Paint Area Calculator: How Much Paint Do You Need?

This paint area calculator works out the total wall and ceiling area you need to cover in square feet or square meters, and then estimates how many gallons or liters of paint to buy. You enter the room’s dimensions, the number of doors and windows, and the number of coats you plan to apply; the calculator returns the paintable surface area and the recommended paint volume. It uses standard coverage rates (about 350 sq ft per gallon, or 8.6 m² per liter) and subtracts typical door and window areas so you don’t overbuy.

Formula

The calculator uses two standard formulas, one for area and one for paint volume.

Wall area: 2 × height × (length + width) gives the total wall area of a rectangular room. Then subtract 21 sq ft for each door and 15 sq ft for each window (standard assumptions). Add the ceiling area, which is length × width.

Paint volume: Total area × number of coats ÷ coverage rate. The coverage rate is 350 square feet per gallon (about 8.6 m² per liter) for one coat of typical latex paint on primed drywall.

• l = room length (ft)
• w = room width (ft)
• h = wall height (ft)
• d = number of doors (each ≈ 21 sq ft)
• win = number of windows (each ≈ 15 sq ft)
• coats = number of coats (usually 2)

Worked Example

Using the default values: length = 12 ft, width = 10 ft, height = 8 ft, doors = 2, windows = 1, coats = 2.

1. Wall area = 2 × 8 × (12 + 10) = 16 × 22 = 352 sq ft
2. Subtract doors: 2 × 21 = 42 sq ft; windows: 1 × 15 = 15 sq ft. Net walls = 352 − 42 − 15 = 295 sq ft
3. Ceiling area = 12 × 10 = 120 sq ft
4. Total area = 295 + 120 = 415 sq ft
5. Paint needed = 415 × 2 ÷ 350 = 830 ÷ 350 = 2.37 gallons

The calculator rounds to 2.4 gallons, so you’d buy 3 gallons for the job.

How to Measure Your Room Correctly

Accurate measurements are the key to a useful estimate. Use a tape measure and record dimensions to the nearest half-foot.

• Measure the length and width of the room at floor level, from wall to wall.
• Measure wall height from floor to ceiling, not including baseboards or crown molding.
• If the room has sloped ceilings, measure the average height or break the wall into rectangles and triangles.
• Count doors and windows that are fully inside the wall area you’re painting. Skip half-walls or openings without doors.
• For ceilings, use the same length × width; if the ceiling is vaulted, calculate the actual surface area (not just the floor projection).

Common Mistakes and How to Avoid Them

Even with a calculator, people often make errors that lead to buying too much or too little paint.

• Forgetting to subtract openings: Each door can be 20–25 sq ft and each window 10–15 sq ft. Skipping them can overestimate by 10% or more.
• Using interior dimensions for exterior walls: If you’re painting the outside, measure the exterior wall area, not the interior room size.
• Ignoring texture and porosity: Rough surfaces (stucco, brick, unpainted drywall) absorb more paint. Coverage can drop to 200–250 sq ft per gallon.
• Rounding too early: Do all calculations in one step, then round up to the nearest quart or gallon. Buying a fraction of a gallon is not possible, so always round up.
• Mixing units: If you measure in meters, use the metric coverage (8.6 m² per liter). Don’t mix feet and meters in the same calculation.

How to Read the Result and Adjust for Your Situation

The output “paint needed� is the minimum for the stated conditions. Real-world jobs often require a bit more.

• If you’re painting a dark color over a light one, you may need an extra coat (3 instead of 2).
• If the wall is unprimed bare drywall, add a primer coat (which is not counted in the calculator).
• If you’re using a sprayer, you’ll lose 10–20% more paint to overspray compared to a roller.
• If the room has many corners, trim, or built-ins, you’ll need extra for cutting in.
• Always buy an extra quart or half-gallon for touch-ups later.

Why Coverage Rates Vary

The 350 sq ft per gallon figure is a typical manufacturer rating for one coat on smooth, primed drywall. Actual coverage depends on:

• Surface texture: Smooth surfaces cover best; textured or rough surfaces need more.
• Paint type: Higher-quality paints often have more solids and cover better. Flat finishes cover more than glossy ones.
• Application method: Rollers and brushes use less paint than sprayers.
• Color change: Going from light to dark or dark to light may require extra coats.
• Primer: Using a primer first reduces the number of topcoats needed.

Alternatives and When to Use a Professional Estimate

This calculator is fine for a single room or a simple rectangular space. For complex projects, consider other methods:

• Use a laser distance measure for large or irregular rooms.
• For multi-room jobs, calculate each room separately and sum the totals.
• For exterior painting, measure the perimeter and height, then subtract large windows and doors.
• If you’re hiring a contractor, they will often provide a free on-site estimate that accounts for surface conditions and access.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most paint labels say one gallon covers about 350–400 square feet with one coat on smooth, primed drywall. That’s the standard used in this calculator. For rough or unprimed surfaces, expect less—around 250–300 sq ft per gallon.

Should I include the ceiling in the paint area?

Only if you plan to paint the ceiling. The calculator includes the ceiling by default because most people paint it at the same time. If you’re only painting walls, subtract the ceiling area from the total.

How do I calculate paint for a room with sloped ceilings?

Measure the wall height at the tallest point and the shortest point, then use the average. For the ceiling itself, calculate the actual surface area of the slope (length × sloped length), not the horizontal projection. You may need to break the room into rectangles and triangles.

Why does the calculator say 2.4 gallons but the store sells 1-gallon cans?

You round up to the next full gallon. For 2.4 gallons, you’d buy 3 gallons. It’s better to have a little extra than to run out mid-project, and you can always return unopened cans.

Does primer count as a coat?

No, primer is separate. The calculator assumes you’re painting over a primed or previously painted surface. If you need primer, add one coat of primer to your total paint volume, or use a paint-and-primer-in-one product if the label says it covers bare drywall.

What if my room isn’t a perfect rectangle?

Break the room into rectangular sections, calculate each separately, and add them together. For L-shaped rooms, treat each leg as its own rectangle. For alcoves or bays, add their wall and ceiling areas to the main room.

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in l | Room length | ft | 12 | 0
in w | Room width | ft | 10 | 0
in h | Wall height | ft | 8 | 0
in d | Number of doors | - | 2 | 0
in win | Number of windows | - | 1 | 0
in coats | Number of coats | - | 2 | 1
out area | Total paintable area | sq ft | (2*h*(l+w) - d*21 - win*15) + l*w | 0
out gallons | Paint needed | gallons | area * coats / 350 | 1