Paint is bought in cans, not in gallons
Every paint calculator on the internet finishes with a number like "you need 12.4 litres" or "you need 3.3 gallons", and then leaves you standing in the aisle. The shop does not sell 3.3 gallons. It sells a five gallon bucket, a one gallon can and a quart, and the question you actually came to answer is which of those to put in the trolley.
That is a small optimisation problem, and it is the part this tool takes seriously. The combination that costs least is not always the one with the least surplus, and it is almost never the one you get by dividing. Say you need 15.2 litres. Five 3.6 litre cans give you 18 litres for the price of five cans. One 18 litre tin gives you the same 18 litres for the price of one, and in most shops that is a third less money for identical paint and identical 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 simply the bucket price divided by the single-can price: at $155 against $38 the bucket wins from about 4.1 gallons up, and 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 the saving is something you can check rather than something you have to believe.
Coverage belongs to the surface, not to the tin
The figure printed on the tin is not a lie, but it is a laboratory measurement: a smooth, sealed test panel, a calibrated roller, no cutting in, no touch-ups. Read it as the best case, because that is what it is.
Your wall is somewhere else on the scale, and the spread is enormous. A previously painted, smooth interior wall really does return the tin figure — 350 to 400 sq ft per gallon per coat is the honest range for a quality flat latex. Bare new plaster or render returns barely half of it. Raw block or brick, with its open pores and its genuinely larger surface area, can return 45%. Texture and heavy coatings are the same trap in a different form: the area your tape measure reports is a flat projection, while the surface the roller has to wet is corrugated.
A calculator with one coverage constant is not slightly wrong on these surfaces, it is wrong by a factor. That is why the surface is a first-class input here and why the results panel reports the effective coverage next to the nominal one. When the tool tells you 400 sq ft per gallon became 220, you know exactly where the extra money went.
The first coat is a different coat
On anything porous the first coat is partly drunk by the substrate instead of forming a film on top of it. Nothing is wrong with the paint; it is doing the job the surface asked for. But it means the classic formula — area times coats divided by coverage — treats the first coat and the third as identical when they are not.
So the first coat gets its own coverage figure, derived from the surface. On bare drywall it lands around 72% of the nominal figure; on new plaster around 70%; on primed and sanded work it goes back to 100%. On a smooth previously painted wall the correction disappears entirely and the arithmetic collapses back to the simple formula everyone knows, which is exactly as it should be: a model should only complicate itself where the world is actually complicated.
The colour change decides the coats, and the coats decide everything
Coats are a straight multiplier on the whole order. Going from two to three is fifty per cent more paint, more labour and more days. No other field on this page moves the total as much, and none is guessed more casually.
- Same colour, refreshing tired paint — two coats.
- Light over light, or dark over a similar dark — two coats.
- Dark over light — three, because the new colour has to build depth.
- Light over dark — four, or three over a grey-tinted primer.
- Strong red, yellow or orange — four in either direction. Those pigments have poor natural opacity, and no amount of care with the roller fixes chemistry. A base tinted toward the final colour is worth more here than any discount.
The selector fills the coats field rather than replacing it. If you have already seen the wall under a work light, your number beats the table.
Doors and windows come out at full size — the opposite of a board estimate
Every opening is deducted here, at the width and height you measured, with no threshold and no minimum. That is worth stating plainly because the rule is the reverse of the one that governs plasterboard, where 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 such granularity. It is a liquid with continuous coverage, so 15 sq ft of glass is 15 sq ft you do not paint, whether it arrives as one patio door or three small windows. What is genuinely wrong is the habit of deducting a flat ten or fifteen per cent "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 does not add back, and on a thick masonry wall that is a real if small amount. And door frames, skirting and architraves are almost always a different paint in a different sheen, bought separately in quarts.
Ceilings are a separate job with a separate paint
A ceiling is not more wall. It is usually painted in a dead flat white chosen for hiding power rather than washability, it takes fewer coats, and it covers slightly less per gallon than a good wall paint because that flatness comes from extra filler.
