Paint Calculator
Four steps stand between a room's dimensions and the number of cans in your trolley: wall area, deductions for doors and windows, the number of coats, and the coverage your product actually delivers. This calculator walks through all four and shows the arithmetic at each stage, so you can see where the number came from instead of trusting a single figure.
Wall area comes from the perimeter times the height: (length + width) × 2 × height.
Rules of thumb: a standard door is about 20 sq ft (1.9 m²) and an average window about 15 sq ft (1.4 m²). Measure yours if they are unusual.
Typical range is 350–400 sq ft per gallon.
Around 10% covers touch-ups and spills.
Your paint estimate will appear here
Enter your room size, subtract the doors and windows, pick your coats, then calculate.
Quick Answer
Wall area is the room perimeter times the ceiling height. Subtract about 20 sq ft per door and 15 per window, multiply what remains by the number of coats, then divide by the coverage per gallon and round up. Interior wall paint covers roughly 350 to 400 sq ft per gallon and primer 200 to 300. A 10 by 12 room with 8 ft ceilings needs 352 sq ft of wall covered twice, which is 704 sq ft, or 2 gallons at 400 sq ft per gallon.
How It Works: Formula & Variables
Step 1: Wall surface area
Wall Area = perimeter × ceiling height = (length + width) × 2 × height
You can also measure each wall on its own and add them together, which is the better approach when the room is not a plain rectangle.
Step 2: Subtract the openings
Net Area = Wall Area − doors − windows
Standard door around 20 sq ft, average window around 15 sq ft. Measure yours if they are far off those.
Step 3: Multiply by the coats
Adjusted Area = Net Area × number of coats
Step 4: Divide by coverage
Gallons Needed = Adjusted Area ÷ coverage per gallon, rounded up
- Wall paint
- About 350 to 400 sq ft per gallon.
- Primer
- About 200 to 300 sq ft per gallon.
- Texture penalty
- Rough or textured surfaces cut coverage by roughly 15% to 25%.
- Metric
- 1 US gallon is 3.785 liters, and coverage lands around 33 to 37 m² per gallon.
Worked Examples
Example 1: The bare division
You have 1,000 sq ft to cover and a product that spreads 400 sq ft per gallon. 1,000 ÷ 400 = 2.5 gallons. In the shop that becomes 3 cans, or 2 gallons plus 2 quarts if the product comes in quart sizes and you would rather not open a third gallon.
Example 2: A 10 by 12 bedroom, two coats
With 8 ft ceilings the perimeter is (10 + 12) × 2 = 44 ft, so the wall area is 44 × 8 = 352 sq ft. Two coats doubles that to 704 sq ft. At 400 sq ft per gallon: 704 ÷ 400 = 1.76, which rounds up to 2 gallons. No deductions taken here, which leaves a little in hand for touch-ups later.
Example 3: A 12 by 14 room with openings subtracted
Perimeter is (12 + 14) × 2 = 52 ft, and at 8 ft high the wall area is 52 × 8 = 416 sq ft. Take off one door at 21 sq ft and two windows at 15 each, so 51 sq ft in total, leaving about 365 sq ft of paintable wall. Two coats brings that to 730 sq ft. Using the conservative end of the coverage range: 730 ÷ 350 = 2.09, which rounds up to 3 gallons. Only just over the 2 gallon mark, but 2 cans would leave you short by roughly a third of a wall.
Key Concepts
Coverage is a lab figure, not a guarantee: The 350 to 400 sq ft on the can comes from a controlled test on a prepared surface. Real walls have patches, porosity, and texture, and a roller loaded by a real person puts down a thicker film than a machine does. Build in a margin.
Coats multiply the area, not the cans: Two coats means painting the same 350 sq ft twice, so you divide 700 by the coverage rather than dividing 350 and doubling. The result is the same here, but keeping the multiplication in the area step avoids errors once deductions and buffers enter the picture.
Primer is a separate calculation: If you are priming first, run the numbers twice: once for primer at its lower coverage, once for the topcoat. Assuming one gallon of each will do the same job is how people end up back at the shop.
Texture changes the real surface area: A rough wall has hills and valleys, and paint has to reach all of them. The flat measurement underestimates how much surface is really there, which is why coverage drops on stucco, popcorn ceilings, and bare masonry.
Round up, then keep the leftovers: Paint stores well in a sealed can, and a matching touch-up later is worth more than the few dollars you would save buying tight. Colors mixed at different times can differ slightly, so having the original batch on hand matters.
Common Mistakes
Treating 350 to 400 sq ft as guaranteed: That range is the ideal-conditions number. Porous surfaces, textured walls, and heavy-handed rolling all pull the real figure down.
Forgetting the second coat: The most expensive mistake on this list, because it halves your estimate. Double the area before dividing by coverage whenever you plan two coats.
Rounding down: 1.9 gallons means 2 cans, not 1. Round up every time and add roughly 10% on top for touch-ups, spills, and the bit that stays in the tray.
Skipping the deductions on a glassy room: A sunroom with five windows loses 75 sq ft of paintable wall. On a small room that can be a whole gallon of unnecessary paint.
Using wall paint coverage for primer: Primer covers noticeably less ground. Calculate it separately at 200 to 300 sq ft per gallon.
Frequently Asked Questions
Around 350 to 400 square feet for interior wall paint. That figure comes from the manufacturer and assumes a smooth, previously painted surface and a normal application. A wall that has never been painted, or one with any texture on it, will drink more than the label suggests, so treat 400 as a best case rather than a promise.
Roughly 200 to 300 square feet, noticeably less than finish paint. Primer is built to soak into the surface and grip it, which is exactly why it does not spread as far. Budget for more cans of primer than you would of topcoat when you are priming bare drywall, patched repairs, or raw wood.
Work out the wall area, subtract the doors and windows, multiply what is left by the number of coats, then divide by the coverage per gallon and round up. Wall area is the room perimeter times the ceiling height, so a 10 by 12 foot room with 8 foot ceilings has (10 + 12) × 2 × 8 = 352 square feet before deductions.
Yes, on any room where they add up to a meaningful chunk of the wall. Standard rules of thumb are about 20 square feet for a door and 15 for an average window. Two doors and three windows come to roughly 85 square feet, which is a quarter of a gallon at two coats. Skip the deduction on a room with one small window and you will barely notice; skip it on a room with a wall of glass and you will overbuy badly.
Two is the normal answer for a wall in reasonable condition. Go to three when you are covering a dark color with a light one, painting over a strong accent wall, or working on a surface that has never been painted. One coat only works when you are refreshing the same color on a wall that is already in good shape.
Quite a bit, yes. A textured surface has more actual surface area than its flat measurements suggest, so the same gallon spreads across less wall. Light texture like orange peel or knockdown costs you around 15%, and rough finishes such as stucco, popcorn ceilings, or bare masonry can take 25% or more. The calculator applies that reduction for you once you pick a surface type.
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