Paint Calculator
Calculate how many gallons of paint you need for any room.
Results
How to Calculate How Much Paint You Need
Calculating paint for a room is straightforward once you understand the variables. The paint calculator above takes your room dimensions and subtracts door and window areas to give you the net paintable wall surface. Divide by your paint's coverage rate, multiply by the number of coats, and round up — that's your purchase quantity.
The Basic Formula
Wall area = 2 × (length + width) × height. For a 15×12 room with 9-foot ceilings, that's 2 × (15+12) × 9 = 486 sq ft. Subtract 20 sq ft per door and 15 sq ft per window. With 1 door and 2 windows: 486 − 20 − 30 = 436 sq ft paintable area.
At 350 sq ft per gallon with 2 coats: 436 × 2 ÷ 350 = 2.49 gallons. Always round up to the nearest half gallon — so buy 2.5 gallons (one quart-size plus two one-gallon cans, or three one-gallon cans with a little leftover).
Coverage Rates by Surface Type
Paint coverage varies significantly by surface. Smooth, previously painted drywall in good condition: 400 sq ft/gal. Standard painted drywall: 350 sq ft/gal. Lightly textured walls (orange peel): 300 sq ft/gal. Heavily textured walls, bare drywall, or deep color changes: 250 sq ft/gal.
When in doubt, use the lower end of coverage. It's cheaper to buy one extra quart than to run out and make another store trip — especially since paint colors can vary between batches.
Ceiling Paint
This calculator covers walls only. For the ceiling, calculate length × width (your room's floor area) and multiply by the number of coats. Ceilings typically require only one coat of flat white paint. For a 15×12 ceiling: 15 × 12 = 180 sq ft ÷ 400 = 0.45 gallons. One quart (0.25 gal) will stretch if you're careful; one gallon gives comfortable coverage.
When to Use Primer
Primer is necessary when painting bare new drywall (without primer, paint soaks unevenly), covering a dramatic color change (dark to light or bright to neutral), painting over stains or water damage, and on previously glossy or oil-based painted surfaces. Self-priming paints save a step but often don't perform as well as a dedicated primer coat on problem surfaces.
Buying Tips
Always buy paint from a single batch (same lot number) to ensure consistent color. Keep the leftover paint in a sealed can stored upside down — this creates an airtight seal and extends shelf life to 2–5 years for future touch-ups. Label the can with the room name, color name, and mix code.
Frequently Asked Questions
One gallon of paint typically covers 350–400 square feet on smooth previously painted walls. Textured walls or color-change projects may yield closer to 250–300 sq ft per gallon. Always check the specific paint's label for its rated coverage.
Two coats are standard for full, even color and better durability. One coat may work when painting a similar color over fresh paint. For color changes, new drywall, or bright/saturated colors, always plan on two coats.
Yes — add 10–15% to your calculated amount for touch-ups, mistakes, and keeping a color-match reserve. Leftover paint stored in a cool, dry place can last 2–5 years for future touch-ups.
No — this calculator covers walls only. For the ceiling, calculate length × width and divide by your paint's coverage rate. Most ceilings only need one coat of flat white paint.
DIY paint costs roughly $25–$60 per gallon depending on brand and finish. A typical 12×15 bedroom requires 2–3 gallons for two coats, so materials run $50–$180. Professional painting labor adds $200–$600+ per room depending on region and prep work needed.