10 = 120 sq ft.
Door & window deductions
A standard interior door covers about 20 sq ft (roughly 3′ × 6.67′). A standard window covers about 15 sq ft. Subtract doors × 20 and windows × 15 from the total area. For 1 door and 1 window: -20 - 15 = -35 sq ft.
Coats & waste
Multiply the net area by the number of coats (1, 2 or 3), then multiply by 1.1 to add 10% for waste. With 2 coats: net area × 2 × 1.1 = final square footage.
Gallons needed
Divide the final square footage by 350 (the typical coverage of one gallon) and round up to the nearest whole number. This ensures you always have enough paint.
Worked example
A 12′ × 14′ bedroom with 8′ ceilings, 1 door, 2 windows, painting walls and ceiling with 2 coats.
Use cases for the paint calculator
Home improvement
Planning to repaint a bedroom, living room, or entire floor? Measure each room and add up the gallons needed. This calculator prevents you from buying too much (wasted money) or too little (extra trips to the store).
Interior design
When designing a color scheme for a space, knowing the exact paint quantity helps budget accurately. Factor in accent walls, ceiling color changes, and trim paint for a complete room transformation.
Contractor estimates
Professional painters can use this tool to generate quick material estimates for client proposals. Include labor, primer, and supplies on top of the paint gallons to build a complete quote from one quick calculation.
New construction
Builders and developers painting multiple units can batch-calculate paint needs room by room. The uniform ceiling heights and standard window sizes in new builds make the estimates especially accurate.
Tips for buying paint
Buy extra for touch-ups
Even with the 10% waste buffer built into the calculator, it is smart to buy one extra quart of each color for future touch-ups. Store it in a cool, dry place with the lid sealed and the can labeled with the room and color name. Minor scuffs and marks happen — having matching paint on hand saves a frustrating color-matching trip later.
Primer adds a coat
If you are painting over a dark color, bare drywall, or a porous surface like fresh spackle, you need a coat of primer first. Primer covers about the same area as paint (350-400 sq ft per gallon), so count it as an extra coat in the calculator. Many paints now include primer, but dedicated primer is still better for blocking stains and ensuring even coverage.
Textured walls need more paint
Popcorn ceilings, knockdown texture, orange peel walls, and brick all absorb significantly more paint than smooth drywall. Expect to use 20-30% more paint for heavily textured surfaces. You can adjust for this by increasing the waste factor mentally — for example, choose a higher coverage estimate or simply add 20% to the gallon total the calculator gives you.
Sheen matters for coverage
Flat and matte paints tend to hide imperfections well and apply evenly. Higher sheens (eggshell, satin, semi-gloss, gloss) may require more careful application but are easier to clean. The coverage per gallon is roughly the same across sheens from the same brand, so the calculator works for any finish.
When to use the paint coverage calculator
This calculator is most useful before any painting project — large or small. For a single bedroom, it tells you whether one gallon is enough or if you need two. For a whole-house repaint, you can run each room separately and add up the totals to place a single accurate order at the paint store. The calculator also helps you compare costs between different paint brands and quality tiers: once you know you need 4 gallons, you can multiply by the per-gallon price of each option to see the true material cost of your project.
Renters painting a room back to its original color before moving out can use the calculator to buy exactly what they need. DIYers planning a weekend project can avoid the frustration of running out of paint halfway through the second coat. Even experienced painters benefit from a quick check — mental math mistakes are easy to make when you are estimating multiple rooms, and a single gallon of premium paint can cost $50 or more, so accuracy matters.
The paint calculator is also helpful when deciding between paint and wallpaper, or when budgeting for a full interior renovation. By converting room dimensions into a precise gallon count, you remove one of the biggest uncertainties in any painting project. Combine it with our other home improvement tools to plan your entire renovation budget from start to finish.
Real-world context
A large living room is a common case that pushes past a single gallon. A 20 ft by 15 ft room with 9-foot ceilings, one door, three windows, ceiling included, and 2 coats works out to walls of 2 × (20+15) × 9 = 630 sq ft plus a 300 sq ft ceiling, minus 65 sq ft of deductions, for a net area of 865 sq ft. Multiplied by 2 coats and 1.1 for waste, that is 1,903 sq ft — six gallons at the standard 350 sq ft spread rate, which is why open-plan living rooms so often need a full case of paint rather than the two cans a smaller bedroom would use.
A small powder room sits at the opposite end. A 6 ft by 8 ft bathroom with 8-foot ceilings, one door, one window, ceiling included, and 2 coats needs only 224 sq ft of wall plus 48 sq ft of ceiling, minus 35 sq ft of deductions, for a net area of 237 sq ft. With coats and waste factored in that is 521 sq ft — just 2 gallons, which is also why a single quart sample is rarely enough to finish even the smallest bathroom on its own.
Great rooms with vaulted or skipped ceiling coverage show how skipping the ceiling changes the math. A 24 ft by 18 ft open room with 10-foot walls, two doors, four windows, no ceiling paint, and a single coat comes out to 840 sq ft of wall minus 100 sq ft of deductions for a net area of 740 sq ft, or 814 sq ft after the 10% waste allowance — 3 gallons for one coat, a useful reference when a homeowner is only refreshing walls and leaving a textured or recently-painted ceiling untouched.
Common misconceptions
"One gallon always covers 400 square feet, so I can just divide my total square footage by 400." Coverage varies by surface. Smooth, previously painted drywall gets close to 400 sq ft per gallon, but new drywall, textured surfaces, and darker base colors absorb more paint and can drop coverage to 250-300 sq ft per gallon — which is why the calculator's 350 sq ft default is a middle-ground estimate, not a guarantee.
"A gallon of premium paint and a gallon of budget paint cover the same area, so price is the only difference." Not necessarily. Higher-quality paints generally carry more solids and pigment per gallon, which often means better hide in fewer coats — a premium paint that lets you skip a third coat on a dark-to-light color change can end up cheaper overall than a budget paint that needs an extra pass.
Related calculators
Frequently asked questions
How much area does a gallon cover?
One gallon typically covers 350-400 sq ft for a single coat.
How many coats do I need?
Two coats are recommended for most colors. Dark-to-light or light-to-dark may need 3.
Should I include windows and doors?
Yes, but we subtract their area — standard door = 20 sq ft, standard window = 15 sq ft.
Does the paint calculator account for waste?
Yes — an extra 10% is added for waste, touch-ups, and roller loss.