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Paint Calculator Guide
Gallons from wall area, adjusted for the things that actually change the number: surface texture, coats, and whether you need primer.
Gallons = area × coats ÷ coverage per gallon
A 12×14ft room at 9ft high, less a door and two windows, is about 417 sq ft. Two coats at 375 sq ft/gal ≈ 2.2 gallons.
The Number on the Can Is a Best Case
That 350–400 sq ft per gallon assumes a smooth, previously painted, similar-coloured wall. It's a laboratory figure, not a promise.
Heavy texture can eat 35% more because the surface area is genuinely larger than the wall's dimensions suggest — you're painting into every dimple. Bare drywall drinks the first coat. Neither is a defect; both are why the can's number rarely survives contact with a real room.
Primer Isn't Paint and It Isn't Optional
People skip primer to save a step and then apply four coats of expensive finish paint trying to cover, which costs more and takes longer.
Primer does two jobs paint can't: it seals porous surfaces so the finish coat sits on top instead of soaking in, and it gives a uniform base so the colour reads the same everywhere. Going dark-to-light without it is the classic way to spend a weekend rolling.
Two Coats Is the Real Answer
One-coat coverage claims are true only in the narrow case they were tested in — same colour family, smooth primed wall, perfect application.
In practice the second coat is what evens out roller marks and gives consistent sheen. If your budget stretches to one coat of premium or two coats of mid-range, take the two coats.
Don't Deduct Small Openings
This calculator deducts a door at about 21 sq ft and a window at about 15, which is deliberately conservative.
You still cut in around every opening, and cutting in uses more paint per square foot than rolling does. Deduct precisely and you'll be short — and running out mid-wall means a visible lap line where fresh paint meets a dried edge.
Buy the Whole Job at Once
Paint is tinted in batches. Two gallons mixed on different days can differ slightly, and on a large wall the difference shows.
If you need three gallons, buy three at once and box them — pour them all into one bucket and stir. That guarantees a single colour across the room regardless of what the machine did on any given day.
Related: wallpaper calculator, deck stain calculator, drywall calculator.
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