House Paint Calculator: Square Feet Coverage Guide

This house paint calculator estimates the total square feet of paintable surface area and the amount of paint needed for your project. You enter the dimensions of each wall (or the total wall area), the number of doors and windows, and the number of coats you plan to apply. The calculator then returns the net paintable area and the gallons of paint required, based on standard coverage rates.

Formula

The calculator uses two simple formulas. First, it finds the gross wall area by multiplying the total wall length by the wall height. Then it subtracts the area taken up by doors and windows, because those surfaces are not painted. The net area is then multiplied by the number of coats and divided by the paint coverage rate (typically 350 square feet per gallon).

In symbols:

• Net Area (sq ft) = (Wall Length × Wall Height) − (Number of Doors × 20) − (Number of Windows × 15)
• Gallons Needed = (Net Area × Coats) ÷ 350

Where:

• Wall Length = total length of all walls to be painted, in feet
• Wall Height = distance from floor to ceiling, in feet
• Doors/Windows = count of each; each door subtracts 20 sq ft, each window subtracts 15 sq ft (standard estimates)
• Coats = number of paint layers you plan to apply
• 350 = average coverage of one gallon of paint on smooth, primed drywall, in square feet per gallon

Worked Example

Using the default values from the calculator: wall length = 60 ft, wall height = 9 ft, doors = 2, windows = 3, coats = 2.

1. Gross wall area = 60 × 9 = 540 sq ft
2. Door area = 2 × 20 = 40 sq ft
3. Window area = 3 × 15 = 45 sq ft
4. Net paintable area = 540 − 40 − 45 = 455 sq ft
5. Total area to cover (with 2 coats) = 455 × 2 = 910 sq ft
6. Gallons needed = 910 ÷ 350 = 2.6 gallons

So for this room, you would need to buy 3 gallons of paint (since paint is sold in whole gallons).

When to Use This Calculator

Use this calculator when you are planning to paint a room, a hallway, or a whole interior floor. It works best for rooms with standard 8–10 foot ceilings and typical door/window sizes. It is also useful when comparing paint quantities for different rooms or when budgeting for a paint job.

For exterior painting, the same formula applies, but you would measure the perimeter of the house and the height of the walls, and subtract larger areas for garage doors and large windows. For a more precise estimate, measure each wall individually and add them together.

How to Measure Your Walls Correctly

Accurate measurements are the key to a good estimate. Here is how to get them:

• Measure the length of each wall at the baseboard, using a tape measure. Add all wall lengths together to get the total wall length.
• Measure the height from the floor to the ceiling. If the ceiling is sloped or vaulted, measure the highest point and use that as a rough average.
• Count all doors and windows that are fully within the walls you are painting. For each door, subtract 20 sq ft; for each window, subtract 15 sq ft. These are standard averages for interior doors (about 3 ft × 6.5 ft) and windows (about 3 ft × 5 ft).
• If you have a large picture window or a sliding glass door, measure its actual area and subtract that instead of the standard estimate.

Common Mistakes and How to Avoid Them

One frequent mistake is forgetting to subtract doors and windows. That can overestimate paint needs by 10–15%. Another is using the wrong coverage rate: 350 sq ft per gallon is a good average for smooth, primed drywall, but rough surfaces (textured plaster, brick, unpainted wood) can absorb much more – sometimes only 200–250 sq ft per gallon. In that case, buy extra paint or apply a primer first.

Also, remember that the number of coats matters. A dark color over a light primer may need two coats; a light color over a dark wall may need three. The calculator lets you adjust the coats, but it does not account for touch-ups, which typically require about 10% extra paint. It is wise to round up to the nearest gallon.

Finally, the calculator assumes you are painting all walls the same color. If you are using multiple colors, calculate each wall group separately.

Understanding Paint Coverage Rates

Paint coverage is usually listed on the can as “350–400 sq ft per gallon.� This number is based on a smooth, non-porous surface applied with a roller at the recommended thickness. In practice, coverage varies with:

• Surface texture: Smooth drywall covers more; stucco, brick, or textured plaster covers less.
• Porosity: New drywall and unprimed wood absorb more paint.
• Application method: Spraying uses more paint than rolling, and brushing uses the least but takes longer.
• Color change: Going from dark to light (or vice versa) may require extra coats.

For most interior rooms, using 350 sq ft per gallon is a safe, conservative estimate. If you are using a high-quality paint with a primer built in, you might get closer to 400 sq ft per gallon on a second coat.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

On average, one gallon of paint covers about 350 square feet of smooth, primed surface. For rough or porous surfaces, coverage can drop to 250 square feet or less. Always check the label on the paint can for the manufacturer’s specific coverage rate.

Do I need to subtract windows and doors when calculating wall area?

Yes, you should subtract the area of windows and doors because you are not painting those surfaces. A standard interior door is about 20 square feet, and a typical window is about 15 square feet. Subtract these from the gross wall area to get the actual paintable surface.

How do I calculate the square footage of a room with high ceilings?

For walls, simply multiply the total wall length by the ceiling height. For example, a 12-foot-high wall that is 20 feet long gives 240 square feet. If the ceiling is vaulted, measure the highest point and use that as an average – or measure each wall section separately for accuracy.

Should I buy extra paint for touch-ups?

Yes, it is wise to buy about 10% more paint than the calculator suggests. This covers touch-ups after the job, spills, and uneven application. If the calculator says you need 2.6 gallons, buy 3 gallons – you will likely use most of it.

Does primer count as a coat?

Primer is not a finish coat, but it does cover the wall and affects how much paint you need. If you are priming new drywall or a dark color, apply one coat of primer and then two coats of paint. In that case, you would enter “2� for the number of paint coats, and buy primer separately based on the same square footage.

What if my walls are textured or unpainted?

Textured or unpainted surfaces absorb more paint. Reduce the coverage rate to 250–300 square feet per gallon. For very rough surfaces like stucco, you may need even more. It is safer to buy an extra gallon and return any unopened cans.

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in length | Wall Length (total of all walls) | ft | 60 | 0
in height | Wall Height | ft | 9 | 0
in doors | Number of Doors (subtract ~20 sq ft each) | - | 2 | 0
in windows | Number of Windows (subtract ~15 sq ft each) | - | 3 | 0
in coats | Number of Coats | - | 2 | 1
out area | Net Paintable Area | sq ft | length * height - doors * 20 - windows * 15 | 0
out gallons | Paint Needed | gallons | (area * coats) / 350 | 1