Paint Calculator
Calculate how much paint you need for any room; just enter your wall dimensions, doors, and windows.
Room Dimensions
Enter your room dimensions and click Calculate.
How Does the Formula Work?
Estimating paint is straightforward once you understand the formula. The calculator computes the total wall area by adding all four walls: 2 × (length + width) × height. It then subtracts standard openings; about 21 sq ft per door and 16 sq ft per window; to produce the net paintable area. Multiply that number by the coat count and you get the total surface the paint must cover.
Finally, divide the total paint area by the coverage rate; the number of square feet one gallon covers. Sherwin-Williams puts a typical gallon at about 350 to 400 sq ft, and the calculator starts at 350; if your can's label gives a different figure, enter that one instead.
For example, a 16 × 13 ft room with 8-ft ceilings has a gross wall area of 464 sq ft. Subtract one standard door and two windows (53 sq ft) and you get 411 sq ft of paintable surface. Two coats bring the total to 822 sq ft. At 350 sq ft per gallon, you need roughly 2.3 gallons. In practice the calculator suggests two 1-gallon cans and two quarts, 2.5 gallons in all, which covers the job with a little left for touch-ups.
Variables That Affect Paint Usage
Surface porosity plays a major role. New drywall and fresh plaster are more absorbent and may require a dedicated primer coat before two finish coats. Colour change also matters: painting a dark wall with a light colour (or vice versa) often needs an extra coat.
Understanding Paint Coverage
Coverage rate is how many square feet one gallon covers in a single coat. Textured or porous surfaces (new drywall, stucco, brick) take more paint than smooth, primed walls. The calculator uses a default of 350 square feet per gallon, which you can adjust based on your paint's label specification.
How the Calculator Works
The calculator computes total wall area (room perimeter × wall height), subtracts door and window openings, multiplies by the number of coats, and divides by the coverage rate to give you total gallons needed. It then picks the mix of 1-gallon cans and quarts that leaves the least paint over; when two mixes leave the same amount, it shows the one with fewer containers. For 3.2 gallons that is three 1-gallon cans and one quart, 3.25 gallons in all. The calculator rounds up to whole container units because paint cannot be purchased in fractional quantities.
Surface Preparation
Paint adhesion and finish quality depend heavily on surface preparation. Wash walls with TSP (trisodium phosphate) solution to remove grease, nicotine, and grime. Fill holes and cracks with lightweight spackling compound, sand smooth with 150-grit sandpaper, and wipe with a tack cloth. Apply primer over bare drywall, repairs, and stains. If painting over a dark color, use a tinted primer that matches your new color family to improve coverage and reduce the number of topcoats needed.
Types of Interior Paint
Interior paint comes in flat, eggshell, satin, and semi-gloss finishes. Flat hides imperfections best and suits ceilings and low-traffic rooms. Eggshell suits living rooms; it is washable and forgiving. Satin works for hallways and kids rooms. Semi-gloss resists moisture for kitchens and bathrooms.
How Much Paint Do You Need
This calculator computes net paintable area by multiplying room perimeter by wall height then subtracting doors and windows. For a 12 by 14 foot room with 8-foot ceilings, one door and two windows: wall area equals 416 sq ft minus 53 sq ft openings equals 363 net sq ft. At 350 sq ft per gallon with two coats that is 2.07 gallons. The calculator rounds up and suggests can sizes; two one-gallon cans and one quart in this case, 2.25 gallons in all.
Tips & Recommendations
Always add a 10 % buffer to your calculated amount for touch-ups, colour matching, and small mistakes. Store leftover paint with the lid sealed for future repairs.
New drywall, bare wood, and patched areas absorb paint unevenly. A coat of primer seals the surface, reduces the total paint needed, and gives a more uniform finish.
Covering a dark colour with a lighter one usually requires three coats or a tinted primer. Budget paint and time accordingly to avoid a patchy result.
Exterior paints have different coverage rates and are formulated for UV and moisture resistance. This calculator is designed for interior walls; use a dedicated exterior tool for outdoor projects.
Frequently Asked Questions
How much paint do I need for a 12×12 foot room?
A standard 12×12 ft room with 8-foot ceilings has roughly 384 sq ft of wall area. After subtracting one door and two windows (about 53 sq ft), you are left with approximately 331 sq ft. At a typical coverage rate of 350 sq ft per gallon, one gallon covers a single coat. For two coats you need about 2 gallons, so two 1-gallon cans cover it with a little left for touch-ups.
Do I really need two coats of paint?
In most cases, yes. A single coat often leaves streaks, shows roller marks, and does not provide full coverage over the primer or existing colour. Two coats deliver a uniform finish, better durability, and true colour accuracy. If you are painting a very similar shade over the old colour, you may get away with one heavy coat, but two thinner coats usually give a superior result.
Does the calculator deduct doors and windows?
Yes. The calculator subtracts a standard door area (about 21 sq ft) for each door and a standard window area (about 16 sq ft) for each window you specify. If your doors or windows are unusually large or small, you can adjust the coverage input to compensate. This deduction gives a realistic net wall area so you do not over-buy.
What does paint coverage rate mean?
Coverage rate tells you how many square feet one gallon of paint can cover in a single coat. Sherwin-Williams puts a typical gallon at about 350 to 400 sq ft. The rate depends on paint quality, surface texture, and application method, so check the label of the paint you buy.
Should I buy extra paint beyond the calculated amount?
We recommend buying 5–10 % more than the calculated quantity. Extra paint is useful for future touch-ups, colour matching (batches vary slightly), and covering unexpected repairs. Store leftover paint in a cool, dry place with the lid sealed tightly.
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