Drywall Paint Calculator: How Much Paint You Need

This drywall paint calculator works out how many gallons (or liters) of paint you need for a room’s drywall walls and ceiling. You enter the room’s dimensions, the number of doors and windows, and the number of coats you plan to apply; it returns the total paintable area and the required paint volume, including a 10% waste allowance. The result is an estimate, not a guarantee—actual coverage varies with paint type, application method, and surface porosity.

Formula

The calculator uses the standard coverage rate of 350 square feet per gallon per coat, which is the typical figure quoted by most paint manufacturers for smooth, primed drywall. The formula is:

Total paintable area = (2 × height × (width + length)) − (doors × 21) − (windows × 15) + (width × length)

Then:

Paint needed (gallons) = (total area × number of coats / 350) × 1.1

Where:

• width, length, height – room dimensions in feet (or meters if you use the metric equivalent of 350 sq ft/gal ≈ 8.6 m²/L).
• doors – each standard interior door is assumed to be 21 square feet (about 2.0 m²).
• windows – each standard window is assumed to be 15 square feet (about 1.4 m²).
• coats – number of paint layers you plan to apply (usually 2 for a color change, 1 for repainting same color).
• 350 – coverage in square feet per gallon for one coat.
• 1.1 – 10% waste factor for spillage, touch-ups, and uneven application.

Worked Example

Using the default values: width = 12 ft, length = 14 ft, height = 8 ft, doors = 2, windows = 3, coats = 2.

1. Perimeter walls area: 2 × 8 × (12 + 14) = 16 × 26 = 416 sq ft.
2. Subtract doors: 2 × 21 = 42 sq ft. Subtract windows: 3 × 15 = 45 sq ft. So net wall area = 416 − 42 − 45 = 329 sq ft.
3. Ceiling area: 12 × 14 = 168 sq ft.
4. Total paintable area = 329 + 168 = 497 sq ft.
5. For 2 coats: 497 × 2 = 994 sq ft.
6. Divide by 350: 994 / 350 = 2.84 gallons.
7. Add 10% waste: 2.84 × 1.1 = 3.12 gallons.

The calculator rounds to 3.12 gallons (11.8 liters). In practice, you’d buy 4 gallons—you can’t buy a third of a gallon, and having a bit extra is useful for touch-ups.

How to Use the Calculator

Measure the room’s width, length, and height in feet (or meters). Count the number of standard doors and windows—skip this if you’re only painting walls, not ceilings. Choose the number of coats: 1 for a same-color repaint, 2 for a dramatic color change or when covering a darker color with a lighter one, and sometimes 3 for very dark to very light transitions. The calculator assumes you’re painting both walls and ceiling; if you’re only painting walls, subtract the ceiling area manually (or set the ceiling contribution to zero by using a width or length of 0, but that will skew the wall area—better to just subtract the ceiling square footage from the result).

The result is in gallons and liters. Paint is sold in quarts (0.25 gal), gallons, and sometimes 5-gallon buckets. Round up to the nearest available container size.

When to Use This Estimate

This calculator is ideal for planning a paint purchase for a single room or a whole house. It gives you a quick ballpark figure to budget and to avoid buying too much or too little. It works best for rooms with standard 8-foot ceilings and typical window/door sizes. For rooms with vaulted ceilings, large picture windows, or sliding glass doors, adjust the window/door square footage accordingly—or just measure the actual openings and subtract those numbers.

Common Mistakes and Misconceptions

• Ignoring the number of coats: The biggest error. Painting a dark wall white often needs two or three coats; the calculator accounts for that, but many people forget to change the coats input.
• Using the wrong coverage rate: 350 sq ft/gal is a typical average, but high-quality paints may cover 400 sq ft/gal, while cheap paints or rough surfaces (unprimed drywall, stucco) may cover only 250–300 sq ft/gal. If you know your paint’s specific coverage, adjust the result proportionally (multiply by 350/actual coverage).
• Forgetting the ceiling: If you’re painting the ceiling too, it adds significant area. Many first-timers only measure walls.
• Not adding waste: Paint gets spilled, dripped, and absorbed unevenly. The 10% waste factor is a safety margin—it’s not excessive.
• Rounding down: Always round up to the nearest quart or gallon. Running out of paint mid-project means a trip to the store and potentially a slight color mismatch if the batch differs.

