How to Calculate Paint Coverage (and Stop Buying Too Much)

Every paint project starts the same way: you stand in the paint aisle, staring at gallon cans, trying to decide if two is enough or if you need three. Buy too few and you're making a second trip mid-project — possibly ending up with a slightly different batch. Buy too many and you've got half a gallon drying out in the garage for the next five years.

Here's the good news: figuring out how much paint you actually need takes about 60 seconds of math. Once you know the formula, you'll nail it every time — whether you're painting a single bedroom wall or the entire exterior of your house. Let's walk through it.

The Basic Formula

The core calculation has three steps, and they're simpler than you think:

1. Calculate your total wall area. Measure the length and width of the room, then use: Wall Area = 2 × (Length + Width) × Height. This covers all four walls.
2. Divide by your paint's coverage rate. Most interior latex paints cover about 350 square feet per gallon on smooth drywall. Check the can — this number is always on the label.
3. Multiply by the number of coats. Two coats is standard for most jobs.

So the full formula looks like this:

Gallons needed = (2 × (L + W) × H) ÷ Coverage per gallon × Number of coats

Let's run through a real example. Say your bedroom is 12 feet long, 10 feet wide, with standard 8-foot ceilings:

• Wall area = 2 × (12 + 10) × 8 = 2 × 22 × 8 = 352 sq ft
• Gallons for one coat = 352 ÷ 350 = 1.01 gallons
• Gallons for two coats = 1.01 × 2 = 2.01 gallons

You'd buy 2 gallons for that room with two coats. You'll have almost nothing left over, which is about as close to perfect as paint math gets. If you want a little touch-up reserve (you should), grab an extra quart.

Quick Reference by Room Size

Don't feel like doing the math? Here's a cheat sheet for common room sizes. All numbers assume 8-foot ceilings and standard coverage of 350 sq ft per gallon on smooth drywall — no doors or windows subtracted (that comes next).

A few things to notice. For a 12×12 room — one of the most common bedroom sizes — two gallons handles two coats with only a tiny shortfall. You'll be fine in practice because doors and windows reduce the actual paintable area (we'll cover that below). But if you're going dark-to-light or working with a porous wall, round up to three gallons.

For larger rooms like a 15×15, three gallons is the safe bet for two coats. You'll have some left over, but that's your touch-up stash for the next few years.

Subtracting Doors and Windows

The quick-reference table above gives you the total wall area, but you're not painting over your doors and windows. If you want a tighter estimate, here are the standard deductions:

• Standard interior door: ~20 sq ft (roughly 3 ft × 6.8 ft)
• Average window: ~15 sq ft (roughly 3 ft × 5 ft)
• Sliding glass door: ~40 sq ft

So for that 12×10 bedroom with one door and two windows, you'd adjust like this:

• Total wall area: 352 sq ft
• Minus 1 door (20 sq ft) and 2 windows (30 sq ft): 352 − 50 = 302 sq ft
• Two coats at 350 sq ft/gal: 302 ÷ 350 × 2 = 1.73 gallons

That's comfortably under 2 gallons, meaning you'll have leftover paint for touch-ups down the road. For most bedrooms and living rooms, subtracting doors and windows saves you from overbuying by about half a gallon — which can be the difference between needing two gallons and three.

That said, for a rough estimate, many painters skip this step entirely. The extra paint accounts for waste, cutting in around trim, and having touch-up paint on hand. If you're on a tight budget, subtract. If convenience matters more, just round up.

Coverage Rates by Surface Type

That 350 sq ft per gallon number? It only applies to smooth, primed drywall. The surface you're painting makes a huge difference in how far your paint stretches. Rough or porous surfaces soak up more paint, so you get less coverage per gallon.

This is where most people underestimate. If you're painting textured walls and you calculate based on 350 sq ft per gallon, you could come up short by an entire gallon on a large room. Always use the coverage rate that matches your actual surface — not the best-case number.

Do You Need Primer?

Primer is one of those things people either swear by or skip entirely. Here's a simple rule: if the surface has never been painted or you're making a big color change, prime it. Otherwise, you can usually skip it.

When you need primer

• New, bare drywall. Unprimed drywall paper is like a sponge — it'll soak up your expensive topcoat and give you blotchy, uneven coverage. A coat of primer seals the surface and lets your paint glide on evenly.
• Dark-to-light color changes. Going from charcoal gray to eggshell white? Without primer, you'll need three or four coats of paint to hide the old color. A tinted primer does the heavy lifting in one coat, saving you both time and paint.
• Bare wood or previously stained surfaces. Wood grain and stains bleed through regular paint. An oil-based or shellac primer blocks the bleed and gives you a clean base.
• Patched or repaired areas. Spackle and joint compound have a different porosity than the surrounding wall. Spot-prime those areas or they'll flash (show through as dull spots) under your topcoat.

