Paint Calculator
Estimate how many gallons of paint you need for any room. Enter the room dimensions, doors, windows and number of coats — the answer updates instantly.
Results
Tip: always round up to the next full gallon and keep leftovers for touch-ups.
How to Use This Paint Calculator
- Measure the length and width of the room in feet.
- Measure the wall height (floor to ceiling).
- Count the doors and windows — standard sizes are subtracted automatically.
- Enter how many coats you plan to apply.
The calculator adds up the four walls, subtracts openings, multiplies by the number of coats and divides by the coverage rate to get the gallons you should buy.
Paint Coverage Explained
A gallon of interior paint typically covers 350–400 square feet per coat. The exact number is printed on every can and depends on the paint quality, color and surface texture:
- Flat/matte paint: usually the most forgiving and close to 400 sq ft per gallon.
- Eggshell and satin: typically 350–400 sq ft per gallon.
- Semi-gloss/gloss: can run slightly lower because they level differently.
- Textured walls (knockdown, orange peel): expect 250–300 sq ft per gallon — lower the coverage number in the calculator.
What About Ceilings and Trim?
This calculator estimates walls only. Add ceiling paint separately: a 12×12 ceiling is 144 sq ft and needs about one gallon per coat. Trim, baseboards and door frames add roughly 5–10% more depending on how much trim the room has.
Doors and Windows: Why 21 and 15 Square Feet?
A standard interior door is about 3×7 feet (21 sq ft) and a typical window about 3×5 feet (15 sq ft). These are industry-average defaults — adjust the counts up or down for unusual openings, and the calculator recalculates instantly.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12×12 room with 8-foot ceilings has about 348 sq ft of paintable wall area after subtracting a standard door and window. With two coats at 350 sq ft per gallon, you need about 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover 350–400 square feet per gallon for one coat. Always check the coverage number on the specific can, because premium paints and textured surfaces can differ.
Do I need primer, and how do I count it?
Primer is recommended for new drywall, bare wood, stains and drastic color changes. Treat it as one extra coat: use the same square footage estimate and add the primer gallons to your total.
Does this calculator include ceilings and trim?
No. The calculator estimates walls only. A typical 12×12 ceiling is 144 square feet and needs roughly one gallon per coat; trim, doors and window frames add a small amount on top.