How the calculation works
The calculator converts each dimension into meters, calculates wall and optional ceiling area, subtracts declared openings, multiplies the net area by the selected coats, and divides by the coverage printed on the product. It then applies the editable extra allowance and rounds up to the selected package sizes.
US example
A 12 ft by 10 ft room with 8 ft walls has 352 sq ft of wall area. At two coats, 350 sq ft per US gallon, and 10% extra, the adjusted need is about 2.21 US gallons before package rounding.
Metric example
A 3.6 m by 3 m room with 2.4 m walls has 31.68 m² of wall area. At two coats, 8.6 m² per liter, and 10% extra, the adjusted need is about 8.1 liters before package rounding.
What the paint estimate includes
Standard mode treats the project as a rectangular room. Its wall area is the room perimeter multiplied by wall height. Advanced options add a second room, measured openings, ceiling area, primer, and trim paint without replacing or resetting the standard calculator.
The result separates exact paint volume from the volume after your extra allowance. The purchase line then rounds up to whole cans or pails. If you enter prices for every selected product, the calculator also adds an estimated total. Prices stay in this browser and do not appear in shared links.
Defaults you can change
The starting coverage is 350 sq ft per US gallon, displayed as its metric equivalent after switching units. Two finish coats and 10% extra are also starting values. These are planning conveniences, not claims about a particular coating. Confirm coverage, coats, primer needs, drying conditions, and surface preparation on the product documentation.
Primer, trim, and package rounding
Primer and trim are separate shopping lines because their coverage, coat count, package size, and price can differ from finish paint. When several package sizes are available, the formula engine can compare combinations. It favors the least purchase cost when every package price is supplied. Otherwise, it favors less surplus and then fewer packages.
Limitations
- Real coverage changes with porosity, texture, color change, application method, and the exact product.
- Openings, vaulted surfaces, built-ins, and trim must be measured or selected explicitly to affect the estimate.
- Preparation can disturb hazardous coatings, so older painted surfaces may require lead-safe practices or professional advice.
Frequently asked questions
Why does the calculator ask for package volume?
Paint is sold in different package sizes around the world. Your product label is the most reliable source for the volume you can actually buy.
Should doors and windows always be subtracted?
Subtract them when their combined area is meaningful, but keep a reasonable extra allowance for touch-ups, texture, and measurement variation.
Is the 350 sq ft per gallon coverage a guarantee?
No. It is an editable starting point. Use the coverage stated for the exact paint, surface, and application method whenever possible.
Sources
Sources support unit conventions, product variability, and project limitations. They do not endorse this calculator.