How the math works
The calculator takes your room's perimeter — 2 × (length + width) — and multiplies it by wall height to get the total wall surface. It then subtracts standard openings (20 ft² per door, 15 ft² per window) to avoid overbuying, and multiplies the result by your number of coats before dividing by the paint's coverage rate. Most quality interior paints cover 350–400 square feet per gallon on a smooth, primed wall — check your specific can label, since the rate can drop to 250–300 ft²/gal on textured or unprimed drywall.
Frequently asked questions
How many square feet does one gallon of paint cover?
Most interior wall paints cover 350–400 square feet per gallon in a single coat, on a smooth, previously painted or primed surface. Textured walls, brand-new drywall, or dark color changes can pull that down to 250–300 square feet per gallon, so it's worth checking the coverage number printed on your specific can rather than assuming the average.
Do I need to count primer separately?
Yes. Primer is a separate product with its own coverage rate (usually similar, around 300–400 ft²/gal), and it's typically applied as one coat before your color coats — not counted as one of your "2 coats" of paint. If your walls are new drywall, a repair-patched surface, or a big color jump (like navy over white), plan on one primer coat in addition to what this calculator estimates.
Should I do one coat or two?
Two coats is the standard for a durable, even finish, especially on walls that have been painted a different color before. A single coat can work for a simple refresh in the same or a very similar color, but it often shows roller marks and uneven patches once it dries. If you're unsure, budget for two — it's the default in this calculator.
Does this work for ceilings too?
Not directly — this calculator is built around wall perimeter × height. For a ceiling, multiply the room's length by its width to get the flat area, then divide by your paint's coverage rate the same way. Ceiling paint is usually a flat finish with similar coverage to wall paint, so you can reuse the 350 ft²/gallon default as a starting point.
How much extra paint should I buy for touch-ups?
A good rule of thumb is to round your estimate up to the next full can rather than buying the exact decimal amount — most stores won't sell partial gallons anyway. If you want dedicated touch-up reserve on top of that, keep roughly an extra half-pint to a pint per room, well-sealed, for scuffs and nail holes down the road.
Do matte, eggshell, and semi-gloss cover differently?
Coverage rates are broadly similar across sheens from the same product line, but glossier finishes (semi-gloss, high-gloss) are often applied more thinly and evenly, which can slightly reduce how far a gallon stretches in practice. Flat and matte finishes are generally the most forgiving and closest to the 350–400 ft²/gallon estimate.
Can I use this calculator for exterior painting?
The area math works the same way, but exterior surfaces (stucco, brick, wood siding) are typically more porous and textured than interior drywall, which lowers real-world coverage — often to 250–300 ft²/gallon or less. For exterior jobs, use the "Advanced settings" field to lower the coverage rate before calculating.