TOOL_01 · Interior paint
Paint calculator
Gallons for the job — doors and windows subtracted, coats counted, primer optional. The math is shown, not hidden.
① Room dimensions
Paint needed
0.0gal
≈ 2 gal + 3 quarts
Total area
0sq ft
453 walls × 2 coats
Material Cost Estimator
Estimated Total for Interior Paint
$144.00
3 units × $48.00 per gallon
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How the math works
Walls: perimeter × height, minus 21 sq ft per door and 15 sq ft per window. Ceiling: room area (estimated from your perimeter), added only if you check the box. Total: area × coats, divided by 350 sq ft per gallon, rounded up — because the store doesn't sell 0.7 gallons.
Primer math is separate: primer covers about 200 sq ft per gallon since it soaks in more. One coat of primer is included when you switch the calculator to “Paint + primer.”
Adjust for reality: rough texture, bare drywall, or deep color changes can eat 20–30% more paint. Buying a quart of buffer beats an emergency second trip with half a wall cut in.
Common questions
How many square feet does a gallon of paint cover?
One gallon of typical interior latex paint covers about 350–400 square feet per coat on a smooth, primed wall. This calculator uses 350 sq ft per gallon — the conservative end — so you don't run short mid-project. Rough or heavily textured walls can drop coverage to 250–300 sq ft per gallon.
Do I really need two coats?
For most repaints over a similar color, two coats is the standard for even color and durability. If you're making a dramatic color change (light over dark or vice versa), plan on two coats plus primer. New drywall always needs primer plus two coats.
How much paint does a 12x12 room need?
A 12×12 room with 8-ft ceilings has 384 sq ft of wall (48 ft perimeter × 8 ft). With two coats and two doors plus one window subtracted, you'll need about 2 gallons. Add a third gallon if you're painting the ceiling.
Should I buy gallons or a 5-gallon bucket?
For 15+ gallons of the same color, a 5-gallon bucket is cheaper per gallon and keeps color consistent across rooms. For 1–3 gallons, buy individual gallons — they're easier to carry, shake, and store for touch-ups.
Estimates use standard coverage assumptions and are for planning purposes. Actual needs vary by surface, paint quality, and application.