Calculators · Paint Calculator

Walls & Finishes

Paint Calculator

Enter a room or single walls, subtract doors and windows, choose your coats, and get the square feet to paint and the gallons to buy.

Room 1

Room length

Room width

Room height

Number of identical rooms

Paint the ceiling too

Opening 1

Type of opening

Opening width

Opening height

Number of identical openings

Opening 2

Type of opening

Opening width

Opening height

Number of identical openings

Coats of paint

The number you need depends on the surface and the color change.

Coverage per gallon (sq ft)

Per coat. Manufacturers publish 250 to 400. Use the figure on your can.

How to calculate how much paint you need

Paint is bought by the gallon, but it is calculated from the area you are going to cover. Work out the walls, take away what you will not paint, add the ceiling if you want it, and then see how many gallons that area takes at the coverage of your paint:

Step	Formula
Walls of a room	2 × (length + width) × height
A single wall	length × height
Door or window	width × height
Ceiling (optional)	length × width
Net area, one coat	walls − openings + ceiling
Total area to cover	net area × number of coats
Gallons needed	total area ÷ coverage per gallon, rounded up

Use Room rows for a normal rectangular room. The calculator multiplies the distance around it by its height. Use Wall rows for a single wall, an accent wall or a room with an odd shape. Rooms or walls that are the same size can share one row: type how many there are in the last box. Each measurement accepts feet and inches written as 9'6", and you can switch any box to inches, yards or meters.

Example: a 12 ft by 14 ft room with 8 ft walls

A room is 12 ft long, 14 ft wide and 8 ft high. It has one door and two windows, which we take at the average sizes Behr gives in its exterior painting guide (20 and 15 square feet). It gets two coats at 350 square feet per gallon. This is the default case the calculator loads when you open the page:

Step	Operation	Result
Walls	2 × (12 + 14) × 8	416 sq ft
Door	1 × 20	− 20 sq ft
Windows	2 × 15	− 30 sq ft
Net area, one coat	416 − 20 − 30	366 sq ft
Two coats	366 × 2	732 sq ft
Gallons, exact	732 ÷ 350	2.09
Gallons to buy	rounded up	3 gallons
Left over	3 − 2.09	0.91 gallons

Tick Paint the ceiling too and the ceiling adds 12 × 14 = 168 square feet. The net area becomes 534 square feet, two coats take 1,068 square feet, and 1,068 ÷ 350 = 3.05 gallons. That crosses the 3-gallon line by a small amount, so the calculator now says 4 gallons. This is why it shows the exact figure and the left over amount next to the whole number.

How many square feet does a gallon of paint cover?

Two manufacturers publish a range, and the ranges are not the same. Sherwin-Williams says a gallon typically covers about 350-400 square feet. Behr says one gallon of any of its paints covers between 250 and 400 square feet of surface with one coat. We checked both pages in October 2026. The calculator starts at 350, the low end of the Sherwin-Williams range and inside the Behr range, and you can type any other number in the coverage box. Here is what the range does to the example above (366 square feet, two coats, no ceiling):

Coverage per gallon	Exact gallons	Gallons to buy
250 sq ft	2.93	3
350 sq ft	2.09	3
400 sq ft	1.83	2

The coverage printed on the can of the paint you choose is the best figure to use. Sherwin-Williams also gives a quart as covering 100 square feet, and Behr lists quarts and five-gallon pails among its sizes. This calculator works in gallons. When the total reaches five gallons or more, it also shows the same amount as five-gallon pails plus loose gallons.

Paint for common room sizes

These are rooms with 8 ft walls, two coats and 350 square feet per gallon, before subtracting any door or window. Taking the openings out lowers the figures a little.

Room	Wall area	Ceiling area	Gallons, walls only	Gallons, walls and ceiling
10 × 10 ft	320 sq ft	100 sq ft	1.83, buy 2	2.40, buy 3
12 × 12 ft	384 sq ft	144 sq ft	2.19, buy 3	3.02, buy 4
12 × 14 ft	416 sq ft	168 sq ft	2.38, buy 3	3.34, buy 4
12 × 16 ft	448 sq ft	192 sq ft	2.56, buy 3	3.66, buy 4
14 × 16 ft	480 sq ft	224 sq ft	2.74, buy 3	4.02, buy 5
16 × 20 ft	576 sq ft	320 sq ft	3.29, buy 4	5.12, buy 6

Gallons by area and number of coats

If you already know the net area to paint, find it here. The figures use 350 square feet per gallon and are rounded up to whole gallons.

