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Paint Calculator for Interior Rooms

Estimate wall paint by room dimensions, doors, windows, coats, and coverage per gallon.

Free planning calculatorUS units
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Calculator

Estimate the material order.

Start with the project dimensions, then adjust defaults if the room or product calls for it.

Enter your room details and calculate.

Use the number well

Plan like a buyer, not a guesser.

The point of the estimate is not just to produce a number. It is to understand what might push the shopping list up or down before checkout.

This calculator works best when you already know the room dimensions and want a quick buying estimate before heading to the paint store. It is built around common U.S. paint labels, where one gallon often covers roughly 250 to 400 square feet depending on paint quality, sheen, and surface condition.

The result is intentionally conservative. It subtracts common door and window areas, then rounds the final purchase amount up so you are less likely to run short in the middle of a wall or have no paint left for touch-ups.

Biggest factors

What changes the estimate most

  • Wall texture and fresh drywall usually increase paint use because the surface absorbs more finish.
  • Deep color changes, strong whites, and low-hide paint often need extra coats even when the room is measured correctly.
  • Large windows, tall ceilings, accent walls, and extensive trim change how much paint belongs in the cart.
  • Manufacturer coverage numbers vary, so the label on the exact can should win over a generic estimate.

Worked example

Example: average bedroom repaint

A 12 x 10 room with 8-foot walls, 1 door, 2 windows, 2 coats, and paint labeled for 350 square feet per gallon.

  • Wall area = 2 x (12 + 10) x 8 = 352 square feet
  • Openings removed = 1 door x 21 + 2 windows x 15 = 51 square feet
  • Paintable area = 352 - 51 = 301 square feet
  • Two coats = 301 x 2 / 350 = 1.72 gallons

A practical shopping quantity is 1.75 gallons, or more commonly 2 full gallons if you want a buffer for touch-ups and roller loss.

Buying tips

What to check before checkout

  • Use the low end of the coverage range when walls are patched, textured, or changing from dark to light.
  • Buy enough of the same batch at once when possible so touch-up color stays consistent.
  • If you are also painting the ceiling, trim, or doors, estimate those separately because they often use different products.

Common mistakes

Where people usually go wrong

  • Using floor area instead of wall area for interior paint planning.
  • Forgetting to increase coats when covering a bold old color or unfinished drywall.
  • Buying the exact calculated amount and leaving nothing for the final wall section or future repairs.

Next steps

What to do after you get the number.

1

Measure the ceiling separately if it will be painted.

2

Check the product label for real coverage and recommended coats before checkout.

3

Read the prep and primer guides below if the walls are repaired, glossy, or heavily stained.

FAQ

Common questions

Does this paint calculator include ceilings?

No. It estimates wall paint only. Add the ceiling as a separate rectangle if you plan to paint it.

Why does the calculator subtract doors and windows?

Most doors and windows are not painted wall surface. The tool uses common estimates of 21 square feet per door and 15 square feet per window.

Should I round up paint?

Yes. Paint is usually bought in gallons and quarts, and a small buffer helps with touch-ups and roller loss.