Wall Paint Calculator: Square Feet Coverage

This calculator works out how much wall paint you need based on the total square footage of the walls you plan to paint. Enter the wall area in square feet (or square meters) and the number of coats, and it returns the required paint volume in gallons and liters, assuming standard paint coverage.

Formula

The calculation uses a simple formula:

Paint needed (gallons) = (Total wall area in square feet × Number of coats) ÷ Coverage per gallon

Where:

• Total wall area – the sum of all wall surfaces to be painted, in square feet (sq ft).
• Number of coats – how many layers of paint you plan to apply (usually 2 for a color change, 1 for repainting the same color).
• Coverage per gallon – the area one gallon covers with one coat, typically 350 sq ft for most quality latex paints (the calculator uses 350).

For metric: 1 gallon ≈ 3.78541 liters, and 350 sq ft ≈ 32.5 square meters. The calculator converts the result to liters automatically.

Worked Example

Using the default values from the calculator (wall area = 400 sq ft, coats = 2):

1. Multiply area by coats: 400 × 2 = 800 sq ft of total surface to cover.
2. Divide by coverage per gallon: 800 ÷ 350 = 2.2857 gallons.
3. Convert to liters: 2.2857 × 3.78541 = 8.65 liters.

The calculator shows 2.29 gallons (or 8.65 liters) as the answer. That is the amount of paint you need for two coats on 400 sq ft of wall.

How to Measure Your Wall Area

To get an accurate square footage, measure each wall separately and add them together. For a rectangular room:

• Measure the length and height of each wall in feet.
• Multiply length × height for each wall to get its area.
• Add all wall areas together.

Example: A room with four walls – two walls 12 ft long and 8 ft high (96 sq ft each) and two walls 10 ft long and 8 ft high (80 sq ft each) – has a total wall area of 96 + 96 + 80 + 80 = 352 sq ft.

Subtract openings: For each window (typically 15 sq ft) and each door (typically 20 sq ft), subtract that area from the total. If you are painting a wall with a large window, measure the window and deduct it.

Why Coverage Varies: Paint Type and Surface

The 350 sq ft per gallon figure is a rough average. Actual coverage depends on several factors:

• Paint quality: Higher-quality paints often have more solids and cover better, sometimes reaching 400 sq ft per gallon.
• Surface texture: Smooth, primed drywall covers more than rough stucco or textured plaster, which can reduce coverage to 200–250 sq ft per gallon.
• Porosity: New drywall or bare wood absorbs more paint, so you may need extra for the first coat.
• Color change: Painting a light color over a dark one usually requires an extra coat or a primer.

Common Mistakes and How to Avoid Them

People often underestimate paint needs because they forget a few key things:

• Forgetting to subtract openings – but be careful: it is safer to subtract only large openings (windows, doors) and ignore small ones like outlets.
• Not accounting for coats – the calculator includes coats, but if you are using a primer, that is a separate coat and should be added.
• Rounding down – always round up to the nearest quart or gallon. You can return unused paint, but running out mid-project is a hassle.
• Mixing units – if you measure in meters, convert to feet (1 m = 3.28084 ft) before using the square-foot formula.

Limitations of the Calculator

This calculator gives an estimate, not a guarantee. It does not account for:

• Waste due to spills, brush marks, or touch-ups (add 10% extra).
• Special surfaces like brick, which may need 2–3 times more paint.
• Primer requirements – primer is a separate product and should be calculated separately.
• Ceilings and trim – those are separate areas and need their own calculations.

For most interior walls, the estimate is accurate within 10–15%. If you are painting a large area or using expensive paint, it is wise to buy one extra gallon and keep the receipt.

Frequently Asked Questions

How many square feet does a gallon of paint cover?
A gallon of most interior latex paint covers about 350–400 square feet with one coat. The calculator uses 350 sq ft, which is a safe conservative estimate for typical drywall.

Do I need to subtract windows and doors?
Yes, subtract the area of large openings. A standard door is about 20 sq ft, a standard window about 15 sq ft. However, if you are painting a wall with many small windows, it is easier to measure the wall and subtract each window individually.

How many coats of paint do I need?
Usually two coats for a color change or when painting over a darker shade. If you are repainting the same color and the surface is in good condition, one coat may suffice. Primer counts as a coat.

What if I am painting a textured wall?
Textured surfaces absorb more paint. Reduce the coverage to about 250 sq ft per gallon, or increase the paint amount by 30–40%.

Should I buy more paint than the calculator says?
It is a good idea to add 10% for waste and touch-ups. If the calculator says 2.29 gallons, buy 2.5 gallons (or 3 gallons if you prefer to have extra).

Does the calculator work for exterior walls?
Yes, but exterior surfaces like wood, brick, or stucco have lower coverage. Use a coverage factor of 200–250 sq ft per gallon for those materials.

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in area | Wall area | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
out gallons | Paint needed | gallons | (area * coats) / 350 | 2
out liters | Paint needed | liters | ((area * coats) / 350) * 3.78541 | 2