Paint Calculator

Paint is cheap. Running out of paint halfway up the last wall is expensive — not in dollars, in Saturdays. This calculator tells you exactly how much wall paint to buy before you crack open the first can. Enter your room dimensions, subtract the doors and windows, set your coats and the coverage figure printed on the tin, and you get gallons (or litres) with a waste buffer baked in. Why does the number matter? A gallon of decent paint — Behr Premium Plus, Benjamin Moore Regal — runs $45 to $85 at Home Depot or your local paint store. Buy two gallons too many of a custom-mixed color and you cannot return it; that money dries on a garage shelf. Buy too little and the second batch may not match the first — tint machines drift, and you will see the seam every morning for years.

How to use this calculator

Measure the length and width of the room at the longest walls, baseboard to baseboard. Painters paint walls, and walls run the full room — do not measure the rug or the furniture zone.
Measure wall height from the floor to the ceiling. If the ceiling slopes, measure at the tallest and shortest points and use the average, since the calculator takes a single height.
Count every door, including the closet door you will definitely forget. Leave the area per door at 21 sq ft for a standard interior door — the default already covers the slab plus trim.
Count the windows and sanity-check the area per window. The 15 sq ft default fits a typical 3×5 window; if you have a big picture window or a sliding glass door, measure it and enter the real number — guessing here is where most estimates go wrong.
Set the number of coats. Two is the standard for a finish that lasts. Handle primer separately (see the FAQ below) — it covers differently than topcoat, so do not just add it as a third coat here.
Read the coverage figure off YOUR paint tin or its data sheet — not a blog, not this page. Cheap contractor paint can be 250 sq ft per gallon while Behr Marquee claims up to 400. This one number swings your result more than anything else you enter.
Leave the waste allowance at 10% unless you are spraying or painting heavy texture. It covers roller absorption, cut-in waste, the drip you swear will not happen, and a little left over for touch-ups.

Formula

The math is straightforward: wall area equals 2 × (length + width) × height — the room's perimeter times its height. Subtract the doors and windows to get paintable area, multiply by the number of coats, divide by the coverage figure from your tin, then add the waste percentage. In one line: gallons = (wall area − openings) × coats ÷ coverage × (1 + waste%). The calculator runs the whole thing in metric internally and converts at the end, so mixing feet and litres across the inputs still gives the right answer.

Worked example

Take a typical primary bedroom: 14 × 12 ft with 9 ft ceilings, two doors (bedroom plus closet), and three windows. Step one — wall area: 2 × (14 + 12) × 9 = 468 sq ft of wall. Step two — subtract the openings: two doors at 21 sq ft each is 42, three windows at 15 sq ft each is 45, for 87 sq ft total. 468 − 87 = 381 sq ft of paintable wall. Step three — two coats means covering 381 × 2 = 762 sq ft. Step four — divide by coverage. The tin says 350 sq ft per gallon: 762 ÷ 350 = 2.18 gallons. Step five — add 10% waste for the roller, the cut-ins, and the inevitable drip: 2.18 × 1.10 = 2.4 gallons. At the store that means two gallon cans plus two quarts — or, simpler, three gallon cans, since paint is cheaper by the gallon and the leftover lives on a labeled shelf for touch-ups. At roughly $45 a gallon for Behr Premium Plus at Home Depot, the paint for this room costs about $135. Skip the waste allowance and you would walk out with 2.2 gallons, run dry on the last wall, and spend your Saturday driving back for a quart that might not match.

Measurement tips

Read the coverage off the actual tin you are buying. Behr Marquee, Sherwin-Williams Emerald, and the $22 contractor special all cover differently — sometimes by 150 sq ft per gallon.
Going from dark to light? A $28 gallon of Kilz or Zinsser primer beats a third $60 coat of topcoat every time. Ask the paint counter to tint the primer toward your finish color — they do it free.
Buy all your paint in one trip and have every can shaken at once. Tint machines drift between batches, and a slightly-off gallon shows up as a visible band where one wall meets the next.
Box your gallons when a room needs more than two — pour them into a 5-gallon bucket and mix. Pros do this so minor tint differences disappear entirely.
Cut in the edges with a brush before rolling each wall, and keep a wet edge. Letting the cut-in dry first is how you get the picture-frame effect around every wall.
Use a 3/8-inch nap roller for smooth drywall and 1/2-inch or thicker for textured walls. The wrong nap either starves the wall or stipples it.
Save a pint from the final coat, write the room, color code, and date on the lid with a Sharpie, and store it indoors. Touch-up paint kept in a freezing garage separates and never quite matches.

