Paint Calculator in Feet: How Much Paint Do You Need?

This paint calculator in feet works out how many gallons or liters of paint you need for a room or surface, based on the dimensions you enter in feet and inches. You input the total wall area (or length and height of each wall), the number of coats, and the paint coverage per gallon or liter, and it returns the exact amount of paint to buy. The calculator below uses square feet for area and gallons for paint, with a standard coverage rate of 350 square feet per gallon, which you can adjust for your specific paint brand.

Formula

The paint calculator uses a simple formula: Paint needed (gallons) = (Total wall area × Number of coats) ÷ Coverage per gallon. In symbols: G = (A × C) / K, where:

• G = gallons of paint required
• A = total wall area in square feet (length × height for each wall, summed)
• C = number of coats (usually 2 for a new color, 1 for repainting same color)
• K = coverage in square feet per gallon (typically 350–400 for most paints)

If you measure in meters, convert to feet first: 1 meter = 3.28084 feet. Or use liters: 1 gallon = 3.78541 liters.

Worked Example

Using the default values from the calculator: a wall 12 feet long and 8 feet high, with 2 coats, and paint covering 350 sq ft per gallon.

1. Wall area = length × height = 12 ft × 8 ft = 96 sq ft
2. Total area for 2 coats = 96 × 2 = 192 sq ft
3. Gallons needed = 192 ÷ 350 = 0.5486 gallons
4. Rounded to 2 decimals: 0.55 gallons

That’s about half a gallon. For a typical 10×12 ft bedroom with 8 ft ceilings, you’d have four walls: two 10×8 and two 12×8, total area = 2×(80+96) = 352 sq ft. With 2 coats, that’s 704 sq ft, needing 704÷350 = 2.01 gallons. So a single gallon would not be enough; you’d need 2 gallons (or 1 gallon plus a quart).

How to Measure a Room in Feet

To use this calculator, measure each wall’s length and height in feet. Use a tape measure and record to the nearest 0.1 foot (or inch). For a rectangular room, you don’t need to measure every wall individually—just the perimeter and height. But if you have windows, doors, or built-in cabinets, subtract their area to avoid overbuying.

• Measure the length and width of the floor in feet; the wall area for a rectangular room = 2 × (length + width) × height.
• For each window: measure height and width, multiply to get square feet, subtract from total.
• For each door: typically 21 sq ft for a standard 3×7 ft door, but measure to be exact.
• If you’re painting the ceiling, add its area (length × width) separately.

For irregular walls, break them into rectangles and triangles, calculate each area, and sum them.

Common Mistakes and How to Avoid Them

One frequent error is mixing units—entering dimensions in meters but using a coverage rate per gallon (which is per square foot). Always convert to feet or use a metric calculator. Another mistake is forgetting to subtract windows and doors, which can lead to buying 10–20% more paint than needed. Conversely, some people subtract too much, forgetting that trim, corners, and texture absorb more paint.

Rounding errors also matter. Paint is sold in gallons and quarts (1 quart = 0.25 gallons). If your calculation gives 1.1 gallons, you can’t buy 1.1 gallons—you’d buy 1 gallon and 1 quart (1.25 gallons). Always round up to the nearest quart, not down.

Coverage rates vary by paint quality and surface porosity. A cheap paint might cover only 250 sq ft per gallon, while a premium paint can cover 400. Check the label on the can; the calculator lets you adjust the coverage field.

When to Use This Calculator (and When Not To)

This calculator is ideal for interior walls, exterior siding, fences, and any flat surface where you can measure length and height. It works for both latex and oil-based paints, as long as you input the correct coverage. However, it does not account for:

• Primer—if you’re priming bare drywall or a dark color, you need a separate coat of primer, which is not included in the “coatsâ€� field unless you add it.
• Textured surfaces (stucco, brick, popcorn) that absorb more paint—reduce coverage by 20–30%.
• Spraying vs. rolling—spraying wastes more paint, so add 10–15% to the total.
• Trim, doors, and windows if you’re painting them a different color—calculate those separately.