Limitations of the Formula

This formula is a simplified model. Real-world paint coverage depends on:

• Surface porosity: Fresh drywall with joint compound absorbs much more paint than a previously painted, primed surface. Primer is recommended for new drywall—this calculator assumes the surface is primed or previously painted.
• Application method: Spraying wastes more paint than rolling (often 20–30% overspray). Brushes and rollers give more control but may leave a thicker coat.
• Paint quality: Higher-quality paints often have more solids and better hiding power, so they may cover more area per gallon.
• Texture: Textured walls (e.g., orange peel, knockdown) have more surface area than smooth walls, so they need more paint.

For new drywall, always use a primer first—it seals the surface and reduces the number of topcoats needed. The calculator does not include primer; add it separately using the same area but with a coverage rate of about 300 sq ft/gal for primer.

Alternative Approaches

If you don’t want to use this calculator, you can:

• Use the paint can’s label: Most cans list coverage in sq ft per gallon. Divide your total area by that number.
• Use an online paint estimator from a paint brand: Many manufacturers offer their own calculators, but they often require more inputs (like number of windows, doors, and ceiling height) and may push their own products.
• Ask a paint store professional: Bring your room measurements to a local paint store, and they can recommend a quantity. This is helpful if you’re using a specialty paint (e.g., limewash, chalk paint) with different coverage.
• Use a simple rule of thumb: For a 10×12 room with 8-foot ceilings, one gallon of paint covers about 400 sq ft, so you’d need about 1 gallon per coat for the walls plus a bit for the ceiling. But this rough method is less accurate than the calculator.

Frequently Asked Questions

Q: How much paint do I need for a 12×12 room?
A: Using the calculator with width=12, length=12, height=8, 1 door, 1 window, and 2 coats, the area is (2×8×24 − 21 − 15) + 144 = 384 − 36 + 144 = 492 sq ft. For 2 coats, that’s 984 sq ft, divided by 350 = 2.81 gallons, plus 10% = 3.09 gallons. So you’d buy 4 gallons.

Q: Does one gallon of paint cover a 12×12 room?
A: One gallon covers about 350 sq ft per coat. A 12×12 room with 8-foot walls has about 384 sq ft of wall area (minus doors/windows) plus a 144 sq ft ceiling. So one gallon might cover the walls with one coat, but you’ll need more for the ceiling or a second coat. Usually you need at least 2 gallons for a single coat of walls and ceiling.

Q: How many square feet does a gallon of paint cover?
A: The typical coverage is 350 sq ft per gallon for one coat on smooth, primed drywall. This varies by paint brand and quality; check the label. Some paints cover 400 sq ft, while others only 250 sq ft.

Q: Should I include the ceiling in my paint calculation?
A: Yes, if you’re painting the ceiling. Ceilings are usually painted with a flat white paint, and they add significant area. For a 12×14 room, the ceiling is 168 sq ft—that’s almost half a gallon per coat.

Q: What is the 10% waste factor for?
A: It accounts for paint lost to spills, drips, roller absorption, and touch-ups. It’s a safety margin; you can reduce it to 5% if you’re an experienced painter, but for DIY, 10% is wise.

Q: Does primer count as a coat?
A: No, primer is separate. Primer seals the drywall and provides a uniform surface for the topcoat. It typically covers about 300 sq ft per gallon. Apply primer first, then the number of topcoats you selected. The calculator does not include primer—add it separately.

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in w | Room width | ft | 12 | 0
in l | Room length | ft | 14 | 0
in h | Wall height | ft | 8 | 0
in d | Number of doors | - | 2 | 0
in win | Number of windows | - | 3 | 0
in coats | Number of coats | - | 2 | 1
out area | Total paintable area | sq ft | (2*h*(w+l) - d*21 - win*15) + (w*l) | 0
out gallons | Paint needed | gallons | (area * coats / 350) * 1.1 | 2
out liters | Paint needed | liters | ((area * coats / 350) * 1.1) * 3.78541 | 2