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How many gallons for your walls and ceiling, with doors and windows subtracted properly.

Enter the room dimensions to see how many gallons you need.

Room

Ceiling paint is usually a separate product.

Doors and windows

21 covers a standard door and frame.

15 is an average window.

Paint

Two is the normal recommendation.

Check the can — 350 to 400 is typical.

Needed over bare surfaces, stains, or a big color change.

Optional

Estimate. A real-world figure based on the assumptions listed below, which you can change.

How this calculator works

Wall area is the room’s perimeter times its ceiling height. Doors and windows are subtracted at their real sizes — a standard door with its frame is about 21 square feet, an average window about 15. Both figures are editable, because a picture window and a bathroom window are not the same hole in the wall.

Coverage defaults to 350 square feet per gallon, the conservative end of the 350–400 range manufacturers print on the can. The low end is the safer default for someone buying paint once: coverage drops on porous, textured or unprimed surfaces, and running out mid-wall leaves a visible lap mark.

Gallons are always rounded up. Paint is sold in whole containers, and a second trip in the middle of a coat is worse than a little left over.

The formula

Exactly what happens to your numbers, step by step.

  1. Wall area

    wall area = 2 × (length + width) × ceiling height

  2. Paintable area

    paintable = wall area − (doors × area per door) − (windows × area per window)
          + ceiling area, if painting the ceiling

  3. Gallons

    gallons = ⌈paintable area × coats ÷ coverage per gallon⌉

A worked example

A 12 × 10 foot bedroom with 8-foot ceilings, one door and two windows, two coats.

What you enter

Room
12 ft × 10 ft
Ceiling height
8 ft
Doors / windows
1 / 2
Coats
2
Coverage
350 sq ft per gallon

Step by step

Perimeter
2 × (12 + 10) = 44 ft
Wall area
44 × 8 = 352 sq ft
Openings
21 + 30 = 51 sq ft
Paintable
301 sq ft
Two coats
602 ÷ 350 = 1.72 gallons

2 gallons

Just under two gallons is needed, so buy two. The remainder is worth keeping for touch-ups — matching a color later is harder than it sounds even with the same paint code.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • Walls are flat and previously painted. Textured or bare surfaces absorb more.
  • A standard door with frame is 21 square feet and an average window 15, both adjustable.
  • Coverage is 350 square feet per gallon unless you change it.
  • The ceiling, if included, is length × width.

What this cannot tell you

  • A dramatic color change, especially light over dark, often needs an extra coat or a tinted primer.
  • Trim, doors and window frames are not included — they are usually a different product and finish.
  • Textured walls, popcorn ceilings and masonry can consume 20% to 50% more paint than a smooth surface.

Questions people ask

How much does a gallon of paint cover?

Usually 350 to 400 square feet on a smooth, previously painted surface. Textured, porous or unprimed surfaces cover considerably less. The can states the manufacturer’s figure.

Do I really need two coats?

Almost always. One coat rarely covers evenly, particularly over a color change, and the finish tends to look patchy in raking light. Two coats is the standard recommendation for a reason.

Should I subtract doors and windows?

Yes — on a small room they can be 15% of the wall area, which is a whole gallon on a larger job. This calculator subtracts them at editable sizes.

When do I need primer?

Over bare drywall, wood or masonry, over stains, and when making a big color change. A tinted primer under a strong color often saves a topcoat.

Related calculators

Sources and review

Sources

  • Manufacturer coverage figures printed on the container, typically 350–400 ft² per gallon

Methodology version 1.0.0 · Last reviewed

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For coursework, a resource list or your own site. How every calculator is built and checked is set out on the methodology page.

Citation

Chalk Calculator. (2026, September 20). Paint Calculator (Methodology version 1.0.0). https://chalkcalculator.com/paint-calculator/

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