When you can skip primer

• Repainting the same or similar color. If your walls are already painted and in good shape, and you're applying a similar shade, a quality paint-and-primer-in-one will do the job.
• Previously primed walls. If someone already primed the surface and it's clean, go straight to paint.

When you do need primer, budget for it separately. Primer coverage is typically 300–400 sq ft per gallon, similar to paint. For that 12×10 bedroom, you'd need about 1 gallon of primer plus your 2 gallons of topcoat.

Interior vs Exterior

The formula is the same whether you're painting inside or outside, but almost everything else changes. Here's what to keep in mind for exterior projects:

Coverage rates are lower outside. Exterior surfaces — siding, stucco, brick — are rougher and more porous than interior drywall. Where you might get 350–400 sq ft per gallon inside, expect 200–300 sq ft per gallon outside depending on the material. Check the coverage rates table above and adjust accordingly.

You're painting different shapes. Instead of four simple rectangular walls, exterior projects often involve gable ends, dormers, soffits, and trim. Measure each section separately and add them up. For a basic rectangular house, you can still use the 2 × (L + W) × H formula for the main walls, then add extra for gables and trim.

Weather matters. Exterior paint needs temperatures between about 50°F and 85°F to cure properly. Too hot and it dries before it can level out. Too cold and it won't bond. High humidity slows drying and can cause drips or bubbles. Plan your project for a stretch of mild, dry weather.

Paint types differ. Exterior paints are formulated with UV stabilizers and mildew inhibitors that interior paints don't have. Never use interior paint outside — it'll peel and chalk within a year. The reverse isn't great either: exterior paints emit more VOCs and aren't designed for indoor air quality.

For a typical 1,500 sq ft single-story house (about 1,200 sq ft of exterior wall area after subtracting windows and doors), you'd need roughly 4–6 gallons for two coats on smooth siding, or 6–8 gallons on textured stucco. That's a meaningful difference in cost, so getting your surface type right really matters.

Common Mistakes

After helping thousands of people estimate paint, here are the mistakes we see over and over:

1. Forgetting the second coat

This is the big one. You calculate perfectly for one coat, buy that amount, and then realize mid-project that one coat looks terrible. Now you're stuck waiting for the store to open. Always plan for two coats unless you're doing a same-color touch-up.

2. Ignoring wall texture

If your walls have knockdown, orange peel, or skip-trowel texture, they have significantly more surface area than flat drywall. That texture eats paint. Using the smooth-drywall coverage rate on textured walls will leave you short by 15–30%.

3. Buying quarts for large rooms

Quarts seem cheaper, but they cost more per square foot than gallons — usually 30–40% more. If your room needs more than about 80 square feet of coverage (basically anything beyond a single accent wall), buy gallons. The math always favors gallons for bigger jobs.

4. Not reserving paint for touch-ups

Walls get scuffed, furniture dings them, kids draw on them. If you use every last drop during your project, you won't have any left for touch-ups. Keep at least a quart in reserve. Label it with the color name, brand, finish, and room. You'll thank yourself in six months.

5. Mixing brands or batches without checking

Even the same color from the same brand can vary slightly between batches. If you're buying multiple gallons, check that the batch numbers match or "box" your paint — pour all the cans into a large bucket and stir them together before you start painting. This ensures perfectly consistent color across every wall.

Frequently Asked Questions

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

A 12×12 room with 8-foot ceilings has about 384 square feet of wall area. At standard coverage of 350 sq ft per gallon, you'd need roughly 1.1 gallons for one coat or 2.2 gallons for two coats. Buy 2 gallons for one coat (with a touch-up reserve) or 3 gallons for two coats.

How many square feet does a gallon of paint cover?

A gallon of standard interior latex paint covers 350–400 square feet on smooth drywall. Coverage drops on textured walls (250–350 sq ft), brick (about 150–200 sq ft), and stucco (about 150–200 sq ft). Always check the label on your specific can for the manufacturer's estimate.

Do I really need two coats of paint?

In most cases, yes. Two coats give you even color, better durability, and proper hiding — especially when changing colors. You might get away with one coat only if you're repainting the exact same color or using a high-quality paint-and-primer-in-one over a very similar shade.

Should I subtract doors and windows from my paint estimate?

For a rough estimate, you can skip it — the extra paint covers trim, touch-ups, and waste. For a precise calculation, subtract about 20 square feet per standard door and 15 square feet per average window from your total wall area before dividing by coverage.

Last updated: March 2026