Paint Calculator

Estimate paint for a rectangular room using wall and ceiling dimensions. Subtract the measured area of doors and windows, set separate coats, and use the coverage printed on your paint label.

Room lengthft Enter a length greater than zero

Room widthft Enter a width greater than zero

Wall heightft Enter a height greater than zero

Total door and window areaft² Measure the openings you want to exclude instead of relying on a standard door or window size. Enter an opening area of zero or more

Wall coatscoats Enter a whole number from 1 to 10

Include ceiling

Ceiling coatscoats Ignored when the ceiling is excluded. Enter a whole number from 1 to 10

Product coverageft²/gal Use the spread rate stated on the selected paint label. Enter coverage greater than zero

Waste allowance% Allows for application loss, touch-ups, and small measurement differences. Enter a percentage from 0 to 50

Calculation Results

Whole gallons to purchase

—gallons total

Wall and ceiling paint are calculated and rounded separately.

How to estimate room paint

Measure the room

Enter room length, width, and wall height in feet. The calculator treats the room as a rectangle.

Measure openings

Add the actual face area of doors and windows you want to exclude. Direct measurement avoids assuming that every opening has a standard size.

Set wall and ceiling coats

Choose whether to include the ceiling and enter its coats separately because wall and ceiling products are often purchased separately.

Use the paint label

Enter the product’s stated square-foot coverage per gallon, then adjust waste if the surface or application method is likely to use more paint.

Wall and ceiling paint formulas

Wall area is the perimeter multiplied by wall height. Measured openings are subtracted from wall area, but the net area cannot be less than zero. Ceiling area is length multiplied by width when selected.

Gross wall area = 2 × (Length + Width) × Height

Net wall area = max(0, Gross wall area − Opening area)

Wall gallons = Net wall area × Wall coats × (1 + Waste ÷ 100) ÷ Coverage

Ceiling gallons = Length × Width × Ceiling coats × (1 + Waste ÷ 100) ÷ Coverage

Wall and ceiling gallons are rounded up separately because they are commonly different products. Coverage varies with paint, substrate, porosity, texture, color change, film thickness, and application method, so the product label should control.

Paint coverage planning examples

Examples use 375 ft² per gallon and show calculated volume before whole-gallon rounding.

Surface area	Coats	No waste	10% waste
375 ft²	1	1.00 gal	1.10 gal
375 ft²	2	2.00 gal	2.20 gal
750 ft²	1	2.00 gal	2.20 gal
750 ft²	2	4.00 gal	4.40 gal

Methodology, assumptions and sources

The calculator uses rectangular-room geometry and a user-entered total opening area. It does not assume standard door or window sizes. Wall and ceiling coating areas are calculated separately at full precision.

The default 375 ft² per gallon sits within the commonly stated 350–400 ft² planning range, but the selected product’s technical data and label take priority. Whole-gallon purchase results use upward rounding.

Sources:

Sherwin-Williams — Paint Calculator — room measurement and paint-quantity guidance.
Sherwin-Williams — How Much Paint to Buy — typical gallon coverage and measuring guidance.

Last reviewed: August 2026. Product instructions take priority, and Sherwin-Williams does not endorse Efficienco.

Frequently asked questions

How much area does one gallon of paint cover?

A commonly stated planning range is about 350–400 square feet per gallon, but the coverage on the selected product label is the project-specific value to use.

Why enter the total opening area instead of door and window counts?

Doors and windows vary in size. Entering measured area avoids hidden standard-size assumptions and makes the deduction transparent.

Why are wall and ceiling gallons rounded separately?

Wall and ceiling paint are often different products. Rounding a combined volume could understate the whole containers required for either product.

Does this calculator include primer?

No. Primer is a separate product with its own coverage and coat requirements and should be estimated separately.