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Formula
Gallons = (Wall length × Height − Doors − Windows) × Coats × (1 + Waste ÷ 100) ÷ Coverage
- Wall length
- combined width of all walls being painted (the room perimeter for a whole room)
- Doors, Windows
- number of openings × area of each (default 20 ft² per door, 15 ft² per window)
- Coats
- number of coats of paint (default 2)
- Waste
- extra allowance in percent for roller and tray loss and touch-ups (default 10%)
- Coverage
- square feet one gallon covers (default 350 ft² per gallon)
How much paint do you need?
Paint is sold by volume but used by area, so the question is always the same: how many square feet of wall are you covering, how many times, and how far does a gallon go? This paint calculator answers it in four steps: wall area, minus openings, times coats, divided by coverage. It then rounds up to whole gallons and shows the cheaper gallons-plus-quarts alternative.
How to use the paint calculator
- Total wall length: add the widths of every wall you’ll paint. For a whole room that’s the perimeter, 2 × (length + width).
- Wall height: floor to ceiling.
- Doors and windows: enter how many. Keep the standard sizes (a 20 ft² door and a 15 ft² window) or choose to enter your own.
- Coats: two is standard.
- Coverage: copy the spread rate from the can, or leave it blank for 350 ft² per gallon. Switch the unit to m² per liter to work in liters.
- Waste allowance: 10% by default.
The result shows the gallons to buy, the exact amount needed, and the gallons-and-quarts combination that covers it.
How to calculate paint by hand
Take an 11 × 13 ft bedroom with 8 ft ceilings, one door and two windows, painted with two coats.
- Perimeter: 2 × (11 + 13) = 48 ft
- Wall area: 48 × 8 = 384 ft²
- Openings: 20 + 2 × 15 = 50 ft²
- Paintable area: 384 − 50 = 334 ft²
- Two coats plus 10%: 334 × 2 × 1.10 = 734.8 ft²
- Gallons: 734.8 ÷ 350 = 2.1 gallons
You need just over 2 gallons. Rounding up gives 3 gallons. The leftover 0.1 gallon is 0.4 of a quart, so 2 gallons plus 1 quart would also do, and the third gallon leaves plenty for touch-ups.
Using liters
The method is identical in metric. Coverage on metric cans is given in m² per liter; 350 ft² per US gallon is about 8.6 m²/L. A 3.2 × 4 m room with 2.4 m walls has a perimeter of 14.4 m and 34.56 m² of wall. Take off a 1.9 m² door and a 1.4 m² window to get 31.26 m². Two coats is 62.52 m², which at 10 m²/L needs about 6.3 L before any allowance.
Paint needed for common room sizes
Calculated with the paint calculator for 8 ft walls, one door, two windows, two coats, 350 ft² per gallon and a 10% allowance:
| Room | Perimeter | Paintable wall | Gallons needed | Buy |
|---|---|---|---|---|
| 10 × 10 ft | 40 ft | 270 ft² | 1.7 | 2 gallons |
| 12 × 12 ft | 48 ft | 334 ft² | 2.1 | 3 gallons, or 2 gallons + 1 quart |
| 14 × 16 ft | 60 ft | 430 ft² | 2.7 | 3 gallons |
| 15 × 20 ft | 70 ft | 510 ft² | 3.21 | 4 gallons, or 3 gallons + 1 quart |
| 20 × 20 ft | 80 ft | 590 ft² | 3.71 | 4 gallons |
At higher coverage, such as 400 ft² per gallon on smooth, primed drywall, you’ll need 12.5% less (350 ÷ 400 = 0.875).
What changes coverage
- Surface texture. Textured plaster, brick and popcorn finishes have more surface area than smooth drywall and soak up more paint.
- Porosity. New drywall, bare wood and fresh plaster absorb the first coat. Prime them first; primer is cheaper and evens out absorption. Run the paint calculator a second time with the primer’s coverage to size that order.
