Area

Coats

Wall area (sq ft)

Doors

Windows

Coverage per gallon (sq ft)

Waste factor (%)

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Coverage per gallon on smooth, textured and unprimed walls — Bare drywall can drop coverage from 350 to as low as 200 sq ft per gallon.

Method & verification

How This Tool Works

Calculation Method

Computes Paint needed from Area, Coats, Wall area, Room length, Room width, Ceiling height, Doors, Windows, Coverage per gallon and Waste factor.

Site methodology

BoringToolsKit methodology and verification standard

No unrelated authority link is added merely to make this page look sourced. Tool-specific external sources are shown only when they directly support the method, rate, or rule.

Assumptions to Verify

Measurements describe the simplified geometry entered.
Verify waste, package yield, product specifications, and supplier minimums before purchase.

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Worked example: a 12 x 14 bedroom

Take a 12 x 14 ft bedroom with 8 ft ceilings, one door, and two windows. Wall area is the perimeter times height, doubled for opposite walls: 2 x (12 + 14) x 8 = 416 sq ft. The door subtracts 20 sq ft and each window 15 sq ft, so 416 - 20 - 30 = 366 sq ft of paintable wall.

Two coats with 10% waste makes the coverage demand 366 x 2 x 1.10 = 805.2 sq ft. At the standard 350 sq ft per gallon rating, that is 805.2 / 350 = 2.30 gallons. Since paint only sells in whole gallons, the calculator rounds up to 3. Note the rounding gap: buying 2 gallons leaves you about 105 sq ft short, roughly a third of a coat, which is exactly the shortfall that forces a mid-job store run.

Change one variable and the answer moves fast. Drop to one coat on a repainted wall in the same color and demand falls to 402.6 sq ft, still 2 gallons. Skip the waste allowance and 2 coats need 732 sq ft, or 2.09 gallons, so rounding up to 3 hides the fact that you are only 18 sq ft over two gallons. That is why the exact coverage line in the result matters: 2.09 versus 2.30 is the difference between 'two gallons plus touch-up' and 'definitely three'.

Where the 350 sq ft rating breaks down

The 350 sq ft per gallon figure is a smooth-wall ideal from manufacturer coverage charts. Real numbers run lower: heavily textured walls knock coverage down 20 to 25%, so budget closer to 275 sq ft per gallon. Bare new drywall is worse still; the first coat soaks into the porous compound and joints, often cutting effective coverage to 200 to 250 sq ft per gallon unless you prime first. Dark paint over a light wall typically needs a tinted gray primer plus two finish coats, effectively three coverage passes.

Other costs the formula does not see. The 20 sq ft per door and 15 sq ft per window defaults are averages; a 3 ft x 6 ft 8 in door slab is 20 sq ft flat, but wide patio doors or picture windows can be 40 sq ft or more, so measure anything oversized. Ceilings are excluded entirely: paint them separately at about length x width, using ceiling paint rated around 300 to 350 sq ft per gallon. Finally, coverage ratings assume 40 to 70 degrees F and a rolled application; spraying loses 10 to 30% to overspray depending on masking, and cold paint can add a full extra coat through poor flow.