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Free room paint calculator

Paint Calculator

Estimate paint gallons for one or more rooms. Include doors, windows, ceilings, coats, product coverage and extra paint with clear, editable math before buying.

Enter each room, copy the coverage from your paint can, and the calculator rounds the result up to whole gallons.

Step 1

Add your rooms

Use the widest length and width. Your rooms stay on this device when you switch tools.

Room 1

Length

Width

Wall height

We subtract 21 sq ft per door and 15 sq ft per window. These are visible average-size assumptions.

Step 2

Read your paint can

Coverage changes by paint, surface and application. Replace our starter value with the number on your product.

Step 3

Your paint shopping amount

We round up because stores sell whole gallons.

Buy2gallons
Wall area301 sq ft
Ceiling area0 sq ft
Math before rounding1.89 gal10% extra included

Simple math, visible assumptions

How the paint estimate is calculated

For each room, wall area is two times length plus width, multiplied by wall height. The calculator subtracts the entered openings, never allowing a negative wall area.

That paintable area is multiplied by the number of coats. Optional ceiling area and your chosen extra percentage are then added before gallons are rounded up.

  1. Measure room length, width and wall height.
  2. Count standard doors and windows, and choose whether to include the ceiling.
  3. Enter coats and the square-foot coverage printed on your paint.
  4. Add your preferred extra amount, then buy only after checking the room and product label.

Before you buy

What this estimate can—and cannot—tell you

It measures quantity

The calculator turns your dimensions and package values into a rounded shopping amount.

It does not inspect the room

It cannot see moisture, substrate damage, lead, asbestos, uneven walls or installation obstacles.

The product label wins

Coverage, batch matching, preparation and installation instructions from the chosen product come first.

Questions people ask

Paint Calculator FAQ

How does the room paint calculator work?

It calculates wall area from each room's perimeter and height, subtracts the doors and windows you enter, adds ceilings if selected, then applies coats, coverage and your extra-paint percentage.

How many square feet does one gallon of paint cover?

Coverage differs by product and surface. The calculator starts at 350 square feet per gallon, but you should replace that number with the coverage printed on your paint can.

Should I subtract doors and windows?

You can. This calculator uses 21 square feet for a standard door and 15 square feet for a standard window. Leave the counts at zero or adjust your room measurements if those starter sizes do not fit your project.

Does this calculate primer too?

No. Primer can have different coverage and preparation rules. Use its own product coverage and calculate it separately rather than treating it as another paint coat.

Sources and scope

Method and safety notes were checked against the following current resources:

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