Manually calculating your paint needs is a straightforward process. Here’s how to do it correctly:

1. Measure the Surface Area: Calculate the total area you need to paint. For a simple wall, multiply its height by its width. For a room, calculate the area of each wall and add them together. Remember to subtract the area of large windows and doors.
2. Determine Total Coverage Needed: Multiply your total surface area by the number of coats you intend to apply. Two coats is standard for most jobs.
3. Check the Paint's Coverage Rate: Look on the paint can for the manufacturer's stated coverage rate, which is typically listed as "square feet per gallon". A gallon of latex paint usually covers about 350-400 sq ft.
4. Adjust for Surface Texture: A rough surface like new drywall or textured stucco will absorb more paint. Reduce the can's stated coverage rate by about 10-20% for these surfaces.
5. Calculate Gallons Needed: Divide your total coverage area (from Step 2) by the adjusted coverage rate (from Step 4) to determine the number of gallons you need. Always round up to the nearest whole gallon.

• Coverage Rate: The amount of area a gallon of paint can cover in a single coat. This varies by paint type and brand.
• Latex Paint: A water-based paint that is easy to clean up, has low odor, and is the most common choice for interior walls.
• Oil-Based Paint: An alkyd paint known for its durability and smooth finish. It's often used for trim, doors, and cabinets but requires mineral spirits for cleanup.
• Primer: A preparatory coating applied before the paint. It ensures better adhesion, increases durability, and provides a uniform surface for a consistent finish.
• Sheen: The level of gloss or shininess in the paint, ranging from flat (no shine) to eggshell, satin, semi-gloss, and high-gloss. Higher sheens are more durable and easier to clean.

"The numbers on the paint can are a best-case scenario. They assume a professional application on a perfectly smooth, sealed surface. For the average DIYer, I always recommend adding a 15% waste factor, not just 10%. This covers paint lost in the roller tray, drips, and the occasional mistake. It's far cheaper to have a little extra paint for touch-ups than to buy a whole new gallon months later and discover the color doesn't quite match because it's from a different batch." - Professional Painter

Painting a Bedroom: The room has 400 sq ft of wall space. You are using latex paint (350 sq ft/gallon coverage) and applying two coats.
Total Coverage: `400 sq ft * 2 coats = 800 sq ft`.
Gallons Needed: `800 sq ft / 350 sq ft/gallon = 2.29` gallons.
You should purchase 3 gallons to be safe.

Priming New Drywall: You have 1200 sq ft of new, unpainted drywall. Primer has a lower coverage rate, about 300 sq ft/gallon.
Gallons Needed: `1200 sq ft / 300 sq ft/gallon = 4` gallons of primer.

• Forgetting to Subtract Windows/Doors: Overestimating your surface area by not subtracting large openings can lead to buying too much paint.
• Ignoring Surface Texture: Applying paint to a rough or unprimed surface will use significantly more paint than the can suggests.
• Not Buying Enough for Touch-Ups: Always aim to have at least a quart of leftover paint for future scuffs and repairs.
• Using the Wrong Roller Nap: Using a thick, 3/4" nap roller on a smooth wall will waste a lot of paint. Use a short nap (1/4" or 3/8") for smooth surfaces.

• Interior Painting: Calculating paint needed for walls, ceilings, and trim in any room.
• Exterior Painting: Estimating paint for siding, fences, and decks.
• Furniture Flipping: Determining how much paint is needed for a small project like a dresser or chair.
• Budgeting: Creating an accurate budget for a renovation by estimating material costs upfront.