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. Most quality interior latex paints, including popular brands like Benjamin Moore Regal Select and Sherwin-Williams Duration, advertise coverage between 350 and 400 sq ft per gallon on smooth drywall. Textured walls, bare plaster, or deep colours can reduce that to 250–300 sq ft per gallon.
For example, a 16 × 13 ft room with 8.2-ft ceilings has a gross wall area of about 476 sq ft. Subtract one standard door and two windows (≈53 sq ft) and you get 423 sq ft of paintable surface. Two coats bring the total to 846 sq ft. At 350 sq ft per gallon, you need roughly 2.4 gallons. In practice you would purchase two 1-gallon cans or one 5-gallon bucket.
Coverage Rates by Brand
Benjamin Moore Regal Select: up to 400 sq ft/gal. Sherwin-Williams SuperPaint: 350–400 sq ft/gal. Behr Premium Plus: 250–400 sq ft/gal depending on colour. Always check the product data sheet for the specific paint you plan to use — the label usually lists coverage under "Spread Rate."
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. Application method counts too — rollers use about 10 % more paint than airless sprayers at the same coverage.
Types of Interior Paint
Interior paint in the US comes in two main bases: latex (water-based) and alkyd (oil-based). Latex accounts for over 90 percent of residential paint sales because it dries faster (1–2 hours), has low odor, cleans up with soap and water, and meets strict VOC regulations. Premium latex lines include Benjamin Moore Regal Select, Sherwin-Williams Duration, Behr Marquee (Home Depot exclusive), and PPG Diamond. Alkyd paint is reserved for trim, cabinets, and high-wear surfaces where a hard, smooth finish is desired — Benjamin Moore Advance is a popular water-based alkyd hybrid. Sheen levels range from flat (no shine, hides imperfections — best for ceilings), matte and eggshell (low sheen — most popular for living areas), satin (moderate sheen — kitchens and hallways), semi-gloss (durable and washable — bathrooms and trim), to high-gloss (maximum durability — doors and furniture).
Understanding Paint Coverage
Coverage rate is how many square feet one gallon covers in a single coat. Most quality latex paints cover 350–400 square feet per gallon on smooth, primed surfaces. Textured or porous surfaces (new drywall, stucco, brick) can reduce coverage to 250–300 square feet. Dark colors painted over a light base may require three coats. The calculator uses a default of 350 square feet per gallon, which you can adjust based on your paint's label specification. One gallon of Benjamin Moore Regal covers about 400 square feet; Behr Premium Plus covers about 350–400 square feet; Sherwin-Williams SuperPaint covers about 350 square feet.
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 shows the optimal combination of 1-gallon cans and 5-gallon buckets for the most economical purchase. For example, if you need 3.2 gallons, buying one 5-gallon bucket would be wasteful — three 1-gallon cans plus one quart sample for touch-ups is more practical. 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 (DAP DryDex or 3M Patch Plus Primer), sand smooth with 150-grit sandpaper, and wipe with a tack cloth. Apply primer over bare drywall, repairs, and stains — Zinsser Bulls Eye 1-2-3 and Kilz Original are the top-selling primers at Home Depot. 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.
Cost Estimation
Paint prices at Home Depot and Lowe's range from $25–$35 per gallon for builder-grade (Glidden, Valspar), $35–$55 for mid-range (Behr Premium Plus, Sherwin-Williams SuperPaint), and $55–$80 for premium (Benjamin Moore Regal, Sherwin-Williams Emerald, Behr Dynasty). A typical 12×14-foot bedroom requires 1.5 to 2 gallons for two coats — costing $50–$160 depending on paint quality. Professional painters charge $2–$6 per square foot (including prep and materials), or $400–$900 for that same bedroom. DIY saves the labor cost but requires rollers ($8–$15), a roller frame ($5), brushes ($10–$20), painter's tape ($5–$8 per roll), drop cloths ($10–$20), and a paint tray ($3–$5).
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. Benjamin Moore Regal Select and Sherwin-Williams Emerald are premium flat options. Eggshell is the most popular for living rooms — washable and forgiving. Satin works for hallways and kids rooms. Semi-gloss resists moisture for kitchens and bathrooms. The finish does not change coverage rates significantly.
Coverage Rates
Most quality paints cover 350 to 400 square feet per gallon per coat. Premium one-coat paints like Behr Marquee and Benjamin Moore Aura offer superior hide over similar colors at up to 400 sq ft per gallon. Two coats are almost always recommended for color changes. The calculator defaults to two coats at 350 sq ft per gallon which you can adjust based on your paint specifications.
Wall Preparation
Fill nail holes with spackling compound like DAP DryDex, sand smooth with 120-grit, and wipe with a damp cloth. Apply primer on new drywall, over dark colors, or on stains. Zinsser Bulls Eye 1-2-3 and Kilz Original are excellent primers at Home Depot and Lowes. Tape off trim and ceilings with FrogTape or ScotchBlue. Use a 2.5-inch angled brush to cut in and a 9-inch roller with appropriate nap for your wall texture.
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 in this case.
Paint Brands and Prices
Economy paints like Glidden cost 15 to 25 dollars per gallon and need more coats. Mid-range like Behr Premium Plus and Valspar Signature cost 30 to 40 dollars with good two-coat coverage. Premium like Benjamin Moore Regal and Sherwin-Williams Emerald cost 50 to 75 dollars but deliver superior results. For a bedroom needing 2 gallons, material costs range from 30 dollars to 150 dollars. Professional painting runs 2 to 6 dollars per square foot of wall area.
Low-VOC and Environmental Options
Low-VOC and zero-VOC paints are now the industry standard. Benjamin Moore Natura, Sherwin-Williams Harmony, and Behr Premium Plus meet or exceed EPA standards. For nurseries and schools, look for GREENGUARD Gold certification. Water-based latex has largely replaced oil-based paint for interiors because it dries faster, cleans with water, and produces fewer fumes.
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 — buying one 1-gallon can and one 5-gallon bucket leaves plenty 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. Most interior latex paints cover between 300 and 400 sq ft per gallon. Premium brands like Benjamin Moore Regal or Sherwin-Williams Duration tend toward the higher end. The rate depends on paint quality, surface texture, and application method — rollers use more paint than sprayers.
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. Most latex paints last several years when stored properly.
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