Paint Calculator: Interior Sq Ft Coverage

This paint calculator works out how much paint you need for an interior room or wall by taking the total square footage you plan to cover, the number of coats, and the paint’s coverage rate per gallon. You enter the square footage of the walls (and ceiling if wanted), choose how many coats, and the calculator returns the exact gallons (and quarts) to buy. It uses the standard industry coverage of 350 square feet per gallon for one coat, which most major paint brands list on their labels.

Formula

The core formula is simple: Paint needed (gallons) = (Area to paint × Number of coats) ÷ Coverage per gallon. Each variable is defined as:

• Area to paint (sq ft) – the total wall and/or ceiling surface you intend to cover, measured in square feet.
• Number of coats – how many layers of paint you plan to apply (usually 2 for a color change, 1 for a touch-up).
• Coverage per gallon (sq ft/gal) – the manufacturer’s stated spread rate, typically 350–400 sq ft per gallon for interior latex paint on smooth, primed drywall.

The result is in gallons, with a secondary output in quarts for smaller jobs. If you’re working in meters, convert your area to square feet first (1 m² = 10.764 sq ft) or use a metric coverage rate (usually 10–12 m² per liter).

Worked Example

Using the calculator’s default values: a 400 sq ft room (e.g., a 12′ × 15′ bedroom with 8′ ceilings, minus doors and windows), 2 coats, and 350 sq ft/gal coverage.

1. Multiply area by coats: 400 × 2 = 800 sq ft of total surface to cover.
2. Divide by coverage: 800 ÷ 350 = 2.2857 gallons.
3. Round up to the nearest quart: 2.29 gallons ≈ 2 gallons + 1 quart (since 0.29 gal ≈ 1.16 quarts).

The calculator gives 2.29 gallons (or 9.14 quarts). In practice, you’d buy 2 full gallons and 1 quart, which gives you a small surplus for touch-ups – always better than running short mid-project.

How to Measure Your Interior Square Footage

Getting the area right is the hardest part. For walls, measure the length of each wall and multiply by the ceiling height. Add all wall areas together. Then subtract the areas of doors (about 20 sq ft each) and windows (about 15 sq ft each). For a ceiling, simply multiply the room’s length by its width. For a room that is 12 ft by 15 ft with 8 ft ceilings:

• Two long walls: 2 × (15 × 8) = 240 sq ft
• Two short walls: 2 × (12 × 8) = 192 sq ft
• Total walls = 432 sq ft; subtract one door (20) and two windows (30) = 382 sq ft
• Ceiling = 12 × 15 = 180 sq ft (if painting it)

Add wall and ceiling areas together if you’re doing both. For irregular rooms, break them into rectangles and sum the pieces.

Common Mistakes and Misconceptions

The most frequent error is forgetting to multiply by the number of coats. A single gallon covers 350 sq ft, but two coats cover only 175 sq ft per gallon. Another mistake is using the floor area as the wall area – a 400 sq ft floor does not mean 400 sq ft of walls; walls are usually 2–3 times that. Also, don’t ignore the coverage rate on the can: premium paints often cover more (400 sq ft/gal) while textured surfaces or unprimed drywall absorb more (250–300 sq ft/gal). Finally, always round up to the nearest quart or gallon. Paint calculators that give exact decimals are theoretical; real-world waste from roller absorption, spills, and touch-ups means you need 5–10% extra.

When to Use This Calculator vs. a Paint Store’s Estimator

This calculator is ideal for planning a DIY project at home: you know the room dimensions, you can measure quickly, and you want a rough gallon count before going to the store. Paint store estimators (online or in-store) often ask for the same inputs but may add extra factors like primer, ceiling texture, or trim color. Use this tool for a fast, transparent calculation. For a large or complex job (multiple rooms, vaulted ceilings, or heavy texture), a store’s estimator or a professional’s on-site assessment is worth the extra time. The calculator’s formula is the same one most pros use – it’s the inputs that matter.

Cost and Buying Considerations

While this calculator doesn’t include prices, you can estimate cost by multiplying the gallons by the price per gallon. Interior latex paint ranges widely in price, but a mid-range gallon typically covers 350–400 sq ft and costs more than a budget brand while offering better durability and easier cleaning. For a 400 sq ft room with 2 coats (2.29 gallons), you’d buy 3 gallons (2 full + 1 quart, or just 3 gallons if you want a uniform can). Buying a quart for touch-ups is usually cheaper than a full gallon if you only need a small amount. Remember that primer is a separate coat – if you’re covering a dark color or new drywall, add a primer coat to your total.

Safety and Preparation Tips

Before painting, ensure the room is well-ventilated – open windows and use fans. Water-based latex paints have low VOCs, but still avoid prolonged inhalation. Use a drop cloth to protect floors, and tape trim with painter’s tape. For surfaces with lead paint (homes built before 1978), do not sand; test for lead or hire a professional. Always stir paint thoroughly and pour into a tray – don’t paint directly from the can. Clean brushes and rollers with soap and water for latex paint, and dispose of leftover paint according to local regulations – never pour it down drains.

Frequently Asked Questions

How many square feet does a gallon of paint cover? Most interior latex paints cover 350–400 square feet per coat on smooth, primed surfaces. Textured walls, unpainted drywall, or highly porous surfaces reduce coverage to 250–300 sq ft per gallon. Always check the label on the specific can you buy.

Do I need 2 coats of paint on interior walls? Yes, for most color changes or when painting over a lighter or darker shade, two coats are recommended. One coat may suffice if you’re painting the same color or using a high-coverage paint over a similar tone, but two coats ensure even color and durability.

How do I calculate paint for a room with a ceiling? Measure the ceiling’s length and width, multiply them for the area, then add that to your wall area. For example, a 12×15 room has a 180 sq ft ceiling. Add it to the wall area (after subtracting doors/windows) and then multiply by coats and divide by coverage.

Should I round up or down when buying paint? Always round up to the nearest quart or gallon. Paint calculators give exact numbers, but real-world waste (roller absorption, spills, touch-ups) means you’ll need 5–10% more. It’s cheaper to have a little extra than to make a second trip to the store.

Does primer count as a coat in the calculator? No, the calculator is for paint only. If you’re priming bare drywall, dark colors, or stained surfaces, add a separate primer coat. Primer typically covers 300–400 sq ft per gallon, so run the calculator again with your area and 1 coat, using the primer’s coverage rate.

What if my room isn’t a perfect rectangle? Break the room into rectangles, measure each, and add the areas. For triangular or angled sections, use the formula (base × height) ÷ 2. Sum all sections for the total area, then enter that into the calculator. This works for walls and ceilings alike.

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in area | Area to paint | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per gallon | sq ft/gal | 350 | 1
out gallons | Paint needed | gallons | area * coats / coverage | 2
out quarts | Paint needed | quarts | (area * coats / coverage) * 4 | 1