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Paint Calculator: How Much Paint Do I Need?

Enter your room's dimensions and we'll tell you exactly how many gallons to buy.

Check your paint can's label — most are 250–450.
Paint needed1.75 galBuy 1 gallon + 3 quarts
Paintable area302 sq ft

How it works

We start from the total wall area — twice the sum of the room's length and width, times the wall height — then subtract space for doors (20 sq ft each) and windows (15 sq ft each), since you don't paint over them. If you're painting the ceiling too, we add the floor area (length × width) on top. Multiply that paintable area by your number of coats, divide by your paint's coverage rate, and round up to the nearest quart — that's your gallon count.

Example

A 12 ft × 10 ft bedroom with 8 ft walls, 1 door, 2 windows, 2 coats, and no ceiling: wall area is 2 × (12 + 10) × 8 = 352 sq ft. Subtract 1 door (20 sq ft) and 2 windows (30 sq ft) for a paintable area of 302 sq ft. At 2 coats and 350 sq ft/gal coverage, that's (302 × 2) ÷ 350 ≈ 1.73 gallons, rounded up to 1.75 gallons — buy 1 gallon + 3 quarts.

Tips

  • Buy slightly more than the calculator says if your walls are textured, unprimed, or a much darker/lighter color than the new paint — rough or high-contrast surfaces soak up more paint.
  • Keep the receipt and extra can lid label; most stores can color-match a top-up months later if you measure a bit short.
  • Primer is a separate budget line — bare drywall, patches, and big color changes usually need one coat of primer first.
  • Buying one 5-gallon bucket instead of several 1-gallon cans avoids visible color variation between batches on large walls.
  • Stir thoroughly before and during the job — settled pigment at the bottom of the can changes the sheen if you don't mix it back in.

Common mistakes

  • Forgetting to subtract doors and windows, which overestimates paint needed by 10–20% in an average bedroom.
  • Using the coverage number printed on a different paint line than the one you're actually buying — always check the specific can.
  • Skipping the ceiling calculation and then running out of wall paint because leftover ceiling paint got used on walls (or vice versa).
  • Not accounting for a second coat — one coat almost never gives even coverage over a color change.
  • Rounding down "to save money" and ending up short mid-wall, which forces a rushed, poorly-matched extra store run.

FAQ

Does this calculator account for doors and windows?
Yes. It subtracts 20 sq ft of wall area per door and 15 sq ft per window by default, since you don't paint over openings. You can adjust these in the advanced settings if your doors or windows are unusually large or small.
Should I count the ceiling separately?
Ceilings are usually a different paint (flat white) and a different coverage rate than walls. Check "Include ceiling" to add it to this calculation, or run the calculator a second time with the ceiling's own square footage if you want a separate gallon count.
Why does the calculator round up to a 5-gallon bucket sometimes?
Once a job needs more than 4 gallons, a single 5-gallon bucket is almost always cheaper per gallon than buying five separate 1-gallon cans, and you avoid tinting-batch color differences between cans.
What if my paint covers less than 350 sq ft per gallon?
Coverage varies by product and by how porous or patched the wall is — cheaper paints and unprimed drywall repairs often cover closer to 250–300 sq ft/gal. Enter the number from your paint can's label in the coverage field for the most accurate result.

Related guides

Paint a Room Like a Pro

Step-by-step technique for clean lines and even coats.

Cost to Repaint a Room

What painters actually charge, and where the money goes.

Best Paint Sprayers

Buying guide for bigger jobs.

Estimates only. Always check your specific paint product's coverage rate and buy a little extra for touch-ups. Local building codes may apply to primers/paints used in certain rooms (e.g. low-VOC requirements) — check before you buy.