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Paint Calculator: Gallons for a Room

Estimate how many gallons of wall paint a room needs from its dimensions, door and window openings, coat count, spread rate, and a waste allowance. Rounds up to the half gallon you can actually buy.

Room paint gallons
Enter the room size, the door and window openings, coats, and the spread rate to get gallons to buy.

Seeds a standard 12x12x8 room. Toggle the ceiling or bump waste for texture.

Room size
ft

Length of the room along one wall.

ft

Width of the room along the adjacent wall.

ft

Floor-to-ceiling wall height.

Doors and windows
pcs

Number of door openings to subtract. Set to 0 for none.

sq ft

Area of one door opening. A standard interior door is about 20 sq ft (roughly 1.86 m2).

pcs

Number of window openings to subtract. Set to 0 for none.

sq ft

Area of one window opening. A common window is about 15 sq ft (roughly 1.39 m2).

Coats, spread rate, and waste

Number of coats. Two is typical over a similar color; use more when covering a big color change.

Manufacturer spread rate per gallon. A common interior figure is 350 sq ft per gallon; check the label.

%

Extra percent for rollers, trays, touch-ups, and texture. About 10% is common; add more for heavy texture.

Add the ceiling (length x width) to the paintable area, or keep walls only.

Gallons to buy

2.5 gal

9.46 l

Raw gallons: 2.1 galPaintable area: 334 sq ftGross wall area: 384 sq ft

What does a room paint calculator tell me?

It converts room dimensions and openings into paintable wall area, then into the gallons to buy for your coats.

Paint is sold by the gallon, but rooms are measured in feet. A room paint calculator bridges the two: find the wall area, subtract the doors and windows you will not paint, multiply by the number of coats, divide by the spread rate on the can, add a little waste, and round up to a can size you can actually buy.

The wall area of a rectangular room is its perimeter times the wall height: 2 x (length + width) x height. Subtracting openings matters most in rooms with several large windows or doors, where they can remove a meaningful share of the surface. Ceilings are optional here because they are often painted with a separate flat ceiling paint and bought on their own.

Two numbers drive the result as much as the room size: the spread rate and the coat count. Manufacturers publish spread rates near 350 to 400 square feet per gallon on smooth, sealed drywall, but porous or textured surfaces cover less. Two coats is the usual recommendation for an even, durable finish. Enter the values from your product and your surface, not just the defaults.

How does this calculator size a room in gallons?

Perimeter times height minus openings, multiplied by coats and waste, divided by spread rate, rounded to the half gallon.

Formula

Gross wall area = 2 x (length + width) x height. Paintable area = gross wall area - (doors x area per door) - (windows x area per window) + ceiling. Raw gallons = (paintable area x coats / spread rate) x (1 + waste / 100). Gallons to buy = ceil(raw x 2) / 2.

Gross wall area is 2 x (length + width) x height, in store units of square feet. Paintable area subtracts the door and window openings and, when you turn it on, adds the ceiling (length x width). Raw gallons multiply paintable area by the coat count, divide by the spread rate per gallon, and scale by the waste factor (1 + waste / 100). Gallons to buy round up to the half gallon with ceil(raw x 2) / 2, since paint is sold in gallons and quarts. Metric users enter meters and square meters; the runner converts to store units and shows a liter reading beside the gallon total.

How much waste should I add?

About 10% is a reasonable default. It absorbs what stays on the roller and in the tray, touch-ups, and the extra a textured surface drinks up. Increase it for heavy texture, cutting-in around lots of trim, or a sprayer, which loses more to overspray. The half-gallon rounding already adds a small buffer on top of the waste factor.

Why round to the half gallon?

Paint comes in gallons and quarts, and four quarts make a gallon, so the half gallon is the smallest practical retail step for a wall project. Rounding up avoids running short partway through a wall, where a fresh can can flash or shade slightly against a drying edge. Keep the labeled leftover for future touch-ups from the same batch. Related leaves: primer amount for the same walls, and trim gallons for baseboards, casings, and doors.

Example

Paint a 12 ft by 12 ft room with 8 ft walls, one 20 sq ft door and two 15 sq ft windows, walls only, 2 coats at 350 sq ft per gallon, 10% waste.
  1. 1
    Gross wall area2 x (12 + 12) x 8 = 384 sq ft.
  2. 2
    Paintable area384 - (1 x 20) - (2 x 15) = 334 sq ft.
  3. 3
    Raw gallons(334 x 2 / 350) x 1.10 = 2.099 gal.
  4. 4
    Gallons to buyceil(2.099 x 2) / 2 = ceil(4.199) / 2 = 5 / 2 = 2.5 gal.
ResultBuy 2.5 gallons (two 1-gallon cans plus two quarts, or a 1-gallon plus a 1-gallon and a half).

FAQ

Wall area, openings, coats, spread rate, and why paint is bought by the half gallon.

How many gallons for a 12x12 room with 8 ft walls at 2 coats?

Gross wall area is 2 x (12 + 12) x 8 = 384 sq ft. Subtracting one 20 sq ft door and two 15 sq ft windows leaves 334 sq ft. Two coats at 350 sq ft per gallon with 10% waste is about 2.10 gallons, which rounds up to 2.5 gallons to buy.

How does the metric mode work for paint?

Switch to Metric and enter the room in meters and the openings in square meters. A 3.66 m by 3.66 m room with 2.44 m walls (the same as 12 by 12 by 8 ft) still needs about 2.5 gallons. Paint is sold by the gallon in the United States, so the primary purchase quantity stays in gallons with a liter reading alongside.

What spread rate should I use?

Use the number on the can. Interior wall paints commonly claim 350 to 400 sq ft per gallon on smooth, sealed drywall. Porous, textured, or previously unpainted surfaces cover less, so choose a lower rate. The first coat over new drywall or a big color change usually spreads less than the second.

Do I really need two coats?

Two coats is the usual recommendation for an even, durable finish, even over a similar color. One coat can work for a minor refresh in the same color. Deep or dramatic color changes may need a tinted primer plus two coats. Set the coat count to match your situation.

Does this include the ceiling?

Only if you turn it on. By default the calculator sizes walls only. Choose Add ceiling to include length times width in the paintable area. Ceilings are often painted with a dedicated flat ceiling paint, so many people size and buy them separately.

Why does it round to the half gallon?

Paint is sold in gallons and quarts, and four quarts make a gallon, so a half gallon is the smallest common retail step for this purpose. Rounding up to the half gallon avoids running short mid-wall and leaves a little for touch-ups. Keep leftover paint labeled for future repairs.

Sources

High-trust standards and documented references.

  1. 1. Painting Industry Standards (coats, spread rate, and workmanship)

    PDCA (Painting Contractors Association)

    Trade-association painting standards behind the two-coat default and the 350 to 400 sq ft per gallon planning range for interior wall paint.

    https://www.pdca.org/

  2. 2. Painting - Interior and Exterior (product data and spread rates)

    Sherwin-Williams

    Manufacturer coverage guidance; typical interior spread rates near 350 to 400 sq ft per gallon used as the default.

    https://www.sherwin-williams.com/en-us/paint-colors/coverage-calculator

  3. 3. How Much Paint Do I Need? (coverage and coats)

    Benjamin Moore

    Manufacturer guidance on estimating gallons, coats, and subtracting openings; corroborates the two-coat default.

    https://www.benjaminmoore.com/en-us/interior-exterior-paints-stains/how-to-paint/how-much-paint-do-i-need

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