Paint Calculator

Buying too little paint means a second trip and a risk that the new batch looks slightly different; buying far too much wastes money. A room paint estimate only needs a few numbers: the size of the walls, the openings you will not paint, how many coats you plan and how far a gallon spreads. The paint calculator above works with feet and gallons or meters and liters, and can include the ceiling when you are painting it with the same product.

How to Use the Paint Calculator

Choose Room Dimensions and enter the length and width of the floor and the height of the walls. The calculator multiplies the perimeter by the height to get the wall area. If you are painting only some walls, or an exterior, a fence or an accent wall, choose Total Area instead and enter the area of the surfaces directly.

Next, enter the number of doors and windows. A standard interior door is about 3 by 6⅔ feet, or 20 square feet, and a typical window is around 15 square feet; change the sizes to match your room. Enter the number of coats and the coverage printed on the can, then pick a can size and, optionally, a price per can to see how many cans to buy and what they cost.

How Much Paint Do I Need?

With the default room, 12 by 12 feet with 8-foot walls, the perimeter is 48 feet and the wall area is 48 × 8 = 384 square feet. One door (20 ft²) and two windows (2 × 15 = 30 ft²) leave 334 square feet to paint. Two coats cover 668 square feet, and at 350 square feet per gallon that takes 668 ÷ 350 ≈ 1.91 gallons, about 7.23 liters. Rounded up, that is 2 one-gallon cans, or $80 at $40 a can.

A useful rule of thumb follows from the same numbers: an average bedroom of about 12 × 12 feet needs roughly one gallon per coat for the walls, and a ceiling of the same room adds a little under half a gallon per coat.

Paint = (Wall area − Door area − Window area + Ceiling area) × Coats ÷ Coverage

Wall area

the perimeter of the room, 2 × (length + width), times the wall height

Coats

the number of coats of paint you plan to apply

Coverage

the area one gallon or liter covers in one coat, from the can label

Round the result up to whole cans: Cans = ⌈Paint ÷ Can size⌉.

Paint Coverage and Number of Coats

Most interior latex wall paints list a coverage of about 350 to 400 square feet per gallon, or roughly 9 to 10 square meters per liter, for one coat on a smooth, primed surface. Real coverage is often lower. Textured walls, popcorn ceilings, bare drywall, unpainted plaster, brick and rough wood absorb more paint or have more surface area, and coverage on them can drop by a quarter or more. Rolling with a thicker nap or spraying also uses more paint than the label suggests.

Two coats are the norm for an even, durable finish, even with paints sold as paint-and-primer in one. One coat can be enough when you are refreshing the same color over a sound surface. Plan on a primer coat, which has its own coverage rating, when painting new drywall, covering stains or water damage, switching from oil-based to latex paint, or going from a dark color to a much lighter one.

• Smooth, previously painted walls: use the label coverage.
• New drywall or bare plaster: prime first, and expect the first coat to cover less.
• Textured or porous surfaces: reduce the coverage by about 20–30%.
• Dramatic color changes: allow an extra coat or a tinted primer.

Choosing Can Sizes and Saving Money

In the US, paint is mostly sold in quarts, one-gallon cans and five-gallon buckets; a quart is a quarter of a gallon. Quarts usually cost far more per gallon, so if your estimate is 1.8 gallons, two gallons are normally cheaper than a gallon and three or four quarts. For large jobs, a five-gallon bucket often costs less than five single gallons and keeps the color consistent across walls. In metric markets, common tin sizes are 1, 2.5, 5 and 10 liters.

When you need several cans of a custom-tinted color, mix them together in a larger bucket, a step called boxing, so slight differences between cans do not show as patches on the wall. Keep a little leftover paint for touch-ups, sealed tightly and labeled with the room and color formula.

Frequently asked questions