Paint Calculator

How much paint do I need for a room?

Measure the walls, subtract the doors and windows, multiply by the number of coats and divide by the coverage printed on the paint label. Then round up to cans you can buy. A 12 × 12 ft bedroom with 8 ft walls, one door and two windows needs about 2.10 gallons for two coats at 350 sq ft per gallon with a 10% allowance, which is 2 gallons and 1 quart.

This calculator does that arithmetic for rooms of any size, wall by wall when a room isn’t a simple box, and for a ceiling and primer as separate products. It picks the mix of cans that leaves the least paint over, or the fewest cans, or the lowest price at the prices you enter, and it works in liters with the can sizes sold where you shop.

How to use it

• Units: feet and US gallons, or meters and liters. Switching converts the measurements and coverage you have typed; each system keeps its own list of can sizes.
• Measure by: Room size takes the room’s length, width and wall height and assumes four walls of that height. Each wall takes one row per wall, for L-shaped rooms, stairwells or a single accent wall. Total area is for a wall area you already know, before doors and windows.
• Lengths in feet and inches: type feet in the first box and inches in the second (leave inches blank for whole feet). The inches box takes fractions such as 7 1/2, and 14.5 in the feet box reads as 14 ft 6 in.
• Doors and windows: one row per size, with a count for openings that share it (two identical windows are one row with a count of 2). Remove every row to subtract nothing, or choose One total area if you only know the total.
• Coverage: the square feet one gallon covers (or square meters per liter), from the can or the product data sheet.
• Coats and waste allowance: the allowance is a percentage added to the paint for trays, rollers and touch-ups. Enter 0 for none.
• Paint the ceiling and add primer: each becomes its own line on the shopping list, with its own coverage and coats. A ceiling painted with the wall paint joins the wall paint instead.
• Cans: choose what matters most (least leftover, fewest cans or lowest cost) and the sizes you can buy. Lowest cost asks for the price of each size.

Results update as you type. The Try buttons add a ceiling, primer or a third coat to the example room, or switch to a single accent wall or a room measured in meters. Save for comparison keeps up to three scenarios side by side, for example flat and eggshell paints with different coverage. When you measure by room size, Continue in the Flooring and Tile Calculator carries the room’s length and width over as its floor.

Paint formula

Paint needed is the area to cover, times the coats, divided by the coverage, plus the allowance:

$$V = \frac{A \times n}{c} \times \left(1 + \frac{w}{100}\right) \qquad A_\text{walls} = 2\,(L + W) \times H - A_\text{openings}$$

• $V$ is the paint to buy for, in gallons (or liters).
• $A$ is the area to cover in sq ft (or m²): the walls after doors and windows, plus the ceiling when it gets the same paint.
• $n$ is the number of coats.
• $c$ is the coverage from the label, in sq ft per gallon (or m² per liter).
• $w$ is the waste allowance in percent.
• $L$, $W$ and $H$ are the room’s length, width and wall height; $2\,(L + W)$ is its perimeter.

The last step rounds $V$ up to whole cans. A US quart is a quarter of a gallon, so cans of 1 gal and 0.25 gal can make up any amount in quarter-gallon steps.

Worked example: a 12 × 12 ft room with one door and two windows

The room is 12 ft by 12 ft with 8 ft walls. It has one 3 ft × 6 ft 8 in door and two 3 × 5 ft windows. The paint label says 350 sq ft per gallon, the plan is two coats, and the allowance is 10%.

1. Perimeter: 2 × (12 + 12) = 48 ft. Walls: 48 ft × 8 ft = 384 sq ft.
2. Door: 6 ft 8 in is 6 + 8 ÷ 12 = 6.667 ft, and 3 ft × 6.667 ft = 20 sq ft. Windows: 2 × 3 ft × 5 ft = 30 sq ft. Openings: 20 + 30 = 50 sq ft.
3. Wall area to paint: 384 − 50 = 334 sq ft.
4. Paint for two coats: 334 × 2 ÷ 350 = 1.91 gal.
5. With the 10% allowance: 1.91 × 1.10 = 2.10 gal.
6. Cans: 2 gallons hold 2.00 gal, which is not enough, so add 1 quart. 2 gallons and 1 quart hold 2.25 gal and leave 0.15 gal over.

Without the allowance the same room needs 1.91 gal, and 2 gallons would do with 0.09 gal to spare. The allowance is what adds the quart: 10% of 1.91 gal is only 0.19 gal, but it tips the total past 2 gallons.

How to measure walls, doors and windows

Measure each wall along the floor, from corner to corner, and the height from the floor to the ceiling. For a rectangular room the two lengths are enough, because opposite walls match. Measuring to the nearest inch is plenty; the label’s coverage figure is far less precise than that.

• Doors and windows: measure the opening, including the frame if you won’t paint it with the wall color. Subtract only what the wall paint won’t cover.
• L-shaped rooms, alcoves and stairwells: switch to Each wall and enter every straight section as a row.
• Triangles: a gable is half of a rectangle, so enter its full width and half its height.
• Accent wall: enter it alone under Each wall and calculate the other walls as a second scenario.

How much does a gallon of paint cover?

The label on the can tells you, and it is the figure to trust for that paint. PaintCare puts most paints at 300 to 400 sq ft per gallon and suggests 350 for a rough estimate if the label doesn’t say. That is roughly 7.4 to 9.8 m² per liter.

The surface changes it. PaintCare notes that rough textures can take up to 50% more paint, which is like dividing the label’s coverage by 1.5: 350 sq ft per gallon becomes about 233. New drywall, plaster and bare wood soak up more paint too, which is what primer is for.

