Answer: Enter your values and the Paint Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Gallons of paint and primer, with cost estimate
This paint calculator estimates how much paint you need for any room or exterior surface. Enter your wall area, number of coats, and the coverage rate printed on your paint can, and the tool converts those inputs into gallons to buy — plus optional primer and a rough cost figure. It runs entirely in your browser, so nothing is uploaded anywhere.
The math behind paint estimating is simple, but the details matter. Coverage varies by product, sheen, surface texture, and application method, so this guide explains where each number comes from and how to adjust for real-world conditions.
Paint volume in the United States is sold by the gallon, and one US liquid gallon equals 128 fluid ounces. Manufacturers print a spread rate on every can, expressed in square feet per gallon. Standard interior latex paints are typically rated at 350 to 400 square feet per gallon on smooth, primed drywall, while primers cover less — usually 200 to 300 square feet per gallon — because they soak into porous surfaces.
The core formula is: gallons needed = (paintable area × number of coats) ÷ coverage rate per gallon. Always round up to the next whole gallon, because partially used gallons are what you buy, and leftover paint is useful for touch-ups. A standard 10 percent overspray and waste allowance is a common contractor practice on spray jobs.
The table below shows how many gallons a 400 square foot surface needs at common spread rates and coat counts. These are computed directly from the coverage formula — for example, 400 square feet times two coats divided by 350 square feet per gallon equals 2.29 gallons, so you buy three.
Notice how much the spread rate changes the answer: the same wall area needs 60 percent more paint at 250 square feet per gallon (a rough, textured, or heavily tinted surface) than at 400.
| Spread rate (sq ft/gal) | 1 coat over 400 sq ft | 2 coats over 400 sq ft |
|---|---|---|
| 250 (rough/textured) | 1.60 gal | 3.20 gal |
| 300 | 1.33 gal | 2.67 gal |
| 350 (standard interior) | 1.14 gal | 2.29 gal |
| 400 (smooth, primed) | 1.00 gal | 2.00 gal |
| Primer at 200 | 2.00 gal | 4.00 gal |
| Primer at 300 | 1.33 gal | 2.67 gal |
Consider a bedroom that is 12 feet by 12 feet with 8-foot ceilings. The perimeter is 2 × (12 + 12) = 48 feet, so the wall area is 48 × 8 = 384 square feet before subtracting a standard door (about 21 square feet) and any windows.
At 350 square feet per gallon, one coat needs 384 ÷ 350 = 1.10 gallons, and two coats need 2.19 gallons — so you buy three gallons to be safe. At a more generous 400 square feet per gallon, two coats need only 1.92 gallons. If you prime the room with two coats of primer at 200 square feet per gallon, that is another 3.84 gallons of primer, or 2.56 gallons at 300.
Ceiling area is simply length × width: 12 × 12 = 144 square feet, one additional gallon covers it with two coats at most spread rates. Don't forget trim and baseboards, which painters usually estimate by linear foot.
['Preparation drives both coverage and finish quality. New drywall and repairs drink primer: a dedicated drywall primer or a PVA primer sealed with one full coat before two finish coats is the standard sequence. Previously painted glossy surfaces should be lightly sanded and deglossed so the new coat bonds instead of peeling.', 'Budgeting works best by gallon, not by room. If paint costs 40 dollars per gallon and a 384 square foot room needs three gallons for two coats, the paint budget is about 120 dollars plus primer. Brushes, rollers, tape, and drop cloths typically add 30 to 60 dollars for a room-sized job. Buying an extra gallon up front is cheaper than a return trip mid-wall, especially with custom-tinted colors that cannot be matched exactly later.', 'Timing and technique also matter. Paint when surfaces are dry and temperatures are moderate — most latex labels specify application between 50 and 90 degrees Fahrenheit. Maintain a wet edge, work top to bottom, and avoid over-brushing or over-rolling, which both wastes paint and leaves lap marks. Spray application goes faster but typically consumes about 10 percent more material due to overspray.']
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 square feet of wall. Two coats at a standard 350 sq ft/gal spread rate need 2.19 gallons, so buy 3 gallons. Subtract roughly 15 sq ft per window and 21 sq ft per door for a more precise figure.
How many square feet does one gallon of paint cover?
One gallon of typical interior latex paint covers 350 to 400 square feet per coat on smooth, primed drywall, per manufacturer spread rates. Primers cover less — about 200 to 300 square feet per gallon — and rough or textured surfaces can drop coverage to 250 square feet or less.
Do I need to count each coat separately?
Yes. Two coats double the paint used: multiply your paintable area by the number of coats before dividing by the coverage rate. Most manufacturers recommend two topcoats over primer for even color and durability.
How do I calculate wall area for painting?
Measure the perimeter of the room, multiply by wall height, then subtract doors (about 21 sq ft each) and windows (about 15 sq ft each). Ceiling area equals length × width. Add all paintable surfaces together before applying the coverage formula.
Should I buy extra paint for touch-ups?
Buying one partial gallon beyond your calculated need is wise. Colors vary slightly between batches, so a stored touch-up gallon from the same batch avoids visible mismatch later. Stored tightly sealed, latex paint typically remains usable for years.