How much paint do I need?

Two coats over a smooth, primed wall is the easy answer. Most walls aren't smooth or primed, and the coverage figure on the can was measured against the one wall yours isn't. Here's what the job actually takes, and how far off the label runs.

01 How the math works

Paint is a coverage problem. Work out the paintable area, multiply by the number of coats, and divide by how far a gallon goes on that surface. Round up to whole cans, because a wall that comes up 5% short still reads as unfinished.

Two numbers move the answer, and neither is on the label. The first is the surface factor above — the 350–400 ft²/gal printed on the can is measured on a smooth, primed wall, and a textured or bare one takes more. The second is container size. Paint sells in gallons and quarts, and a quart holds a quarter of a gallon, so it covers a quarter of the area. The can prints a range and lets you keep the number you like best; the wall gets a vote too.

02 Worked example

Take the walls of a 12 ft × 14 ft bedroom with 8 ft ceilings (3.66 m × 4.27 m × 2.44 m), ceiling left out. Wall area is 2 × (12 + 14) × 8 = 416 ft²; knock off a door and one window at about 33 ft² and the paintable area is 383 ft² (35.6 m²). Two coats need 766 ft² (71.2 m²) of coverage.

Read the can flat. At 350 ft²/gal (32.5 m² per 3.79 L), 766 ÷ 350 = 2.19 gallons, which rounds up to 3 gallons. That's the number nearly every calculator hands back.

Now paint the room that's actually there. New, unprimed drywall covers at about 250 ft²/gal (23.2 m² per US gallon), so 766 ÷ 250 = 3.06 gallons. Same room, same two coats — but the honest order is 4 gallons, a full gallon past what the flat figure predicted. In metric the same job climbs from 9 L to 12 L.

The 3.06-gallon figure is barely over three. Rather than a fourth full can, buy 3 gallons plus one quart: a quart covers about 87.5 ft² at the smooth rate, which closes the gap and leaves less on the shelf when the job is done.

03 When this calculator is wrong

Manufacturer coverage figures are for smooth, primed walls under ideal conditions, which describes almost no wall anyone is actually painting. Textured walls cut coverage by 20–30%. Unprimed new drywall cuts it by 30%+ on the first coat. Take the can figure at face value and you'll under-order by one or two gallons on a typical room. The exception runs the other way: a repaint in the same color over a sound, previously painted wall often covers 10–15% better than spec, so there the label is the conservative number.

• A room is rarely one surface. Patch a wall and the repair reads as bare drywall at 250 ft²/gal while the rest covers at 350. The calculator applies a single factor; order to the thirstiest surface in the room, not the average.
• The textured band is wide. Light knockdown sits near the 20% end; heavy orange-peel or a popcorn ceiling runs past 30%. The ×0.75 factor is the midpoint, so on heavy texture it still under-orders — treat it as a floor.
• The unprimed penalty is really a first-coat penalty. Bare drywall drinks the first coat; once that coat seals the paper face, later coats cover close to normal. Applying the 250 ft²/gal rate to both coats over-orders a little — which beats coming up short — but priming the board first resets the whole job to the 350–400 ft²/gal rate and can drop you back a gallon.
• A drastic color change hides in the coat count. Covering a deep red or going dark-to-white sometimes takes three coats where two would otherwise do. The math sizes the coats you enter; it can't see the color underneath.

04 What to do with the result

Order the surface-adjusted number, not the flat one, and set the surface to the wall you have rather than the one on the label. When the honest figure lands just over a whole gallon — 3.06 in the example — a gallon plus a quart closes it for less than a second full can, since a quart goes about 87.5 ft² at the smooth rate.

On new drywall, price the primer against the extra finish paint before you skip it. A coat of primer converts the board from the 250 ft²/gal rate to the 350–400 ft²/gal rate, so the priming decision is a paint-quantity decision, not only an adhesion one — and primer is usually the cheaper gallon.

Buy the color in one trip when it's custom-mixed. Tinted paint is matched to a batch, and a second order weeks later can come back a shade off — close enough to see in daylight on the same wall. Round up the first time so you're not chasing a match on the last 40 ft².

05 Common questions

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

A 12 × 12 ft room with 8 ft walls is 384 ft² before openings. Two coats on a smooth, primed wall at 350 ft²/gal land at 3 gallons. On new, unprimed drywall the same room needs 4 gallons — switch the surface setting to match what you're painting.

How much does one gallon of paint cover?

A US gallon of standard interior latex covers 350–400 ft² (32.5–37.2 m²) per coat on a smooth, primed wall. Drop that by 20–30% on texture, and expect 250–300 ft² per gallon on the first coat over bare drywall.

Do I subtract doors and windows?

Yes, but only the big ones. A standard door is about 21 ft² and an average window about 12 ft²; subtract those. Skipping the deduction pads your order, which is the safe direction to be wrong, but on a room with several large openings it can add a whole unnecessary gallon.

Do I need one coat or two?

Two is the default for a color change or bare drywall — one coat rarely reads as even. A repaint in the same color over a sound surface can be a single coat plus touch-ups. The math scales directly with the coat count you enter.

Does priming save paint or just help it stick?

Both. On bare drywall, primer seals the porous paper face so the finish coats cover at the smooth 350–400 ft²/gal rate instead of 250. That can be a gallon of finish paint saved on a single room, and primer is usually cheaper per gallon than the color coat.