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Stop guessing in the paint aisle. Calculate exactly how many cans of paint you need to buy, deduct doors and windows automatically, and estimate total costs. This free online tool allows you to calculate paint coverage quickly and accurately. No sign-up or installation required.
Buying paint is a Goldilocks problem: buy too little, and you are rushing back to the hardware store mid-project. Buy too much, and you have wasted money and are left storing toxic chemicals in your garage for a decade.
Most DIYers severely underestimate how much paint is required for a room. They often forget to account for the second coat, or they fail to deduct the massive square footage taken up by doors, windows, and closets. This guide will teach you the exact math professional painters use to estimate jobs.
The baseline metric every painter must memorize is the coverage rate. Assuming you are painting standard, smooth drywall with a high-quality interior latex paint:
The golden rule only applies to previously painted, smooth drywall. If you are painting new, unprimed drywall (often called "bare rock"), masonry, stucco, or heavily textured walls, the surface is incredibly porous. It will literally drink your first coat of paint. On these surfaces, your first coat may only yield 250 sq ft per gallon. Always use a dedicated primer on bare surfaces before applying expensive top-coat paint.
You do not need to measure the area of every single wall individually. The fastest way to measure a room is to use the Perimeter Method.
Next, you must deduct obstructions. A standard interior door removes about 21 square feet of paintable wall space. A standard window removes about 15 square feet. Our calculator handles this step automatically—simply input the number of doors and windows, and we deduct the math from your gross area to find your Net Paintable Area.
Once you know exactly how many cans of paint to buy, you need to budget the financial cost.
| Paint Quality Tier | Average Cost (Per Gallon) | Expected Durability |
|---|---|---|
| Contractor Grade | $20 - $30 | Low (Hard to clean, thin coverage) |
| Mid-Tier (DIY Standard) | $35 - $55 | Good (Washable, smooth finish) |
| Premium / Designer | $60 - $100+ | Excellent (Scrubbable, rich pigments) |
Using our built-in Budget Estimator, simply select your local currency and slide the price-per-unit based on the tier of paint you intend to buy. The calculator will instantly multiply this by your required paint volume to give you a total project cost.
If your calculator output tells you to buy 4 gallons of paint, do not paint wall-by-wall using individual cans. Even computer-matched paint can have slight color variations between cans. Take a large 5-gallon bucket and pour all 4 individual gallons into it, mixing them together thoroughly. This professional technique is called "boxing," and it guarantees a 100% uniform color across your entire room.
Common questions about paint math, coverage, and application techniques.