Home & Garden · Maintenance planning

How to estimate paint for a room

By TechHomeTips Editors Published September 11, 2026 Updated October 24, 2026 3 min read Report a correction

Estimate paint from wall area, coats, and the coverage printed on the can, not from the calculator’s 350 stand-in.

Estimate paint from wall area, minus openings, times the number of coats, divided by the coverage printed on the can you will buy. Until that can is in your hand, the paint estimator uses 350 square feet per gallon only so you can see the arithmetic. Replace 350 with the label before you treat the result as a shopping list.

This guide is for one rectangular room. Vaulted ceilings, stairs, and brick need a different measure. If the building was built before 1978 in the United States, read the current EPA lead-safe renovation guidance before you sand or scrape. This page does not tell you how to disturb old paint.

Step 1: Measure the walls

  1. Measure length and width of the floor in feet. Perimeter is 2 × (length + width).
  2. Measure wall height in feet, from the top of the baseboard to the ceiling line you will paint.
  3. Wall area is perimeter times height. A 12 by 10 foot room with 8 foot walls is 2 × (12 + 10) × 8 = 352 square feet before openings.

Step 2: Subtract doors and windows you will not paint

The estimator subtracts 20 square feet per door and 15 square feet per window as placeholders. Those are stand-ins for a typical interior door and a small window. Measure an odd opening yourself: a sliding door, a wide window, or a fireplace. Do not subtract a door you plan to paint the same color if you will use the same paint on it. Add that door’s area back, or leave it in the wall total.

Step 3: Multiply by coats

Two coats are a common plan on a color change or on bare drywall with primer counted separately. A same-color refresh might be one coat. Deep colors and a jump from dark to light often need more than two. The can or the paint line’s data sheet is the source for that, not a fixed rule here. Primer, if you need it, is a separate product with its own coverage.

Step 4: Divide by the can, not by 350

Coverage on the label might be 350, 400, or something else per gallon, and it assumes a smooth, primed surface. Rough texture drinks more paint. Divide your net area by the label number and round up to a can size you can buy. Buy a little extra from the same batch when you can, for touch-ups. Batches can differ in color even under the same name.

Ceiling paint is a second area: length times width, if the ceiling is in the job. Do not hide it inside the wall total.

Step 5: Note the color next to the lights

A darker wall makes the same bulbs look dimmer. If you are changing both, read LED brightness by room before you decide the paint is “not bright enough” and buy a second gallon of a lighter color you did not plan.

Before you pay

  • Perimeter and height are written in feet, not a mix of feet and inches.
  • Openings you will not paint are measured or marked as the 20 and 15 placeholders.
  • Coats and primer are separate lines.
  • The divisor is the number on the can, not 350, once the can is chosen.
  • You rounded up and kept the same batch number for touch-ups.

Example: the 12 by 10 room

Walls are 352 square feet. One door at the placeholder 20 and one window at 15 leave 317. Two coats make 634. At the tool’s stand-in of 350 square feet per gallon, 634 / 350 is about 1.8 gallons, so two gallons if you round up. The can you actually buy says 400 square feet per gallon on smooth walls, which is 634 / 400 = about 1.6 gallons, still two gallons once you round up and keep a little for touch-up. The 1.8 figure is not the order. The label is. Texture or a porous new wall can push you over two gallons. That is why you round up and recheck the can.

Last updated: October 21, 2026

Last reviewed October 22, 2026. Suggest a correction

Questions readers ask

Why does the calculator say 350?

It is a placeholder so the math is visible. Replace it with the square feet per gallon on your can.

Do I subtract every door?

Subtract openings you will not paint with this paint. Measure anything that is not a normal door or a small window.

Should I buy the exact fraction?

No. Round up to a can size, and keep some of the same batch for touch-ups.

Does this include primer and ceiling?

Only if you add them as their own areas and their own coverage numbers. They are not inside the wall total automatically.

What about lead paint?

In U.S. housing built before 1978, stop and read current EPA renovation guidance before you sand or scrape. This page is a quantity estimate, not a lead-safe work method.

Sources and methodology

Claims that depend on an outside authority are tied to the references below. Product details change. Confirm the current specification sheet before you buy or install anything.

  1. Paint quantity estimator — Tech Home Tips (original method), accessed September 20, 2026. States the 350 square foot assumption and the opening sizes used in the tool.

Information on this site is for general educational purposes and is not professional advice.

Parent topic: Home & Garden · Related tool: Room paint quantity estimator

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