To see how much the coverage figure matters, the at other coverage rates table under the result reruns your job at 50 and 100 sq ft per gallon either side of your figure. For the example room, 250 sq ft per gallon needs 2.94 gal and 450 needs 1.63 gal.

How many coats of paint do you need?

As many as it takes to hide the old color evenly. PaintCare’s guidance is that a similar color needs fewer coats, a switch between very dark and very light can take three or four, and vivid reds, oranges and purples may need more. Sheen matters as well: glossier paint covers less, so where flat paint hides the wall in two coats, semi-gloss or gloss often needs three or more.

Each coat is another full layer, so paint grows in step with the coats. The 3 coats button shows the example room with a third coat: 334 × 3 ÷ 350 × 1.10 = 3.15 gal, or 3 gallons and 1 quart.

Do you need primer, and how much?

Use primer on new drywall, bare plaster or bare wood, and consider a tinted primer for a big color change. PaintCare explains that unprimed surfaces absorb more paint and that primer cuts the number of top coats.

Primer is calculated like paint, with the coverage from the primer’s own label. It covers every surface on the list, so the ceiling too when you paint it. The With primer button adds one coat at an example 300 sq ft per gallon to the example room: 334 ÷ 300 × 1.10 = 1.22 gal, or 1 gallon and 1 quart, with 0.03 gal over.

Gallons, quarts or both: choosing cans with the least leftover

Rounding up to whole gallons is the simplest rule, but it can leave most of a gallon unused. The calculator offers three ways to choose:

• Least leftover: the smallest amount of paint that covers the job, using any mix of the sizes you list. On a tie, fewer cans win.
• Fewest cans: as few cans as possible, then the least left over.
• Lowest cost: the cheapest mix at the prices you type in. The price per gallon differs between can sizes and from store to store, so the calculator uses your prices instead of assuming a ratio.

The One accent wall button shows why the rule matters. A 12 × 8 ft wall with two coats needs 96 × 2 ÷ 350 × 1.10 = 0.60 gal. Least leftover buys 3 quarts (0.75 gal, 0.15 gal over); fewest cans buys 1 gallon (0.40 gal over). Which is cheaper depends on your store’s prices. The Ways to buy table shows each rule’s cans whenever they differ.

Ceiling paint and metric rooms

Tick Paint the ceiling to add it. For a room, the ceiling is the floor’s length × width; with the other measuring modes you enter its length and width. A ceiling in a different paint gets its own coverage, coats and cans; choose Same as walls to add it to the wall paint instead. The Ceiling too button adds the example room’s 144 sq ft ceiling at two coats: 144 × 2 ÷ 350 × 1.10 = 0.91 gal, so 1 gallon of ceiling paint.

In liters, can sizes differ from country to country, so the calculator uses the sizes you list (1, 2.5, 5 and 10 L to start). A 3.78 L can holds about one US gallon and a 946 mL can about one US quart, so enter those sizes if that is what your store sells. The Metric room button works a 4 × 3.5 m room with 2.5 m walls, a 0.9 × 2.1 m door and a 1.2 × 1.2 m window, at 9 m² per liter:

1. Walls: 2 × (4 + 3.5) × 2.5 = 37.5 m². Openings: 1.89 + 1.44 = 3.33 m². To paint: 34.17 m².
2. Paint: 34.17 × 2 ÷ 9 × 1.10 = 8.35 L.
3. Least leftover: 1 × 5 L, 1 × 2.5 L and 1 × 1 L, which hold 8.5 L and leave 0.15 L.

Reading the result

• The headline is the cans to buy for the wall paint (or for the walls and ceiling together), with the amount needed and what will be left over.
• Area to paint is the wall area after doors and windows, plus the ceiling. Paint needed is the amount before rounding to cans, and left over is what the cans hold beyond it. With lowest cost, the last tile is the total price instead.
• The shopping list appears when you add a ceiling paint or primer, with a line per product. Download it as a CSV file, or use Copy results or Print to take it to the store.
• How this was calculated repeats every step with your numbers, including any inches turned into feet.
• Ways to buy compares the three can rules for the wall paint when they give different cans.
• At other coverage rates shows how much the paint, the cans and the leftover change if the paint covers less or more than the label says.

Assumptions and limitations

• Coverage is the figure you enter and applies to every coat. Real coverage depends on the surface, the color change, the tools and how thickly you apply it.
• A room is four walls of one height. Sloped ceilings, arches and bay windows need the Each wall mode or a total area.
• Doors and windows are subtracted in full. Trim, baseboards, door faces and cabinets are not included; buy for them separately.
• Gallons are US gallons. An imperial gallon is about 20% larger, so a coverage figure per imperial gallon needs converting first.
• The number of coats is your call, and the result can’t check it. If the old color still shows through after the coats you planned, the extra coat needs more paint than this result.

Common mistakes

• Using the floor area. A 12 × 12 ft room has a 144 sq ft floor but 384 sq ft of wall with 8 ft walls. Paint follows the walls.
• Typing inches in the feet box. 96 in the feet box is a 96 ft wall. Put 96 in the inches box instead; the calculator flags any wall height of 30 ft or more.
• Forgetting the second coat. Coverage on the label is for one coat. Two coats need twice the paint.
• Using the label’s coverage on a rough wall. Rough or textured surfaces can take up to 50% more paint than the label’s figure; lower the coverage before you buy.
• Rounding before adding up. Rounding each wall’s paint up to a gallon can add almost a gallon per wall. Add up the paint first and round once, at the cans.
• Counting a quart as a third of a gallon. Four quarts make a gallon, so 3 quarts are 0.75 gal.