Paint Calculator
Measure your walls, type in the numbers and you’ll get the amount of paint to buy in gallons or litres.
The calculator subtracts doors and windows and multiplies by the number of coats, so you don’t end up with half a can left over or a second trip to the store halfway through the last wall.
How much paint do you need?
Enter your room size and you'll see how much paint to buy right here.
How this is calculated
Wall area is the distance around the room multiplied by the wall height. Doors and windows are taken off that, using a typical size you can change under "Fine-tune the numbers".
That area is multiplied by the number of coats and divided by the coverage on the can. Textured and rough walls soak up more paint, so they get a bit added on top. Last comes the extra for touch-ups.
The shopping list picks the mix of can sizes that usually costs the least, so you might see a gallon where three quarts would technically do.
Did this save you a trip back to the store? You can buy me a coffee.
Buy me a coffeeHow to use the paint calculator
- Measure the length of each wall and the ceiling height. Running a tape measure along the floor is fine.
- Enter how many doors and windows the room has. The calculator takes their area off for you.
- Choose the number of coats. Two is normal for a color change. One can work if you’re refreshing the same color.
- Pick your units and read the result.
Painting the ceiling too? Add it as a separate surface. Ceiling area is the room’s length times its width.
How the calculation works
You can check the result by hand with three lines of math:
Wall area = room perimeter × wall height
Paintable area = wall area − doors − windows
Paint needed = paintable area × coats ÷ coverage per gallon (or litre)
A standard door counts as about 20 sq ft (1.9 m²) and an average window as about 15 sq ft (1.4 m²). Most interior paints cover 350 to 400 sq ft per gallon, or about 10 m² per litre, on a smooth primed wall. The calculator uses the low end of that range on purpose. Running short is more annoying than having a bit left over.
A worked example
Your bedroom is 12 × 14 ft with 8 ft ceilings, one door and two windows.
- Perimeter: 12 + 14 + 12 + 14 = 52 ft
- Wall area: 52 × 8 = 416 sq ft
- Minus the door and windows: 416 − 20 − 30 = 366 sq ft
- Two coats: 732 sq ft
- At 350 sq ft per gallon: about 2.1 gallons
Buy two gallons and a quart.
In metric, a 4 × 5 m room with 2.5 m walls, one door and one window has about 41.6 m² of wall. Two coats is 83 m², or roughly 8.3 litres. A 10 litre tin does the job and leaves some for touch-ups.
How surface type changes coverage
The number on the can assumes a smooth, sealed wall. Rough or porous surfaces soak up more paint, so plan for less coverage on these:
| Surface | Approx. coverage per gallon | Approx. coverage per litre |
|---|---|---|
| Smooth, primed drywall | 350–400 sq ft | 9–10 m² |
| New drywall or bare plaster | 250–300 sq ft | 6–7 m² |
| Light texture (orange peel, knockdown) | 300–350 sq ft | 7–8 m² |
| Heavy texture or popcorn ceiling | 250–300 sq ft | 6–7 m² |
| Brick, block or stucco | 150–250 sq ft | 4–6 m² |
Prime bare plaster and fresh drywall before you paint. Primer costs less than paint, and without it the wall drinks your first coat. Paint-and-primer-in-one cans help on walls that were already painted, but they don’t replace a real primer on bare surfaces.
Tips for buying the right amount
Add about 10% to whatever the calculator says. Spills happen, and you’ll want some paint for touch-ups a year from now.
Get all your cans from the same batch, or pour them together into one bucket before you start. Color can shift slightly between batches, and on a flat wall in daylight you’ll see it.
Going dark over light, or light over dark, often takes a third coat. A tinted primer usually saves you that one.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window, you have about 350 sq ft of wall. Two coats need roughly 2 gallons.
How much paint do I need for a ceiling?
Multiply the room’s length by its width. A 12 × 14 ft ceiling is 168 sq ft, so one gallon covers two coats with some to spare.
Should I subtract doors and windows?
In a room with several of them, yes. It can save you a whole gallon. In a small room with one door the difference is minor, and some people skip it and keep the extra as a buffer.
Do I really need two coats?
Usually. One coat can look fine when you repaint in the same or a very close color. Changing color or painting over patched spots almost always takes two.
How much does one gallon of paint cover?
Around 350 to 400 sq ft on smooth primed walls, which is about one coat on an average bedroom. Textured or unprimed surfaces get less.
How long does leftover paint keep?
Water-based paint lasts several years if the lid is sealed tight and the can is stored somewhere it won’t freeze. Stir it well before reusing it, and throw it out if it smells sour or has gone lumpy.