Paint Calculator — How Much Paint Do I Need?

Enter your room or wall dimensions to instantly estimate how many gallons or liters of paint you need — with doors and windows subtracted and coats and wastage built in, so you buy once and don't run short.

How Do You Calculate How Much Paint You Need?

Every paint estimate starts from one question: what is the total area you're covering? That area is then divided by the paint's coverage rate — how many square feet (or square meters) a single gallon (or liter) covers with one coat — and finally multiplied by the number of coats plus a wastage allowance. The area calculation depends on what you're painting: A single accent wall that's 10 ft wide and 8 ft tall, for example, has only 80 sq ft of paintable area — small enough that even four coats stay under a single gallon at the standard 350 sq ft per gallon coverage rate, which is why sample-size and quart cans exist for jobs exactly this small. That's exactly the situation the calculator's "single wall" mode is built for, so you don't have to run the full room formula just to paint one wall on its own.

The paintable area is the full wall (width × height) minus any doors or windows inside it.

Surface	Area Formula
Single wall	Area = Width × Height
Room (4 walls)	Area = 2 × (Length + Width) × Wall Height
Ceiling	Area = Length × Width
Doors & windows	Subtract 21 sq ft per door, 15 sq ft per window

Once the paintable area is known, the remaining math is straightforward. Paint needed per coat equals the area divided by the coverage rate (350 sq ft per US gallon, or about 8.6 m² per liter — the figures printed on most paint cans). Total paint equals that per-coat amount multiplied by the number of coats, multiplied by 1 plus the wastage percentage:

Total = (Area ÷ Coverage) × Coats × (1 + Wastage ÷ 100)

How Much Paint Does a 12 × 10 ft Bedroom Need?

Let's work through a typical bedroom: 12 ft long, 10 ft wide, 8 ft walls, one door, one window, two coats, and a 10% wastage allowance — the calculator's default setup. Swap in a larger 14 × 16 ft primary bedroom with the same 8 ft walls, one door, two windows, and three coats instead, and the math scales the same way: 2 × (14 + 16) × 8 = 480 sq ft of raw wall area, minus 21 sq ft for the door and 30 sq ft for the two windows, leaves 429 sq ft of paintable surface — which needs 429 ÷ 350 × 3 × 1.10 ≈ 4.04 gallons, rounded up to 5 gallons to buy, using exactly the same steps the calculator runs behind the scenes for any dimensions you enter above.

Calculate the wall area: 2 × (12 + 10) × 8 = 352 sq ft.
Subtract openings: one door (21 sq ft) plus one window (15 sq ft) = 36 sq ft, leaving 316 sq ft of paintable wall.
Divide by coverage: 316 ÷ 350 = 0.90 gallons per coat.
Multiply by coats: 0.90 × 2 = 1.81 gallons.
Add wastage: 1.81 × 1.10 = 1.99 gallons.
Round up to a can: 2 gallons — exactly what the calculator reports.

Re-enter those numbers above and the widget will reproduce every step instantly — and update live if you change any dimension.

Reference Tables

Common Room Sizes → Gallons of Paint (Two Coats, 10% Wastage, 1 Door + 1 Window)

Room	Wall Area	Paint Needed (2 coats)	Buy
10 × 10 ft (small bedroom)	320 sq ft	≈ 1.8 gal	2 gal
12 × 12 ft (bedroom)	384 sq ft	≈ 2.2 gal	2–3 gal
12 × 14 ft (living room)	416 sq ft	≈ 2.4 gal	3 gal
14 × 16 ft (master bedroom)	480 sq ft	≈ 2.8 gal	3 gal
20 × 20 ft (large living room)	640 sq ft	≈ 3.8 gal	4 gal

Typical Coverage Rates by Surface

Surface	Sq ft per Gallon
Smooth interior drywall	350–400
Primed or previously painted walls	300–350
Textured / stucco surfaces	200–250
Raw, unprimed drywall	250–300

Troubleshooting & Practical Tips

Check the Can, Not Just the Formula

Coverage figures vary by brand, finish, and surface. A flat finish on smooth, primed drywall stretches further than a satin finish on rough or textured walls. Read the coverage statement on the actual can you bought — if it differs from 350 sq ft per gallon, adjust your estimate accordingly, or simply raise the wastage slider to compensate.

Textured and Porous Surfaces

Orange-peel, knockdown, and popcorn textures roughly double the surface area the paint has to cover. Unprimed drywall and fresh plaster absorb noticeably more paint on the first coat. For either case, plan on 20–30% more paint than a smooth-surface estimate — the wastage slider is the easy way to add it.

Buy a Little Extra, Save the Receipt

Two identical-looking cans from different batches can differ slightly in color. Buying all your paint at once — with a small surplus — means touch-ups after the job still match. Many stores accept returns on unopened cans, so rounding up to a whole gallon is rarely wasted money.

Common Mistakes to Avoid

Using the room's floor area (L × W) instead of its wall area — underestimates by roughly half.
Forgetting to subtract doors, windows, and built-in cabinetry.
Ignoring ceiling height changes (9 ft and cathedral ceilings add a lot of area).
Planning one coat when the color change is dark-to-light, or the surface is porous.
Rounding the total down instead of up — it's cheaper to have a little extra than to run out mid-roll.

Frequently Asked Questions

How much paint do I need for a 12×12 ft bedroom?

A 12×12 ft room with 8 ft ceilings has 384 sq ft of wall area. After subtracting one door (21 sq ft) and one window (15 sq ft), you get 348 sq ft of paintable surface. At 350 sq ft per gallon, that's just under one gallon per coat — so two coats need about 2 gallons, or about 2.2 gallons with a 10% wastage allowance.

Should I subtract doors and windows from the area?

Yes. A standard interior door is about 21 sq ft and a typical double-hung window about 15 sq ft — trim them off the wall area or you'll overestimate by roughly a quarter to a half gallon on an average room. Our calculator does this automatically based on how many doors and windows you enter.

How many coats of paint do I need?

Most repaint jobs need two coats: the first builds coverage and evens out the old color, the second gives a uniform finish. Dark-to-light color changes and bare drywall or primer over patches usually need two or even three. One coat is generally only enough when you're painting the exact same color over a color that's already in good condition.

Why does the estimate come out in gallons when paint also comes in quarts?

Gallons are the most common retail size for wall paint and give you a clean "cans to buy" number that's easy to round. If your total is under one gallon — common for a single accent wall or a small bathroom — buy quarts instead, and remember that four quarts equal one gallon.

Does ceiling height change how much paint I need?

Yes, and it's the single biggest source of underestimates. Raising the wall height from 8 ft to 9 ft adds roughly 12% more wall area in an average room, and cathedral or vaulted ceilings add far more. Always enter the actual wall height — and tick "include the ceiling" if you're painting it too.

Is 350 sq ft per gallon an accurate coverage figure?

It's the standard coverage manufacturers print on most interior and exterior paint cans, so it's the right default for estimating. Real-world yield is usually a bit lower — 300–330 sq ft per gallon — because of roller texture, drywall porosity, and uneven application. If you want to be safe, bump the wastage slider to 15–20% or check your can's exact number.