- Color change. Going from dark to light, or to a deep red or yellow, may need a third coat. Tinted primer helps.
- Application. Rollers with a thicker nap hold more paint, and sprayers lose a share to overspray, so add more allowance if you spray.
Common mistakes
- Using floor area instead of wall area. A 12 × 12 ft room has 144 ft² of floor but over 380 ft² of wall at 8 ft high. For the floor, use the square footage calculator.
- Forgetting the second coat. Coverage figures are per coat. Two coats double the paint.
- Taking the coverage figure at face value on rough walls. The number on the can assumes a smooth, sealed surface.
- Buying many quarts. Four quarts nearly always cost more than one gallon, so round up to a gallon when the remainder is large.
- Leaving out the ceiling or trim when you plan to paint them. Calculate the ceiling as length × width and trim separately; they usually use different paint.
Planning the rest of the room
If you’re redoing the floor at the same time, the flooring calculator turns room areas into boxes, and the tile calculator counts tiles for a bathroom or backsplash.
Glossary
- Coverage (spread rate): the area one gallon or liter covers in a single coat, printed on the can.
- Coat: one complete layer of paint.
- Primer: a base coat that seals the surface and helps paint adhere and cover evenly.
- Quart: a quarter of a US gallon, about 0.95 L.
Worked examples
A 12 × 14 ft bedroom, two coats
Perimeter 52 ft (12 + 14 + 12 + 14), 8 ft ceilings, one door and two windows at the default sizes, two coats at 400 ft² per gallon, 10% extra. It needs just over 2 gallons, so the alternative to a third gallon is 2 gallons plus a quart.
Inputs: Total wall length = 52; Total wall length unit = Feet; Wall height = 8; Wall height unit = Feet; Doors = 1; Windows = 2; Coats = 2; Coverage (spread rate) = 400; Coverage unit = ft² per gallon (results in gallons); Waste allowance = 10 %
Wall area: 52 ft × 8 ft = 416 ft² Openings: 1 × 20 + 2 × 15 = 50 ft² Paintable: 416 − 50 = 366 ft² To cover: 366 × 2 coats × 1.1 = 805.2 ft² Paint: 805.2 ft² ÷ 400 ft²/gal = 2.01 gal
Answer: 366 ft² of wall, 2 coats: 2.01 gallons needed, so buy 3 gallons (or 2 gallons + 1 quart).
A living room with a 9 ft ceiling
66 ft of wall, 9 ft high, two doors and four windows at the default sizes, two coats at the default 350 ft² per gallon, 10% extra.
Inputs: Total wall length = 66; Total wall length unit = Feet; Wall height = 9; Wall height unit = Feet; Doors = 2; Windows = 4; Coats = 2; Coverage unit = ft² per gallon (results in gallons); Waste allowance = 10 %
Wall area: 66 ft × 9 ft = 594 ft² Openings: 2 × 20 + 4 × 15 = 100 ft² Paintable: 594 − 100 = 494 ft² To cover: 494 × 2 coats × 1.1 = 1,086.8 ft² Paint: 1,086.8 ft² ÷ 350 ft²/gal = 3.11 gal
Answer: 494 ft² of wall, 2 coats: 3.11 gallons needed, so buy 4 gallons (or 3 gallons + 1 quart).
A metric room in liters
15.6 m of wall, 2.5 m high, one 1.8 m² door and one 1.5 m² window, two coats at 10 m² per liter, 10% extra.