Net area, one coat	1 coat	2 coats	3 coats
200 sq ft	1	2	2
400 sq ft	2	3	4
600 sq ft	2	4	6
800 sq ft	3	5	7
1,000 sq ft	3	6	9
1,500 sq ft	5	9	13

Doors, windows and the surface you paint

Subtracting openings gives a more accurate estimate, and Behr gives averages for it: 15 square feet for a window and 20 square feet for a door. In the calculator, set an opening to Avg. door or Avg. window to use them, or choose Custom size and type the real width and height. Those averages come from a Behr guide for house exteriors, so for an unusual door or window measure it.

The surface changes how far the paint goes. Behr says a porous, rough, textured or previously unpainted surface will generally require more paint, as will a dramatic color change. Sherwin-Williams also says product requirements vary with the application method and the surface type or condition. We could not verify a number for how much a texture lowers the coverage, so the calculator does not apply one. If your walls are textured or porous, lower the coverage per gallon or add a coat.

What this calculator does not do

It gives the area to paint and the gallons. It does not give prices and it does not count trim, baseboards, crown molding, doors and window frames to be painted, or primer. For reference, Sherwin-Williams says a gallon of primer covers about 200-300 square feet (checked October 2026). If you want a rough primer figure, you can type the coverage printed on your primer can in the coverage box and choose 1 coat. A gable is a triangle, so for a house exterior add a Wall row with the gable base as the length and half the gable height as the height. Exterior paint may have a different coverage than interior paint, so check your can.

Frequently asked questions

How much paint do I need for 1 square foot?

With the default coverage of 350 square feet per gallon, one square foot takes 1/350 of a gallon per coat, which is about 0.37 fluid ounces (a gallon is 128 fluid ounces). The coverage of a real product can be lower or higher, so check the can and type your own figure in the calculator.

How many square feet does 1 gallon of paint cover?

Sherwin-Williams says a gallon typically covers about 350-400 square feet. Behr says one gallon of any of its paints covers between 250 and 400 square feet with one coat. Both depend on the surface and the product, so the calculator starts at 350 and lets you change it.

What is the formula for calculating paint?

Wall area is the distance around the room times its height. Subtract the doors and windows, add the ceiling if you are painting it, multiply by the number of coats, then divide by the coverage per gallon and round up to a whole gallon. For a 12 ft by 14 ft room with 8 ft walls, one door and two windows (Behr average sizes), two coats and 350 square feet per gallon, that is (416 − 20 − 30) × 2 ÷ 350 = 2.09, so 3 gallons.

How much paint do I need for a 3,000 sq ft house?

The floor area alone does not tell you, because paint goes on walls and ceilings. What counts is the height of the walls and the distance around each room. Add every room in the calculator and set the number of identical rooms where it applies. As an arithmetic example only, 3,000 square feet of net wall and ceiling area at two coats and 350 square feet per gallon needs 6,000 ÷ 350 = 17.14, so 18 gallons. That is 3,000 square feet of painted surface, not a 3,000 square foot house.

Should I subtract doors and windows?

For a more accurate estimate, yes. Behr suggests subtracting them and, in its exterior painting guide, gives an average window of 15 square feet and an average door of 20 square feet. In the calculator you can pick those averages for an opening or type the real width and height.

How many coats of paint do I need?

It depends on the surface and on the color change. Behr says a porous, rough, textured or previously unpainted surface will generally need more paint, as will a dramatic color change. Choose 1, 2 or 3 coats in the calculator. The result starts at 2 coats only as a starting value.

Why does the calculator round up to whole gallons?

Because paint is sold in whole cans. The exact figure (for example 2.09 gallons) is shown next to the rounded one (3 gallons) and the left over amount, so you can decide. Sherwin-Williams also notes that, depending on wall texture and desired coverage, having a little more than a gallon on hand may be best.