Paint needed (gallons) = paintable wall area × coats ÷ coverage per gallon. Paintable area is the wall area minus doors and windows. Coverage is usually 350 to 400 square feet per gallon for one coat. A typical 12×12 room with 8-foot walls and two coats needs a little under two gallons.
Paint is bought in cans, not in gallons
Most paint calculators online end with a number like "you need 12.4 liters" or "you need 3.3 gallons" and leave you standing in the aisle. The store doesn't sell 3.3 gallons. It sells a five gallon bucket, a one gallon can and a quart, and what you came to find out is which of those go in the cart.
That is a small optimization problem, and this tool treats it seriously. The cheapest combination isn't always the one with the least left over, and you almost never get it by dividing. Say you need 15.2 liters. Five 3.6 liter cans give you 18 liters for the price of five cans. One 18 liter container gives you the same 18 liters for the price of one, and in most stores that is a third less money for the same paint and the same leftovers. Divide-and-round-up never finds this. Searching the combinations finds it every time.
It cuts the other way too. If you need 4.2 gallons and the bucket is priced badly, four gallon cans plus a quart can beat it. The break-even is the bucket price divided by the single-can price: at $155 against $38 the bucket wins from about 4.1 gallons up. Below that, you are paying for paint that will skin over in the garage. The results panel prints what a single-size purchase would have cost next to the recommendation, so you can check the saving instead of taking it on faith.
Paint coverage belongs to the surface
The coverage printed on the can is true, for a lab. It is measured on a smooth, sealed test panel with a calibrated roller, no cutting in and no touch-ups. Treat it as the best case.
Your wall sits somewhere else on the scale, and the spread is huge. A smooth interior wall that was painted before really does hit the can figure: 350 to 400 sq ft per gallon per coat is the honest range for a quality flat latex. Bare new plaster or stucco gets barely half of it. Raw block or brick, with its open pores and its larger true surface, can get 45%. Texture and heavy coatings are the same trap in another form. Your tape measure reports a flat projection, and the roller has to wet a corrugated surface.
On these surfaces, a calculator with one coverage constant is off by a factor, far beyond a rounding error. So the surface is a main input here, and the results panel shows the effective coverage next to the nominal one. When the tool says 400 sq ft per gallon became 220, you know exactly where the extra money went.
The first coat of paint behaves differently
On anything porous, the substrate soaks up part of the first coat before it can form a film on top. The paint is fine; it is doing what the surface asked for. It does mean the classic formula, area times coats divided by coverage, treats the first coat and the third as identical, and they aren't.
So the first coat gets its own coverage figure, set by the surface. On bare drywall it lands around 72% of nominal, on new plaster around 70%, and on primed and sanded work it goes back to 100%. On a smooth wall that was painted before, the correction disappears and the math falls back to the simple formula everyone knows. That is how it should be. A model should only get complicated where the job does.
The color change sets the coats, and the coats set the paint order
Coats multiply the whole order. Going from two to three is 50 percent more paint, more labor and more days. No other field on this page moves the total as much, and no other field gets guessed as casually.
- Same color, refreshing tired paint: two coats.
- Light over light, or dark over a similar dark: two coats.
- Dark over light: three, because the new color has to build depth.
- Light over dark: four, or three over a gray-tinted primer.
- Strong red, yellow or orange: four in either direction. Those pigments have poor natural opacity, and careful rolling can't fix chemistry. A base tinted toward the final color is worth more here than any discount.
The selector fills the coats field, and you can still change it. If you have already looked at the wall under a work light, your number beats the table.
Doors and windows come out at full size, the reverse of a drywall estimate
Every opening is deducted here at the width and height you measured, with no threshold and no minimum. That is worth saying plainly, because drywall follows the opposite rule: a small window is cut out of the middle of a full sheet, and deducting it makes you buy too little board.
Paint has no sheet size. It is a liquid with continuous coverage, so 15 sq ft of glass is 15 sq ft you don't paint, whether it is one patio door or three small windows. The real mistake is deducting a flat ten or fifteen percent "for openings". That is a guess standing in for a measurement you already have.
Two honest caveats. Reveals, the returns around a deep window or door, are extra surface the deduction doesn't add back, and on a thick masonry wall that is a real but small amount. And door frames, baseboards and casing almost always take a different paint in a different sheen, bought separately in quarts.
Ceilings are a separate job with a separate paint
Don't count the ceiling as more wall. It usually gets a dead flat white, picked for hiding power over washability. It takes fewer coats, and it covers slightly less per gallon than a good wall paint because the flatness comes from extra filler.
Lumping the two into one area and one product is the most common shortcut in online paint estimators. The order comes out wrong in both directions at once: too much of the expensive wall paint and none of the cheap ceiling paint. So the ceiling here has its own area (the floor area of each room), its own product, its own coverage and its own coat count. It gets its own row in the shopping list and its own can optimization.
