Paint smarter.
Every formula public. Every source cited. Know exactly what you need before you go to the store.
How we calculated this
The calculator figures total wall area as perimeter times ceiling height, then subtracts a standard area for each door (21 sq ft / 2 sq m) and window (15 sq ft / 1.4 sq m) you enter. The net area is multiplied by the number of coats and divided by coverage per gallon or liter.
Coverage is set at 350 square feet per gallon (approximately 9 square meters per liter), which is the average for most interior latex paints on primed drywall. Check your specific paint can — coverage can range from 250 to 400 sq ft per gallon depending on the paint type, finish, and surface texture.
The result is rounded up to the nearest tenth of a gallon or liter — because you can't buy 2.67 gallons. The exact calculated amount is shown alongside so you can see the difference.
Ceilings are not included in this calculation. Use the ceiling paint calculator separately, or add the ceiling area (length × width) manually and recalculate.
How to never buy the wrong amount of paint again
Last spring a friend of mine painted her living room Benjamin Moore Revere Pewter. She bought two gallons based on what the guy at the paint counter told her, got halfway through the second coat, and ran out with one wall unfinished. She went back the next day for another gallon. The new can was from a different batch. You can see the seam where the two batches meet if you stand in the doorway at sunset. She's lived with it for a year because fixing it means repainting the entire wall.
A 2023 Sherwin-Williams retailer survey found that about a third of DIY painters buy the wrong amount on their first trip. Most of them underbuy by a half gallon. The fix takes 90 seconds with a tape measure.
How we calculated these numbers▾
Coverage rates in this guide use 350 sq ft per gallon for standard interior latex on primed drywall — the realistic average from Benjamin Moore, Sherwin-Williams, and Behr product data sheets. Manufacturer labels often claim 400 sq ft/gal under ideal conditions. We use the lower number because your walls aren't lab conditions. Cost figures reflect 2026 US retail pricing from Home Depot and Lowe's.
How to measure a room for paint
Forget the fancy stuff. You need a tape measure and 90 seconds. Walk the room with the tape and write down the length of each wall. Don't measure the wall surface itself — measure at the floor from corner to corner. This gives you the perimeter. Multiply by your ceiling height. That's your gross wall area.
Subtract 20 square feet for each standard interior door and 15 square feet for each average window. These numbers come from standard residential dimensions (a 3×7 door is 21 ft², rounded to 20; a 3×5 window is 15 ft²). If you have oversized windows or French doors, measure them individually.
One thing people consistently get wrong: they try to measure the actual painted surface, crawling along crown molding and around outlets. Don't. The perimeter-times-height method is more accurate because it captures the geometric area, and the small deductions for doors and windows are close enough for paint estimation. You're buying gallons, not milliliters.
Coverage rates: what the can says vs reality
Most paint cans list 350 to 400 square feet per gallon. That number is accurate under lab conditions — smooth primed drywall, a single coat applied at the manufacturer's recommended thickness with a quality roller. Your walls are probably not lab conditions.
The 350 ft²/gal number this calculator uses is the realistic average for interior latex on previously painted smooth drywall. Adjust down for three situations:
- Textured walls (orange peel, knockdown): reduce to 300 ft²/gal. The texture creates more surface area and the valleys soak up extra paint.
- Bare drywall or new construction: reduce to 250 ft²/gal for the first coat. The paper face absorbs heavily. This is why primer exists — it seals the surface cheaply so your expensive paint doesn't disappear into the paper.
- Dark-over-light or light-over-dark color changes: you'll need three coats regardless of coverage rate. Budget accordingly, or use a tinted primer as your first coat.
How much does it cost to paint a room?
Materials are the cheap part. Here's what a standard bedroom actually costs in paint alone, by quality tier:
DIY | Professional | |
|---|---|---|
| Paint (2 gal mid-range) | $90 | $90 |
| Supplies (roller, tape, drop) | $35 | Included |
| Primer (if needed) | $30 | $30 |
| Labor | $0 (your Saturday) | $300–500 |
| Total per room | $125–155 | $420–620 |
DIY saves 70-80% vs hiring but takes a full day per room including prep, two coats with drying time, and cleanup.
The biggest cost variable isn't the paint — it's whether you need primer. One coat of quality primer ($25–35 per gallon) on bare drywall or over dark existing colors saves you from needing a third coat of expensive topcoat. The math almost always favors priming.
Mistakes that waste paint and money
Four errors account for most paint buying mistakes. All are easy to avoid once you know what to watch for.
The worst of these is forgetting the second coat. One coat of paint looks fine in artificial light and terrible in sunlight. Roller marks, thin spots, and primer bleed-through all show through a single coat. Two coats is the standard for a reason: the first coat seals, the second coat colors.
The second-worst mistake is using the manufacturer's maximum coverage rate. Behr claims 400 sq ft per gallon on their Ultra line. Under perfect conditions, it achieves that. On your walls, with your roller technique, after cutting in around three windows and a closet door, you'll get 330-350. Use 350 as your planning number and you'll buy the right amount the first time.
Quick reference: how much paint for common room sizes
If you don't want to measure anything and just want a fast answer, here are the standard room sizes with paint quantities. These assume 8-foot ceilings, one door, two windows, standard latex paint at 350 ft² per gallon, and two coats.
