Paint Room Calculator
Calculate how much paint it takes to paint a whole room, including walls and ceiling.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This estimate subtracts 20 sq ft per door and 15 sq ft per window, regardless of their actual size. The ceiling is included only when selected. Enter the paint label coverage; actual coverage varies by product and surface.
Worked example
calculated with the default values below
Room length 14 ft · Room width 12 ft · Wall height 8 ft · Number of doors 2 · Number of windows 2 · Number of coats 2 · Paint the ceiling Walls only · Coverage per gallon 350 sq ft Wall area 416.0 sq ft · Ceiling area 0.0 sq ft · Paintable area per coat 346.0 sq ft · Gallons needed 1.98 gal · Cans to buy 2
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
A = max(0, 2 H (L + W) + c L W − 20 D − 15 N); G = A × K ÷ R; cans = ceil(G)
- L
- Room length (ft)
- W
- Room width (ft)
- H
- Wall height (ft)
- D,N
- Numbers of doors and windows
- K
- Number of coats
- c,R
- Ceiling flag (1 included, 0 not); coverage (sq ft/gallon, default 350)
Wall area = 2(length + width) × height; optional ceiling = length × width; modeled openings subtract 20 sq ft per door and 15 per window. Rejects opening estimates exceeding modeled surface area; actual opening sizes and surface conditions vary.
Frequently asked questions
What happens if the estimated openings are larger than the room surface?
The calculator shows an input error instead of silently clamping paintable area to zero. Its door and window deductions are fixed averages, so enter actual dimensions or adjust the counts if the estimate is larger than the modeled walls and ceiling.
Why does the calculator subtract twenty square feet for each door?
It treats a door as 20 sq ft and a window as 15 sq ft of non-paintable area, so a 14×12 ft room with 8 ft walls has 416 sq ft of wall minus 2 doors and 2 windows (70 sq ft), leaving 346 sq ft per coat. At 2 coats and 350 sq ft per gallon that is about 1.98 gallons, rounded to 2 cans. Adding the ceiling would add another 168 sq ft.