Inputs: Total wall length = 15.6; Total wall length unit = Meters; Wall height = 2.5; Wall height unit = Meters; Doors = 1; Windows = 1; Door and window sizes = Enter my own sizes; Area of each door = 1.8; Area of each door unit = Square meters; Area of each window = 1.5; Area of each window unit = Square meters; Coats = 2; Coverage (spread rate) = 10; Coverage unit = m² per liter (results in liters); Waste allowance = 10 %
Wall area: 15.6 m × 2.5 m = 39 m² Openings: 1 × 1.8 + 1 × 1.5 = 3.3 m² Paintable: 39 − 3.3 = 35.7 m² To cover: 35.7 × 2 coats × 1.1 = 78.54 m² Paint: 78.54 m² ÷ 10 m²/L = 7.85 L
Answer: 35.7 m² of wall, 2 coats: buy about 7.9 L of paint (10% extra included).
One accent wall, one coat (edge case: no openings)
A single 11 ft × 8 ft wall with no doors or windows, one coat at 350 ft² per gallon and no waste: 88 ft² needs just over a quarter gallon, so buy 2 quarts or a single gallon.
Inputs: Total wall length = 11; Total wall length unit = Feet; Wall height = 8; Wall height unit = Feet; Doors = 0; Windows = 0; Coats = 1; Coverage (spread rate) = 350; Coverage unit = ft² per gallon (results in gallons); Waste allowance = 0 %
Wall area: 11 ft × 8 ft = 88 ft² To cover: 88 × 1 coats × 1 = 88 ft² Paint: 88 ft² ÷ 350 ft²/gal = 0.25 gal
Answer: 88 ft² of wall, 1 coat: 0.25 gallons needed, so buy 1 gallon (or 2 quarts).
Assumptions and limitations
- This is an estimate only, not professional or engineering advice. Actual coverage depends on the product, surface texture, porosity and color change, so check the coverage printed on your can.
- Default coverage is 350 ft² per gallon, the low end of the 350–400 ft² per gallon that Sherwin-Williams gives for a typical gallon of paint. Rough, porous or unprimed surfaces can cover far less.
- Default opening sizes are 20 ft² per door (3 ft × 6 ft 8 in) and 15 ft² per window (3 ft × 5 ft) with the Standard sizes option. Choose "Enter my own sizes" for patio doors or large windows.
- The waste allowance defaults to 10% if left empty, for paint left in the roller, tray and can and for touch-ups.
- Ceilings, trim and primer are not included. Primer usually covers less (about 200–300 ft² per gallon) and should be calculated separately.
- Gallons are rounded up to whole gallons for the headline figure; the gallons-plus-quarts option rounds the remainder up to whole quarts (4 quarts per gallon). In liter mode the amount is rounded up to 0.1 L.
Frequently asked questions
How many square feet does a gallon of paint cover?
Sherwin-Williams puts a typical gallon at about 350–400 square feet on a smooth, previously painted wall. Textured, porous or never-painted surfaces can drop toward 250 ft², and primer typically covers 200–300 ft².
How much paint do I need for a 12x12 room?
With 8 ft ceilings, one door, two windows and two coats, a 12 × 12 ft room has about 334 ft² of wall and needs roughly 2.1 gallons including a 10% allowance at 350 ft² per gallon. Buy 3 gallons, or 2 gallons plus a quart.
Should I buy gallons or quarts?
Quarts suit small top-ups and accent walls, but four quarts usually cost more than one gallon. If you need more than about two quarts beyond your last full gallon, another gallon is usually the better buy.
Do I need two coats of paint?
Two coats is the norm for an even finish, especially when changing color. A single coat can work when repainting the same color over a sound, clean surface.
Should I subtract doors and windows when calculating paint?
Yes, for an accurate figure. A standard door removes about 20 ft² and an average window about 15 ft². For a room with a few small openings, some painters skip the subtraction and keep the difference as extra.
How much paint for a 5-gallon bucket?
At 350–400 ft² per gallon, a 5-gallon bucket covers about 1,750–2,000 ft² in one coat, or about 875–1,000 ft² with two coats.
Sources
- Paint Calculator (How much paint do I need?) — Sherwin-Williams. Checked October 12, 2026.
- NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity — National Institute of Standards and Technology. Checked October 12, 2026.