Read the coverage on your chosen paint
Coverage describes the area a stated volume of paint is expected to cover under specified conditions. Sherwin-Williams gives 350–400 square feet per gallon as a typical general range, while also noting that substrate texture and application losses affect real use. Use the label for your specific product rather than treating a general range as universal.
The default in our calculator is 350 ft² per US gallon per coat. It is an editable starting value. In metric mode, coverage is shown as square meters per liter. US gallons and imperial gallons are different volumes; the imperial option on this site uses US gallons.
Primer is a separate product with its own coverage. A room that needs primer should have a separate primer quantity estimate. Paint coverage does not include primer, trim, tools or labor.
Follow one estimate from wall to shopping list
Take a 12 × 10 ft room with 8 ft walls. The four walls have 352 ft² of area. Subtract 40 ft² of doors and windows, leaving 312 ft². For two coats, the paint needs to cover 624 ft² of application area.
If you choose a 10% extra allowance, that becomes 624 × 1.10 = 686.4 ft². Divide by 350 ft² per US gallon: 1.96 US gallons. With 1-gallon cans, the shopping quantity is two cans. At an example price of $42 per can, the material estimate is $84.
312 ft² is the physical wall area. 686.4 ft² includes two coats and the chosen buffer. 2 US gallons is the whole-can purchase quantity. Each number answers a different question.
Coats and extra allowance are separate decisions
Two coats is an example setting, not a rule for every product or color change. Follow the paint manufacturer’s instructions for the intended surface, preparation and application. Changing from a dark color to a light one may require a different product or preparation plan, not just a guessed extra percentage.
The allowance adds a buffer after the coat calculation. The 10% default is a planning choice you can change to zero or another figure. Avoid adding a buffer twice without realizing it—for example, deliberately using a low coverage value and then adding a large allowance for the same concern.
Ceilings, baseboards and doors are not included in the four-wall calculation. Keep their estimates on separate lines, especially when the finish, product or number of coats differs.
Round to the package you can actually buy
Enter the can volume sold by your shop. If the estimate is 3.2 liters and your chosen can contains 2.5 liters, the whole-can result is two cans, or 5 liters purchased. It does not mean two liters.
Switching units converts the same physical can. A 1 US gallon can becomes approximately 3.785 liters, rather than silently becoming a 5-liter can. If the local package is actually 5 liters, edit the can-size field after changing units. The price belongs to that whole can, not to a single liter.
Before ordering, compare the estimate with the product instructions and your measurement sheet. The calculator is transparent arithmetic; it is not a product performance test or a quotation from a painting contractor.
References & further reading
Manufacturer and retailer references inform the product context. The worked examples and calculator formulas are our own explanations.