Rolling the two together into one area and one product is the most common shortcut in online paint estimators, and it produces an order that is wrong in both directions at once: too much of the expensive wall paint, 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, and it gets its own row in the shopping list and its own can optimisation.
There is also a practical reason to keep them separate: ceilings are painted first. If the estimate treats them as one purchase you will find out at the wrong moment that the single bucket you bought has to do both.
Primer on new work is not optional, and it pays for itself
On new plaster, bare board, patched repairs or raw masonry, primer is not a finishing nicety — it is the layer that stops the substrate drinking your finish paint.
The arithmetic is unusually favourable, which is why it is worth showing rather than asserting. Priming new render 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 litre than finish paint, and it hands back more litres of finish paint than it consumes of itself. Tick the box in the tool with a porous surface selected and watch both the litres and the total fall — that is not a modelling artefact, it is why the trade does it.
Filler and skim coats are a different question and deliberately more conservative here. How much surfacing compound a wall swallows depends on how far out of true it is, which no table knows. The field is editable and it is the most volatile number in the tool; treat the default as a placeholder until you have skimmed a square metre and measured.
Sheen changes more than the look
Flat, eggshell, satin and semi-gloss are usually chosen on appearance alone, but each one also moves the estimate and the maintenance.
Flat hides surface defects best and touches up almost invisibly, which matters on ceilings and on old walls; it is 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, where it earns its place by being scrubbable — and punishes you by showing every roller mark and every wave in the wall it is stretched across. Gloss on a badly prepared wall is the most expensive way to advertise the preparation you skipped.
What this estimate does not cover
It sizes wall paint, ceiling paint, primer and filler, and it prices them by the container. It does not size trim paint, caulk, masking tape, drop cloths, sandpaper, rollers, brushes or the tray, and those together are a real fraction of a small job.
It does not tell you whether the existing surface needs washing, whether a glossy old finish needs to be dulled before anything will stick, or whether the damp patch in the corner needs solving before a single litre is opened. It does not check whether an exterior product is rated for your exposure. And coverage figures here are purchasing estimates, not manufacturer specifications: check them against the tin before you commit to a pallet, and let the tin win.
Frequently asked questions
How many gallons of paint do I need for a room?
Take 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 — which means two one-gallon cans, because 1.87 gallons is not something anybody sells.
Is a five gallon bucket cheaper than five one-gallon cans?
Almost always per gallon, but not always for your job. At $155 a bucket against $38 a gallon, the bucket is worth it from about 4.1 gallons upward — below that you are paying for paint you will not open. The calculator does not use a rule of thumb here: it searches every combination of bucket, gallon and quart that covers your requirement and reports the cheapest one, then shows what buying a single size would have cost so you can see the difference rather than trust it.
Why does the first coat use more paint than the second?
On a porous surface the first coat is partly absorbed instead of forming a film. Bare drywall, new plaster, patched repairs and raw block all drink the first coat, so its effective coverage can drop to 70% of the figure on the tin while later coats behave normally. That is why this tool prices the first coat separately, and why priming changes the arithmetic: primer takes the absorption hit so the finish paint does not.
How many coats do I need to go from a dark colour to white?
Usually four, sometimes three if you use a grey-tinted primer first. Hiding power is the limiting factor, not durability: white over a deep colour keeps letting the old shade read through, and each extra coat multiplies the whole paint order. Going the other way — dark over light — normally needs three. Strong reds, yellows and oranges need four regardless of direction, because those pigments have poor natural opacity; a base tinted toward the final colour saves more than any discount will.
Should I subtract doors and windows from the paint estimate?
Yes, at their real measured size, all of them. This is the opposite of a drywall estimate, where 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 an opening of 15 sq ft saves exactly 15 sq ft of painting. What you should not do is deduct a flat 10% for openings, which is a guess in a place where you already have a tape measure.
Are my numbers sent anywhere?
No. Everything is arithmetic running in your browser. Nothing is uploaded, nothing is stored and there is no account. The prices you type are your supplier's prices and they stay on your machine.