There is a practical reason too: ceilings get painted first. Treat them as one purchase and you find out at the worst moment that the single bucket you bought has to cover both.
Primer on new work pays for itself
Prime new plaster, bare board, patched repairs and raw masonry every time. Primer is the layer that stops the substrate from soaking up your finish paint, and on these surfaces it is mandatory.
The math is unusually favorable, which is why it's worth showing. Priming new stucco lifts the effective coverage of the finish coats from around 55% of nominal to around 90%, and lifts the first coat from 70% to 95%. Primer costs less per liter than finish paint, and it saves more liters of finish paint than it uses itself. Tick the box in the tool with a porous surface selected and watch both the liters and the total drop. The model didn't invent that. It is why painters prime.
Filler and skim coats are another matter, and the tool is deliberately more conservative about them. How much surfacing compound a wall swallows depends on how far out of true it is, and no table knows that. The field is editable, and it is the shakiest number in the tool. Treat the default as a placeholder until you have skimmed a square meter and measured what it took.
Paint sheen changes more than the look
People pick flat, eggshell, satin or semi-gloss on looks alone, but each one also moves the estimate and the upkeep.
Flat hides surface defects best and touches up almost invisibly, which matters on ceilings and old walls. It is also the worst at surviving a wet cloth. Eggshell and satin are the working compromise for living areas, and they cover slightly more per gallon because they carry less filler. Semi-gloss belongs on trim, kitchens and bathrooms. You can scrub it, and in exchange it shows every roller mark and every wave in the wall under it. Gloss on a badly prepped wall is the most expensive way to advertise the prep you skipped.
What this paint estimate does not cover
It sizes wall paint, ceiling paint, primer and filler, and it prices them by the container. It doesn't size trim paint, caulk, masking tape, drop cloths, sandpaper, rollers, brushes or the tray. On a small job those add up to a real share of the bill.
It won't tell you whether the existing surface needs washing, whether a glossy old finish needs dulling before anything will stick, or whether the damp patch in the corner has to be fixed before you open a single can. It doesn't check whether an exterior product is rated for your exposure. The coverage figures here are purchasing estimates. They are not manufacturer specifications, so check them against the can before you commit to a pallet, and when they disagree, go with the can.
For the prep and the brush work that come before and after the numbers, see the step-by-step guides on how to paint a room and how to paint kitchen cabinets.
Frequently asked questions
How many gallons of paint do I need for a room?
Measure the perimeter of the room, multiply by the ceiling height, subtract every door and window at its real size, then divide by the coverage and multiply by the coats. A 12 by 14 ft room with an 8 ft ceiling has a 52 ft perimeter and 416 sq ft of gross wall. Take out one door and two windows and you are at about 374 sq ft. Two coats at 400 sq ft per gallon is 1.87 gallons, so buy two one-gallon cans. Nobody sells 1.87 gallons.
Is a five gallon bucket cheaper than five one-gallon cans?
Per gallon, almost always. For your job, not always. At $155 a bucket against $38 a gallon, the bucket pays off from about 4.1 gallons up. Below that you are paying for paint you will never open. The calculator skips the rule of thumb here: it searches every combination of bucket, gallon and quart that covers what you need and reports the cheapest one. Then it shows what buying a single size would have cost, so you can check the difference yourself.
Why does the first coat use more paint than the second?
On a porous surface the first coat is partly absorbed before it can form a film. Bare drywall, new plaster, patched repairs and raw block all soak up the first coat, so its effective coverage can drop to 70% of the figure on the can, while later coats behave normally. That is why this tool prices the first coat on its own, and why primer changes the math: the primer takes the absorption hit so the finish paint doesn't.
How many coats do I need to go from a dark color to white?
Usually four, sometimes three if you put down a gray-tinted primer first. Hiding power sets the count here, and durability has nothing to do with it: white over a deep color keeps letting the old shade show through, and every extra coat multiplies the whole paint order. Dark over light normally needs three. Strong reds, yellows and oranges need four in either direction, because those pigments have poor natural opacity. A base tinted toward the final color saves more than any discount will.
Should I subtract doors and windows from the paint estimate?
Yes, all of them, at their real measured size. Drywall works the other way: a small window is cut out of a full sheet, and deducting it makes you buy too little board. Paint is a liquid with continuous coverage, so a 15 sq ft opening saves exactly 15 sq ft of painting. What you should skip is the flat 10% deduction for openings. That is a guess, and you already have a tape measure in your hand.
Are my numbers sent anywhere?
No. Everything is arithmetic running in your browser. Nothing is uploaded or stored, and there is no account. The prices you type are your supplier's prices, and they stay on your machine.