How much paint a room needs
Wall area, less the doors and windows, times the number of coats, over what a gallon covers. A bedroom 12 by 12 feet with 8 foot walls has 2 × (12 + 12) × 8 = 384 square feet of wall. Take out one door at 20 square feet and one window at 15 and 349 are left. Two coats is 698 square feet, and at 350 square feet a gallon that is 1.99 gallons: two 1-gallon cans.
Make the window a smaller 12 square feet and the same room is 352 square feet, 2.01 gallons. Rounded up to whole gallons that is three cans; the page buys two gallons and a quart instead, which covers it with about a quarter gallon to spare and costs less. It works out every can size — quart, gallon, 5-gallon pail — and picks the cheapest mix, unless one with fewer sizes is within 2% of it. A quart covers about 100 square feet, by Sherwin-Williams' figure.
What a gallon covers
Sherwin-Williams publishes 350 to 400 square feet a gallon; Behr gives 250 to 400; Benjamin Moore's Regal Select data sheet gives 400 to 450. The page works at 350, the low end, which is also the figure Benjamin Moore's and Lowe's calculators use, and shows beside it what the same walls take at 400. If your can says something else, type it under Assumptions. A coverage below 200 or above 500 is almost always a figure in square meters a liter; multiply that by 40.7.
How many coats
Two is the default, as in Benjamin Moore's calculator. Inch Calculator advises one or two coats, and two or three when the color changes a lot and there is no primer underneath. The gallons on this page are every coat: Sherwin-Williams' calculator prints gallons per coat and has no box for the coats, so its answer has to be multiplied by hand, and Lowe's works one coat only.
Doors and windows
No two calculators agree on how big a door or a window is, which is why both are boxes you can change:
| Who | Door | Window |
|---|---|---|
| Sherwin-Williams | 21 ft² | 15 ft² |
| Benjamin Moore | 21 ft² | 12 ft² |
| Behr | 20 ft² | 15 ft² outside |
| Inch Calculator | 20 ft² | 12 ft² |
| Lowe's | 16.66 ft² | 15 ft² |
A 3 by 6′8″ door is 20 square feet and a 3 by 7′ door 21. A window or a garage door of its own size is a rectangle set to subtract.
Where calculators go wrong
GIGAcalculator's default coverage is 107.64 square feet a gallon — ten square meters turned into square feet but still per liter — so for this room with two windows it says 7 gallons. Worked properly it is 334 square feet, twice over, 1.91 gallons: two cans.
More than one room, and the rest of the job
Add a room for each room, or a rectangle for a single wall and a triangle for a gable end. The ceiling, the trim and the primer have their own pages, and on this one you can switch them on from "What to paint" and "Primer". The prices start at estimates of our own, not a store's shelf price: type yours beside each can. We are not connected with any brand or store named here.