Common mistakes

Planning one coat for a color change. One coat covers; two coats finish. A single coat over a dark color looks streaky in daylight, and you will buy the second round of paint anyway — plus lose a weekend.
Using one paint's coverage number for a different paint. The 400 sq ft per gallon on the premium tin does not apply to the budget gallon you actually bought. That is a full gallon of error on a bedroom.
Forgetting the closet. Walk-in closets add 100+ sq ft of wall. Measure it or skip painting it — but decide before you buy, not when the roller goes dry.
Measuring the room once, with a sagging tape. On a 14-ft wall, a drooping tape reads long by a few inches. Pull it taut or use a $30 laser measure — it pays for itself on the first project.
Skipping the waste allowance to “save money.” Ten percent on a bedroom is a quarter of a gallon — about $11. Running out costs a second trip and a possible color mismatch. Bad trade.
Counting primer as one of your two coats in the calculator. Primer and topcoat cover differently; calculate primer separately and enter only finish coats in the coats field.

Factors that affect accuracy

Surface porosity. Fresh, unprimed drywall drinks paint — the first coat can use 20–30% more than the tin's coverage claims. Previously painted walls in good shape behave; bare drywall, plaster, and patched areas do not.
Color change severity. A same-color refresh might genuinely take one coat. Dark navy to warm white is three coats, or a tinted primer plus two — the calculator cannot see your colors, so set the coats field honestly.
Application method. Spraying is fast, but 10–20% of the paint becomes overspray that never touches the wall. Brushes lay it on thick; rollers lose paint into the nap and the tray.
Paint quality and solids. Higher-solids paints — the expensive tins — genuinely cover more per gallon. The coverage number on the tin already reflects this, which is why reading YOUR tin matters.
Texture. Orange-peel and knockdown texture add real surface area — a heavily textured wall can need 10–15% more paint than a smooth one of the same dimensions. Popcorn ceilings are worse, which is one more reason people finally scrape them.
Temperature and humidity. Paint laid in a hot, dry room flashes off fast, forcing you to reload the roller more often and overlap more — you will use more than the math says. Aim for 50–85°F and moderate humidity.

Please note: material estimates are approximate. Always confirm product coverage with the manufacturer and buy a little extra for cuts, waste and future repairs.

Frequently asked questions

How much paint do I need for a 12×12 bedroom with 8 ft ceilings?

Walls only: 2 × (12 + 12) × 8 = 384 sq ft of wall. Subtract one door (21) and two windows (30) to get 333 sq ft paintable. Two coats at 350 sq ft per gallon is 666 ÷ 350 = 1.90 gallons, plus 10% waste = about 2.1 gallons. Buy two gallons and a quart, or three gallons if you would rather have touch-up paint on hand.

Should I include the ceiling in this calculation?

No — ceilings get their own math and their own paint. Ceiling area is just length × width (144 sq ft for a 12×12 room), and ceiling paint is a flat, spatter-resistant formula sold separately for around $25–$30 a gallon. It also covers differently, roughly 350–400 sq ft per gallon. Run this calculator for the walls, then do the ceiling as its own quick calculation.

How do I account for primer?

Calculate it separately. Primer covers about 200–300 sq ft per gallon on bare drywall, and it costs $25–$35 a gallon versus $45–$85 for topcoat. So on fresh drywall you would buy a gallon of Kilz PVA primer for that 14×12 bedroom, then enter 2 in the coats field here for the finish coats. Do not enter 3 coats in this calculator and call one of them primer — the coverage math is different.

How many square feet does a gallon of paint cover?

Between 250 and 400, depending on the paint. Budget contractor paint sits around 250–300; mid-range like Behr Premium Plus claims about 400 on smooth, primed walls; Sherwin-Williams Emerald lands around 350–400. The honest answer is printed on your tin — and it is measured on smooth, sealed surfaces, so knock 10–15% off for texture or bare drywall.

I am painting over dark red walls. How many coats?

Plan on a tinted primer plus two finish coats, minimum. Deep reds and navies bleed through white like a rumor — each coat looks like it is working until it dries and the pink ghosts back. A gray-tinted primer (ask the paint counter; it is free) kills the color in one coat and lets your two topcoats actually finish the job. Enter 2 in the coats field and budget the primer separately.

Can I return paint I did not use?

Unopened, untinted cans: yes, Home Depot and Lowe’s take them back within 90 days with a receipt. Custom-tinted paint: no — once they squirt the colorant in, it is yours forever. That is the real cost of overbuying a custom color, and it is why the waste allowance here is 10%, not 25% — a little extra for touch-ups, not a spare gallon you will never open.

Why does the calculator ask for door and window sizes instead of just the count?

Because a 6-foot sliding glass door and a small bathroom window do not subtract the same wall. Counting “3 openings” treats them equally; measuring them does not. The defaults (21 sq ft per door, 15 per window) are honest averages for standard sizes — adjust them when your openings clearly are not standard, like a wall of windows or double doors.