One gallon of interior paint covers about 350–400 square feet on smooth, primed walls, and roughly 250–300 square feet on textured or unprimed drywall.
Paint math goes wrong in the same place almost every time: someone buys one gallon for a 12-by-12 room, gets halfway through the second coat, and drives back to the store. Coverage is not a fixed number pulled from the can — it moves with the surface under it, the number of coats on top of it, and how much the color has to change. Getting the estimate right the first time comes down to three variables you can actually measure.
What Decides How Far A Gallon Goes?
The surface under the paint decides coverage before anything else does. Sherwin-Williams puts typical interior coverage at 350–400 square feet per gallon, and that range exists because smooth, previously painted drywall absorbs far less paint than fresh, porous drywall or a heavy knockdown texture.
Color change matters just as much. Going from a light beige to a deep navy is a multi-coat job no matter how good the paint is, while refreshing a wall in nearly the same shade can sometimes close out in one coat. Sheen plays a smaller role — flatter finishes hide touch-ups better, but they don’t change how far the gallon stretches.
Here’s the surface-by-surface breakdown. These are planning figures, not guarantees; the label on your specific can always wins.
| Surface | Coverage Per Gallon | What Changes It |
|---|---|---|
| Smooth, primed drywall | 350–400 sq ft | Best case; one coat often enough for similar colors |
| Previously painted drywall | 350 sq ft | The standard planning number |
| Fresh, unprimed drywall | 250–300 sq ft | Porous paper soaks up the first coat |
| Heavy texture (knockdown, orange peel) | 250–300 sq ft | More surface area per square foot of wall |
| Primer | 200–300 sq ft | Runs thinner than finish paint by design |
| Major color change | Plan 3 coats | Deep or saturated colors need extra passes |
| Popcorn or acoustic ceiling | 200–250 sq ft | Deep texture eats volume fast |
How Do You Calculate Gallons For A Room?
Multiply the total paintable wall area by the number of coats, then divide by your coverage rate. That’s the whole formula, and it takes about two minutes with a tape measure and a phone calculator.
- Measure the length of each wall, then add those lengths together for the room’s perimeter.
- Multiply the perimeter by ceiling height to get gross wall area.
- Subtract doors (about 20 sq ft each) and windows (about 15 sq ft each).
- Multiply the paintable area by your coat count — usually two.
- Divide by 350 for smooth walls, or by 275 if the surface is textured or fresh.
A 10-by-10 room with 8-foot ceilings has about 320 square feet of wall after openings. One gallon covers that in a single coat; two coats push you into the second gallon, or close to it. Sherwin-Williams notes a gallon can handle a 10-by-10 room by raw area, with wall height and texture deciding whether it actually finishes the job.
Buy a little extra on purpose. Half a gallon of leftover paint covers the inevitable scuffs and nail holes that show up a year later, and a matched touch-up beats repainting a whole wall. If you’d rather not gamble on a brand you haven’t used, our roundup of the top-rated one-gallon interior paints compares coverage claims and real-world finishes side by side.
How Many Coats Should You Actually Plan For?
Two coats is the standard answer for color uniformity and durability, and it’s what most manufacturers assume when they quote coverage. Skimping to one coat shows up as patchy sheen under raking light, especially on walls that get direct sun.
Priming first is a separate step, not a substitute. Primer covers only 200–300 square feet per gallon — less than finish paint — because it’s formulated to seal and bond rather than build color. Skipping it on bare drywall usually costs more in topcoat than the primer would have.
The honest caveat: every number on this page is an estimate. The coverage figure printed on your paint can, checked against your actual wall texture and color change, is the only number that’s specific to your project.
FAQs
Is one gallon enough for a 12×12 room?
Usually, for a single coat. A 12-by-12 room with 8-foot ceilings has roughly 400 square feet of wall after subtracting doors and windows, which sits right at the top of the 350–400 square foot range. Two coats means buying two gallons, or one gallon plus a quart if the color change is minor.
Does paint color affect how much I need?
Yes. Going lighter over a dark wall, or from a pale shade to a deep saturated color like red or navy, typically needs three coats instead of two. That extra pass adds roughly 50% to your total paint estimate. Similar colors over similar colors are the cheapest scenario.
Should I buy extra paint for touch-ups?
Buying about 10% more than your calculated amount is a reasonable buffer. Leftover paint stored properly lasts for years and gives you a seamless color match for scuffs, furniture dings, and nail holes. Mixing a fresh can later rarely matches exactly, even with the same formula and brand.
References & Sources
- Sherwin-Williams. “Paint Calculator FAQs.” Source for the 350–400 sq ft per gallon figure and room-size guidance.
- Lowe’s. “Paint Calculator.” Reference for wall measurement and coat-count estimating.
- HowStuffWorks. “Estimating Interior Paint.” Background on surface conditions and primer coverage rates.