How to Calculate Paint for a Room

Figuring out how much paint you need is not rocket science, but it is also not as simple as multiplying length by width. Here is the exact method contractors use, and it is the same logic built into our calculator above.

Step 1: Measure the Wall Area

Add up the length of all four walls, then multiply by the ceiling height. For a 12×10 room with 8-foot ceilings, that is (12 + 10 + 12 + 10) × 8 = 352 square feet.

Step 2: Subtract Doors and Windows

A standard interior door is about 21 sq ft. A typical window is roughly 15 sq ft. Do not bother measuring to the inch—rough estimates are fine for paint planning. In our example, one door and one window removes 36 sq ft, leaving 316 sq ft of paintable wall.

Step 3: Account for Coats

Most projects need two coats. If you are painting a light color over a dark wall, you might need three. Multiply your net wall area by the number of coats. Two coats on 316 sq ft = 632 sq ft of "paint work."

Step 4: Divide by Coverage Rate

Check the paint can label. Standard interior paint covers about 350 sq ft per gallon. Premium paints can hit 400 sq ft, while cheap paint or rough texture might drop to 300. Divide your total paint-work area by the coverage rate. In our example, 632 ÷ 350 = 1.8 gallons. Round up to 2 gallons because you cannot buy 0.8 gallons.

Step 5: Add a Waste Buffer

Professionals always add 10% for cutting in, roller drips, and touch-ups. Two gallons plus 10% = 2.2 gallons. Since paint is sold in whole gallons, you need 3 gallons total for a safe, stress-free job.

How Many Square Feet Does a Gallon of Paint Cover?

This is the most common question we get, and the honest answer is: it depends. The coverage rate varies based on paint quality, surface texture, and whether you are using a primer.

• High-quality latex paint on smooth drywall: 350–400 sq ft per gallon
• Mid-grade paint on lightly textured walls: 300–350 sq ft per gallon
• Budget paint or rough/stucco surfaces: 250–300 sq ft per gallon
• Primer on bare drywall: 300–400 sq ft per gallon
• Ceiling paint (flat finish): 350–400 sq ft per gallon
• Trim paint (semi-gloss, brushed): 300–350 sq ft per gallon, but trim is measured in linear feet, not square feet

Pro tip: always check the specific product label. Brands like Sherwin-Williams, Behr, and Benjamin Moore list exact coverage on every can. If you are buying online, the coverage is almost always listed in the product description. When in doubt, assume 350 sq ft per gallon for walls and you will rarely be short.

One Coat vs. Two Coats: What Is the Difference?

We have all seen the "one-coat coverage" claims on paint cans. In reality, one coat is almost never enough for a professional-looking finish, unless you are repainting with the exact same color on a wall that is already in perfect condition.

One coat is enough when:

• You are repainting with the same or a very similar color
• The existing paint is smooth, clean, and not faded
• You are using a high-quality paint with excellent hide
• You do not mind a slightly uneven finish (acceptable for garages, basements, or rental units)

Two coats are needed when:

• You are changing colors (especially light over dark or vice versa)
• The wall has patches, repairs, or primer spots
• You want a uniform, durable finish that will last 5+ years
• You are using a lower-quality paint that has weak hide

Three coats are needed when:

• You are covering a very dark color with a light one (e.g., navy to white)
• You are painting over wallpaper glue residue or stains
• You are using a highly saturated or bold color like red, deep blue, or bright yellow

The bottom line: for most homeowners, budget for two coats. It takes more time and money upfront, but the finish will look better and last longer, which saves you from repainting sooner.

Paint Coverage by Brand: A Quick Reference

Not all paints are created equal. Here is how the major brands stack up in terms of coverage per gallon, based on manufacturer specs and contractor feedback:

• Behr Premium Plus Ultra: 350–400 sq ft/gal. Widely available at Home Depot. Good balance of price and quality. Often on sale for $30–35/gallon.
• Sherwin-Williams Duration Home: 350–400 sq ft/gal. Higher price point ($50–65/gallon) but exceptional durability and washability. A favorite among professional painters.
• Benjamin Moore Regal Select: 400–450 sq ft/gal. Premium option ($55–70/gallon). Excellent hide and a huge color selection. Available at independent retailers.
• Valspar Signature: 350–400 sq ft/gal. Mid-range option sold at Lowe's. Decent quality, often discounted. Good for DIYers on a budget.
• Glidden Essentials: 300–350 sq ft/gal. Budget option ($15–20/gallon). Fine for utility spaces, but expect to use more paint and possibly need a third coat for full coverage.

Keep in mind that manufacturer coverage numbers assume ideal conditions: smooth, primed drywall, applied with a roller at the recommended thickness. Real-world coverage is usually 10–15% less.

Paint by Room Size: Quick Reference Table

Use this table as a starting point for common room sizes. All estimates assume 8-foot ceilings, two coats, standard 350 sq ft/gallon coverage, and include a 10% waste buffer.

These are rough estimates. Always use our calculator above for your exact room dimensions, door/window count, and paint quality.

Exterior Paint Calculator: What You Need to Know

Exterior painting is a completely different beast from interior painting. The surface materials, weather exposure, and preparation requirements all affect how much paint you need.

• Surface material matters more: Smooth siding (vinyl, aluminum) covers 350–400 sq ft/gal. Rough wood or stucco covers 250–300 sq ft/gal. Brick covers 200–250 sq ft/gal because it is so porous.
• Primer is almost always required: Exterior surfaces are exposed to sun, rain, and temperature swings. A quality exterior primer seals the surface and prevents peeling. Budget for one coat of primer plus two coats of paint.
• Paint quality is critical: Cheap exterior paint fades and peels in 2–3 years. Quality exterior paint (Sherwin-Williams Duration, Benjamin Moore Aura) lasts 10–15 years. The upfront cost is higher but the long-term value is enormous.
• Trim and accents need separate paint: Exterior trim is usually painted with semi-gloss or gloss paint, which has different coverage rates than flat siding paint.
• Weather window: Paint when temperatures are between 50°F and 85°F with low humidity. Painting in direct sun or high humidity causes the paint to dry too fast or too slow, leading to poor adhesion.

For exterior projects, always add 15–20% waste instead of 10%. Exterior surfaces are irregular, and you will waste more paint on cutting in around windows, doors, and trim.

Tips to Save Money on Your Paint Project

• Buy during holiday sales: Memorial Day, Labor Day, and Black Friday are when Home Depot and Lowe's run their biggest paint promotions. You can often save 30–40%.
• Use paint-and-primer-in-one: For most repainting jobs, a quality paint with built-in primer eliminates the need for a separate primer coat, saving you time and money.
• Measure twice, buy once: Use our calculator above to avoid over-buying. Unused paint is hard to return and takes up garage space.
• Choose the right sheen: Flat paint is cheaper than semi-gloss and hides imperfections better. Save semi-gloss for trim, bathrooms, and kitchens where washability matters.
• Buy in 5-gallon buckets: If you need more than 3 gallons, a 5-gallon bucket is usually cheaper per gallon than individual cans. Plus, you avoid slight color batch variations.
• Don't skip the ceiling: A freshly painted ceiling makes walls look brighter and cleaner. Ceiling paint is cheaper than wall paint (about $20–25/gallon), and the impact is huge.
• Paint in the right order: Ceiling first, then walls, then trim. This prevents drips on finished surfaces and reduces touch-ups.