For a quick estimate, many paint stores use a rule of thumb: one gallon covers about 350 sq ft with one coat. But this calculator gives a more precise number based on your actual measurements.

How to Choose the Right Paint Quantity

Once you have the gallons from the calculator, consider these practical tips:

• If you’re between 0.5 and 1 gallon, buy 1 gallon—you’ll need it for touch-ups.
• For large areas, it’s often cheaper to buy 5-gallon buckets (which cost less per gallon) if you need more than 3 gallons.
• If you’re painting a new color over a dark one, you may need an extra coat—add 1 to the coats field.
• Keep leftover paint for future repairs; store it in a cool, dry place with the lid sealed.

Remember that paint is not returnable once mixed, so it’s better to overestimate slightly than to run out mid-project. Most calculators, including this one, round up to the nearest gallon or quart in real life, but the decimal output helps you compare options.

Limitations of the Formula

The formula assumes a flat, smooth surface with even absorption. In reality, new drywall soaks up more paint than repainted walls; unprimed surfaces can absorb up to 20% more. Also, the coverage rate on the can is based on ideal conditions—applied with a roller at the recommended thickness. If you apply paint too thickly, you’ll cover less area; if too thinly, you might need more coats.

Temperature and humidity also affect drying and coverage. Painting in very hot or windy conditions can cause paint to dry too fast, leading to uneven coverage and more paint usage. This calculator doesn’t factor in environmental conditions, so for outdoor projects in extreme weather, add 10% to your total.

Finally, the calculator gives the amount for the wall area only. If you’re painting ceilings, trim, or doors, you need to add those areas separately. The calculator can handle that if you enter the combined area as “length� and set height to 1, but it’s easier to just add the extra square footage to the length field.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most paint cans state a coverage of 350 to 400 square feet per gallon for one coat. This is for smooth, primed surfaces. For rough or porous surfaces, coverage drops to 250–300 sq ft per gallon. Always check the label and adjust the coverage field in the calculator.

How do I calculate paint for a room with windows and doors?

Measure the total wall area (perimeter × height), then subtract the area of each window (width × height) and each door (usually 21 sq ft for a standard door). Enter that net area into the calculator. For example, a 12×12 ft room with 8 ft ceilings has 384 sq ft of wall; subtract a 3×5 ft window (15 sq ft) and a 3×7 ft door (21 sq ft) = 348 sq ft.

Do I need to use primer? Does the calculator account for it?

The calculator only counts the paint coats you enter. If you need primer, add an extra “coat� to the coats field, or calculate primer separately using the same area. Primer typically covers less than paint (about 300 sq ft per gallon), so use a separate calculation if you want accuracy.

What if my wall is not a perfect rectangle?

Break the wall into rectangles and triangles. For a triangle, area = 0.5 × base × height. Add all areas together and enter the total as the “length� with height set to 1 (since you’ve already combined the area). For example, a wall with a sloped ceiling: measure the rectangular part and the triangular part separately, sum them.

How much paint do I need for a 10×10 room?

A 10×10 ft room with 8 ft ceilings has four walls, each 10×8 = 80 sq ft, total 320 sq ft. For 2 coats, that’s 640 sq ft. At 350 sq ft per gallon, you need 640÷350 = 1.83 gallons, so buy 2 gallons. If you subtract a door and window (about 36 sq ft total), you’d need about 1.6 gallons—still 2 gallons.

Can I use this calculator for exterior paint?

Yes, but exterior surfaces like wood siding, stucco, or brick are more porous and absorb more paint. Reduce the coverage to 250–300 sq ft per gallon. Also, account for the fact that you may need 3 coats on bare wood. Measure the total exterior wall area, subtract windows and doors, and enter the values.

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in length | Wall length | ft | 12 | 0
in height | Wall height | ft | 8 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Paint coverage | sq ft per gallon | 350 | 1
out gallons | Paint needed | gallons | (length * height * coats) / coverage | 2