Paint Calculator

How this calculation works

Wall area = room perimeter x wall height, minus roughly 20 sq ft per standard door and 15 sq ft per standard window. Divide the net wall area by your paint's labeled coverage rate (350-400 sq ft/gallon for most latex wall paint; some premium and eco-formula paints run lower, closer to 250-300 sq ft/gallon, so check your specific can) to get gallons needed for one coat.

Then multiply by coat count. Two coats is the safe default for any color change, and mandatory when you're going from a dark wall to a light one or vice versa — a single coat of white over red or navy will look patchy no matter how good the paint is. If you're repainting the same or a very similar color, one coat can be enough, but budget a touch-up gallon regardless.

• Primer changes the math: unprimed drywall or a big color swing (dark-to-light) usually needs a primer coat first, which is priced and bought separately from your finish coat.
• Textured walls (orange peel, knockdown) eat 10-15% more paint than smooth drywall because of the extra surface area in the texture.
• Always round up to the next full gallon — paint stores don't sell partial gallons, and running out mid-wall means a dry-time delay while you go back for more.

Common mistakes

• Assuming 'one gallon per room' regardless of room size — a 10x10 bedroom and a 16x20 great room need very different amounts; see how much paint does a room really take for why that rule of thumb fails.
• Forgetting to subtract door and window area, which overstates gallons needed on rooms with a lot of glass or multiple doorways.
• Buying exactly the calculated amount with zero cushion — a small pour-off for touch-ups later (especially with custom-tinted colors that are hard to rematch exactly) is worth the extra few dollars.
• Skipping primer on new drywall to save a trip, then needing three finish coats instead of two to get even coverage.
• Not checking the can's actual coverage figure before calculating — budget and premium lines from the same brand can have meaningfully different coverage rates, so a number pulled from memory of a past project can be off for this specific product.
• Painting over glossy trim or a slick prior finish without scuff-sanding or a bonding primer first, which can cause peeling regardless of how accurately the gallon count was calculated.

Shopping summary

Buy in whole gallons (or quarts only for very small rooms/closets), one can per coat calculated above, plus a primer gallon if you're covering new drywall or making a major color change. If your room's walls are a single custom-mixed color, buy it all in one batch — different mixing batches of the same color name can shade slightly differently.

Worked example: a 12x14 bedroom with an 8-foot ceiling has a perimeter of 52 feet, so gross wall area is 52 x 8 = 416 sq ft. Subtract one door (20 sq ft) and two windows (15 sq ft each, 30 sq ft total) for a net area of 366 sq ft. At 375 sq ft/gallon coverage for two coats, that's (366/375) x 2 ≈ 1.95 gallons — round up to two gallons, which leaves a small margin for touch-ups without buying a third can.

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