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Paint Calculator: How Much Paint Do I Need?
Buying paint is one of those tasks where guessing gets expensive in both directions: too little and you risk a mid-project store run (and a slightly different tint batch), too much and unused gallons sit in the garage for years. This calculator takes the guesswork out by working from your actual wall area.
Enter your room dimensions, subtract the doors and windows you will not be painting, choose how many coats you plan to apply, and you will get a practical estimate of the gallons to buy.
How to calculate how much paint you need
Measure the room
Measure the length and width of the room in feet, and the wall height from floor to ceiling. For rooms with alcoves or bump-outs, use the largest dimensions to stay on the safe side.
Count doors and windows
Count the doors and windows on the walls you are painting. The calculator subtracts a standard area for each, since you will not be painting over them.
Choose the number of coats
Two coats is the norm for a color change or bold hues. One coat can work for a same-color refresh; drastic changes over dark walls may need a third coat or a tinted primer.
Read your result
The calculator shows the total paintable area and the gallons to buy, rounded up so you never come up short mid-wall.
The math behind this calculator
The math is straightforward: wall area = 2 × (length + width) × height. From that we subtract 21 sq ft per standard door and 15 sq ft per average window. The remaining paintable area is multiplied by your number of coats and divided by your paint’s coverage per gallon — 350 sq ft by default, which is what typical interior latex delivers on a smooth, primed surface, but adjust it to match the number printed on your can.
The result is rounded up to the next whole gallon. Textured walls, unprimed drywall, and porous surfaces absorb more paint, so if that describes your room, treat the estimate as a floor and consider one extra quart. Ceilings and trim are separate jobs with their own paint types, so measure those separately.
Frequently asked questions
How much area does a gallon of paint cover?
A gallon of interior latex paint covers roughly 350 to 400 square feet on a smooth, primed surface. This calculator uses 350 square feet per gallon, the conservative end of that range, so textured or thirsty walls are less likely to leave you short.
Do I really need two coats of paint?
For any color change, yes — two coats give uniform coverage and true color. A single coat is usually only enough when repainting with the same or a very similar color. Going light over dark may require a tinted primer plus two coats.
Should I subtract doors and windows from my paint estimate?
Yes. A standard interior door accounts for about 21 square feet and an average window about 15 square feet of wall you will not paint. The calculator subtracts these automatically based on your counts.
How much paint do I need for a 12x12 room?
A 12 by 12 foot room with 8-foot ceilings has about 384 square feet of wall area. After subtracting a door and a window, two coats need roughly 700 square feet of coverage — right at 2 gallons of paint.
Does ceiling paint count in this estimate?
No. This calculator estimates wall paint only. Ceilings use a dedicated flat ceiling paint; a 12 by 12 ceiling is 144 square feet, which one gallon covers in two coats with room to spare.
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