Paint Calculator

Gallons of paint and primer for rooms and exteriors. Updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Coverage uses the industry standard of 350 sq ft per gallon per coat for paint (real-world range 300–400 depending on texture and porosity) and 250 sq ft per gallon for primer. Results round up to whole gallons per product, the way you actually buy it. Costs use 2026 averages: about $45/gallon for mid-grade interior paint ($28 economy, $70+ premium).

The waste allowance

This calculator rounds up to whole gallons per product, because that is how paint is sold. It does not add a blanket percentage on top. The real variability in paint is not spillage, it is coverage: a published spread rate assumes a particular film thickness on a surface that is already sealed, and a surface that drinks material uses more of it before the film reaches that thickness. The honest allowance is therefore a judgement about the wall rather than a fixed number. On new drywall, bare wood, stucco or concrete block, expect the published figure to overstate what you will get, and buy on the generous side.

What 350 to 400 square feet per gallon actually means

The figure is real and it is published. Sherwin-Williams gives 350 to 400 square feet per gallon on the data sheet for ProMar 200 Zero VOC interior latex primer. It is not a round number pulled from the air, but it comes with a condition attached: that is the coverage at 4 mils wet, drying down to 1.0 mil dry. The condition is the part that gets dropped every time the number is repeated.

Coverage is not a property of the paint alone. It is the relationship between a volume of material and the thickness of the film that volume is spread into. Spread thin, a gallon covers a large area badly. Spread at the thickness the manufacturer tested, it covers the published area and performs the way the data sheet says. The rate and the film thickness are one specification, not two facts.

So the number holds on the surface it was measured on: smooth, sound, already sealed. Porous surfaces change the arithmetic. New unprimed drywall, stucco, bare wood and concrete block absorb material into the substrate before a continuous film forms on top of it. That paint is not building film, so you spend material getting to the starting line, and real coverage comes in lower than published. Texture compounds it, because a rough wall has more actual surface area than its measured dimensions suggest.

Colour change, and why some colours need a third coat

Hide is a property of pigment, not of price or effort. Bright and transparent pigments hide poorly by their nature. Yellows, oranges, reds and clean primaries are the usual offenders, and they often need two coats or more whatever you put underneath, because the pigment itself does not block what is behind it. Muted and greyed shades hide better, because the pigments that produce them are more opaque. That is chemistry, not quality control.

The practical consequence is that coat count depends on the colour you have chosen as much as the colour you are covering. People plan carefully for dark to light and are then caught out by a clean yellow over white, which sounds like it should be easy.

Sherwin-Williams recommends a grey-tinted primer matched to the topcoat to cut the number of finish coats on a dramatic change. The primer does the hiding, at primer prices, so the finish coats only have to deliver colour and film rather than burying what is underneath. A tinted primer plus two finish coats is cheaper and faster than three finish coats, and the result is more even because the base is uniform.

Why two coats is standard even when one looks fine

Two coats is standard professional practice, and hide is only half the reason. The other half is durability. A single coat, even one that reads as solid from across the room, is a thinner and less continuous film. It has more weak spots, it burnishes sooner where it is touched, and it does not scrub as well because there is less material to scrub.

There is also consistency. Rolling is not a precision process, and film thickness varies across a wall more than most people assume. A second coat evens that out in a way that working slowly on the first one does not. Sheen shows this before colour does, which is why a one-coat job in satin or semi-gloss looks patchy in raking light even when the colour is uniform.

Buying for one coat is the most common reason people end up back at the store. For a same-colour refresh on a sound surface, one coat is defensible. For anything else, buy for two and treat a third as the contingency rather than the plan.

Doors, windows and the deduction problem

Wall area is easy: perimeter multiplied by ceiling height. What to subtract for openings is not, and the state of the evidence is worth stating plainly. There is no manufacturer standard formula for door and window deductions. Paint companies publish spread rates because they can test them; they do not publish a deduction table, because the door in one house has nothing to do with the door in another.

The approach this calculator uses is its own methodology, not an industry rule. Treat it as a reasonable default that saves you measuring every opening, and override it when your openings are unusual. A wall of glass or a pair of wide sliders is badly served by any average.

Erring towards subtracting less is the cheaper mistake. Overstating wall area buys a little more paint than you need, and the spare sits in the basement for touch-ups. Understating it sends you out mid-job, possibly to a different batch.

How the surface changes your coverage

The published spread rate applies at a specific film thickness on a sealed, smooth surface. This table gives the direction and the reason each surface pushes that figure, not a substitute set of numbers.

Spread rate of 350 to 400 sq ft per gallon at 4 mils wet and 1.0 mil dry from the Sherwin-Williams data sheet for ProMar 200 Zero VOC interior latex primer. The surface commentary explains why real coverage departs from that figure and is not itself a published coverage table.

What goes wrong when buying paint

Treating 350 to 400 as universal. It is a tested figure at a specific film thickness on a sealed surface. New drywall, stucco, bare wood and block absorb material before a film forms, so real coverage is lower.

Skipping primer on a dramatic colour change. The saving disappears if you then need three finish coats. A grey-tinted primer matched to the topcoat is Sherwin-Williams' own recommendation for cutting the coat count.

Buying for one coat. Two coats is standard practice for durability as much as hide. A single film is thinner, less even, and does not scrub as well.

Expecting a bright colour to behave like a muted one. Yellows, oranges, reds and clean primaries hide poorly because of the pigments involved. That is inherent to the colour, not a fault in the paint.

Deducting openings too generously. There is no manufacturer standard for door and window deductions. Subtracting less buys a spare gallon; subtracting too much sends you back to the store mid-job.

Frequently asked questions

Why your state changes the price (but not the quantity)

How much paint you need is physics — it doesn't care where you live. What it costs does. Labor rates, delivery distance, fuel, and local demand move installed prices by roughly 45 points from end to end: Arkansas, Mississippi, North Carolina sit near the bottom of the range, while Alaska, Hawaii, New York sit at the top. The Price this for selector at the top of the calculator applies a published construction cost index for your state to the 2026 national averages, so the dollar figures move and the material list stays put.

Treat the result as a budgeting range, not a quote. Two metros inside the same state can differ by more than the statewide index suggests, and a single busy contractor can outweigh both. See the full cost index for all 50 states and where the price data comes from.