Paint Calculator: How Much Paint Do I Need?

This paint calculator works out how many gallons or liters of paint you need for a room or wall. You enter the total wall area (or the 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. It also shows the total surface area and the number of paint cans required, so you can plan your purchase without guesswork.

Formula

The basic formula for paint quantity is:

Paint needed (gallons) = (Wall area × Number of coats) ÷ Paint coverage

• Wall area (a) – the total surface you plan to paint, in square feet (or square meters).
• Number of coats (c) – how many layers of paint you’ll apply. Usually 2 for a color change, 1 for a touch-up.
• Paint coverage (cov) – the area one gallon covers, typically 250–400 sq ft per gallon for most interior paints. Check the can label.

If you measure in metric, use square meters and liters. Coverage is often listed as 10–12 m² per liter. The formula stays the same: area × coats ÷ coverage.

Worked Example

Using the default values from the calculator:

• Wall area = 320 sq ft
• Coats = 2
• Coverage = 350 sq ft per gallon

Step 1: Multiply area by coats: 320 × 2 = 640 sq ft of total surface to cover.

Step 2: Divide by coverage: 640 ÷ 350 = 1.8286 gallons.

So you need about 1.83 gallons of paint. Since paint is sold in whole gallons, you’d round up to 2 gallons (the calculator’s “Cans to buy� shows 2).

How to Measure Your Wall Area

To get an accurate estimate, measure each wall you plan to paint. Use a tape measure to find the height and width of each wall, then multiply them for the area. Add all wall areas together.

• For a rectangular room: measure the perimeter (sum of all wall lengths) and multiply by the wall height.
• Subtract areas you won’t paint: windows, doors, and built-in cabinets. A standard door is about 21 sq ft; a typical window is 12–15 sq ft.
• If you’re painting a ceiling, measure the floor length × width.

If you only know the room’s dimensions, you can estimate wall area as: (perimeter × height) – (window/door areas). For a 12×12 ft room with 8 ft ceilings, the perimeter is 48 ft, so walls = 48 × 8 = 384 sq ft, minus doors/windows.

Choosing the Right Coverage Number

Paint coverage varies by paint type and surface. Most interior latex paints cover 300–400 sq ft per gallon. High-quality paints often cover more because they have more solids. Rough or porous surfaces (drywall, brick, stucco) absorb more paint, so coverage drops to 200–250 sq ft per gallon. Glossy or sealed surfaces need less paint because they don’t absorb.

Always check the can label for the manufacturer’s coverage figure. If you’re unsure, use a conservative 250 sq ft per gallon. It’s better to overestimate slightly than to run out mid-project.

Common Mistakes and How to Avoid Them

People often make these errors when calculating paint:

• Forgetting to subtract windows/doors – this overestimates paint by 10–20%.
• Mixing units – don’t use square feet with liters, or square meters with gallons. Convert everything to one system.
• Rounding down – always round up to the nearest whole can. Partial gallons are usually not available.
• Ignoring primer – if you’re painting bare drywall or a dark color, you’ll need primer, which is a separate coat. Add that to your total.
• Not accounting for texture – textured walls (popcorn, knockdown) use 20–30% more paint.

Also, remember that coverage numbers are for smooth, primed surfaces. If you’re painting over a darker color, you may need an extra coat.

When to Use This Calculator vs. a Pro

This calculator is perfect for DIY projects: painting a bedroom, an accent wall, or a small office. It gives you a quick estimate for buying paint. However, for large commercial jobs or unusual surfaces (like highly textured plaster), a professional painter will do a more detailed takeoff that includes waste, primer, and touch-up. For a typical home project, this calculator is accurate enough.

One tip: buy about 10% extra paint to account for spillage, touch-ups, and uneven application. You can also keep leftover paint for future repairs.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

For a 12×12 ft room with 8 ft ceilings, the wall area is about 384 sq ft. Subtract a door (21 sq ft) and a window (15 sq ft) to get 348 sq ft. With 2 coats and 350 sq ft per gallon coverage, you need 348 × 2 ÷ 350 = 1.99 gallons, so buy 2 gallons. If you also paint the ceiling (144 sq ft), add 144 ÷ 350 = 0.41 gallons, so you’d need about 2.5 gallons total.

How many square feet does a gallon of paint cover?

Most interior latex paints cover 300–400 sq ft per gallon on smooth, primed surfaces. Budget brands may cover less (around 250 sq ft), while premium paints can cover up to 400 sq ft. Always check the label for the exact coverage.

Do I need to use primer before painting?

Primer is recommended for bare drywall, stained surfaces, or when painting over a darker color. It seals the surface and provides a uniform base, which improves paint adhesion and coverage. If you use primer, count it as one coat in your calculation.

How do I calculate paint for a wall with a window?

Measure the wall’s height and width, multiply for the total area, then subtract the window’s area (height × width). Do the same for doors. Then multiply by the number of coats and divide by coverage.

What if my paint coverage is different from the default?

Simply change the coverage value in the calculator to match your paint can. Most paints list coverage in square feet per gallon or square meters per liter. Use that number for accuracy.

Should I buy extra paint?

Yes, buying 10% extra is wise. It covers mistakes, spills, and future touch-ups. If you’re mixing colors, having extra ensures you can match the exact shade later.

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in a | Wall area | sq ft | 320 | 1
in c | Number of coats | - | 2 | 1
in cov | Paint coverage | sq ft per gallon | 350 | 1
out need | Paint needed | gallons | a * c / cov | 2
out cans | Cans to buy (1 gal) | gallons | ceil(a * c / cov) | 0