Paint Coverage Calculator — Free & Instant

What this calculator does

Gallons of paint needed for a room, allowing for doors, windows and a second coat. Fill in the fields in the calculator above and the results update as you type — nothing is sent anywhere, and there is no sign-up or paywall.

Most people reach for this after a quick mental estimate has already gone wrong: the numbers are close enough to feel right, but the decision behind them is worth real money or real time. Working it out properly takes seconds here.

How to use the tool above

1. Enter your own numbers in these fields: room length, room width, wall height, doors + windows, coats.
2. Read Gallons needed first — that is the figure most people came for.
3. Change one field at a time to see how sensitive the outcome is. That single habit is what turns a calculator into a decision tool.
4. Note the result somewhere before you compare options, so you are comparing like with like.

The method, in plain words

• Paintable wall area = the larger of (2 × (room length + room width) × wall height - doors + windows × 20, 0)
• Gallons needed = the larger of (2 × (room length + room width) × wall height - doors + windows × 20, 0) × coats ÷ 350
• Cans to buy (1 gal) = ceil(the larger of (2 × (room length + room width) × wall height - doors + windows × 20, 0) × coats ÷ 350)

Nothing here is hidden: the same arithmetic runs in the tool above, so you can reproduce any number by hand if you want to check it.

Worked example 1

What you enter

• Room length: 14.00 ft
• Room width: 12.00 ft
• Wall height: 9.00 ft
• Doors + windows: 3
• Coats: 2

What you get

• Paintable wall area: 408 sq ft
• Gallons needed: 2.33 gal
• Cans to buy (1 gal): 3 cans

Worked example 2 — one input doubled

The fastest way to understand any calculation is to move a single input and watch what happens.

What you enter

• Room length: 28.00 ft
• Room width: 12.00 ft
• Wall height: 9.00 ft
• Doors + windows: 3
• Coats: 2

What you get

• Paintable wall area: 660 sq ft
• Gallons needed: 3.77 gal
• Cans to buy (1 gal): 4 cans

Comparing the two runs tells you which input actually drives the outcome — and therefore which one is worth negotiating, adjusting or planning around.

Common mistakes to avoid

• Mixing time periods. Monthly, weekly and annual figures cannot be compared until they are converted to the same basis.
• Rounding too early. Round only the final answer; rounding an input first quietly shifts everything after it.
• Forgetting what the number excludes. A clean calculation still leaves out fees, taxes or extras that are specific to your situation — add them separately.
• Treating a single run as the answer. Run a slightly pessimistic version too, then plan against that.

Every figure here is an estimate for planning purposes, not professional financial, tax or legal advice. Confirm anything that affects a contract, a filing or a loan with a qualified professional.

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