If you're painting over new drywall, use the drywall calculator to estimate sheets first — bare drywall needs primer before paint. For rooms not on this list, the calculator at the top of the page handles any dimensions. Enter your wall lengths, ceiling height, and door/window count and it does the arithmetic — showing every step so you can check the math.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Choosing the right sheen
Sheen doesn't change how much paint you need, but it changes how the room looks and how long the paint lasts. Quick guide:
Sheen | Best for | Durability | |
|---|---|---|---|
| Flat / matte | Ceilings, low-traffic bedrooms | Low — marks easily, hard to clean | |
| Eggshell | Living rooms, dining rooms, adult bedrooms | Medium — wipes clean, hides imperfections | |
| Satin | Kitchens, bathrooms, kids' rooms, hallways | Good — moisture-resistant, scrubable | |
| Semi-gloss | Trim, doors, cabinets, high-moisture areas | High — very washable, shows every wall flaw |
Most residential rooms use eggshell or satin. Flat is only for ceilings and low-traffic spaces. Semi-gloss is for trim and millwork, not walls.
The most common mistake here is using flat paint on walls in high-traffic rooms. Flat paint looks great on day one and terrible by month three — every fingerprint, scuff, and chair bump is permanent. Eggshell or satin in hallways, kitchens, and kids' rooms saves you from repainting in two years.
How to store leftover paint
Leftover paint is not waste — it's your touch-up supply for the next two to three years. Store it correctly and it stays usable indefinitely. Store it wrong and it's a clump of dried latex in six months.
Keep the can tightly sealed (tap the lid down with a rubber mallet, not a hammer — hammers dent the rim and break the seal). Store upside down so paint forms a seal against the lid. Keep it in a climate-controlled space — garages that freeze in winter will ruin latex paint. Write the room name and date on the can with a marker. Label the color code and brand too, in case you need to buy more later.
For small amounts (under a quart), transfer to a mason jar instead. Less air in the container means less skin formation on the paint surface.
How much primer do I need?
Primer covers 300 to 400 square feet per gallon, roughly the same as paint. A 12 × 14 room with 8-foot ceilings has about 350 square feet of wall after subtracting a door and two windows. That is one gallon of primer for one coat. You need primer in three situations: bare drywall (the paper face absorbs paint unevenly without it), drastic color changes (dark to light or light to dark), and stain blocking (water marks, smoke damage, marker). For a same-color repaint over existing paint in good condition, skip primer entirely and save $25 to $35 per gallon plus the time for an extra coat.
Exterior house paint calculator: how much paint for the outside
Exterior paint coverage is slightly lower than interior because textured surfaces (wood lap siding, stucco, brick) absorb more. Budget 250 to 350 square feet per gallon depending on surface texture. Smooth siding gets 350. Rough-sawn cedar or stucco gets 250. The exterior wall area formula: house perimeter times wall height, plus gable triangles, minus windows and doors. A 1,500 square foot single-story home with 9-foot walls and a 160-foot perimeter has about 1,440 square feet of paintable exterior wall (after deductions). At 300 square feet per gallon and two coats, that is 10 gallons of exterior paint. At $40 to $70 per gallon for quality exterior latex, material runs $400 to $700. Thesiding calculator uses the same wall area measurement if you are comparing paint versus new siding. For full exterior painting cost including labor, see thecost to paint a house guide.
Sources
- Benjamin Moore — Paint Coverage Chart — Industry-standard 350-400 sq ft/gal for interior latex
- Sherwin-Williams — How Much Paint Do I Need — Reference for coverage assumptions and coat recommendations
Frequently asked
How many gallons of paint do I need for an average room?
For a typical 12 × 14 ft bedroom with 8 ft ceilings and two coats, you'll need about 2.5 gallons. Larger living rooms with 9-10 ft ceilings usually need 3 gallons for two coats. The calculator above gives you the exact amount for your specific dimensions.
Should I buy one or two coats' worth?
Always plan for two coats unless you're painting the same color over existing paint in good condition. Two coats give you uniform color, better coverage over primer marks, and a more durable finish. If you're changing color drastically (e.g., dark to light), you may need three coats plus primer.
How much extra paint should I buy?
The rounded-up amount above already gives you a small buffer. For very large rooms or textured walls, consider adding 10-15% extra — textured surfaces absorb more paint than smooth drywall. Keep leftover paint for touch-ups; stored properly it lasts 2-3 years.
Does this calculator include the ceiling?
No, this calculator covers walls only. Ceiling paint is usually a different product (flat white) with different coverage rates. Calculate the ceiling separately: length × width ÷ 350 × coats.
Why does paint cover less than the can says?
Manufacturer coverage claims assume ideal conditions: smooth primed drywall, recommended film thickness, single coat. In real-world conditions with textured walls, heavy-body paint, or absorbent surfaces, actual coverage is often 10-20% lower. The 350 sq ft/gal figure used here is a realistic average.
What if my walls have a heavy texture?
Textured walls (orange peel, knockdown, stucco) increase actual surface area and absorb more paint. Add 10-15% to the calculated amount. For heavy texture like popcorn or smooth concrete, add 20%.
How accurate is the 350 sq ft per gallon number?
It's the widely cited industry average for interior latex paint on primed drywall. Premium paints often claim 400-450 sq ft/gal and can achieve it under good conditions. Budget paints may only cover 250-300. Check your specific paint can and adjust expectations accordingly.
Do I need primer? Does the calculator include it?
The calculator is for paint only, not primer. You need primer if you're painting over bare drywall, dark colors (going lighter), patched repairs, or stained surfaces. Primer typically covers the same area as paint — use the primer calculator for a separate estimate.
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