How the calculation works
- Wall area = (room length + room width) × 2 × wall height.
- Each door and each window is deducted as 20 sq ft, the standard allowance in Sherwin-Williams' estimating guide.
- Gallons = paintable area × number of coats ÷ coverage per gallon, rounded up to a whole gallon.
- The default coverage is 350 sq ft per gallon, the low end of the 350–400 sq ft range Sherwin-Williams gives, so you are less likely to run short.
Example
Room length: 12 ft · Room width: 15 ft · Wall height: 8 ft · Doors: 1 · Windows: 2 · Coats: 2 · Coverage per gallon: 350 sq ft
Result: 3 gallons.
Tips before you buy
- Rough, porous or textured walls and big color changes use more paint than the label suggests.
- Check the coverage printed on your paint can and enter it if it differs from 350 sq ft.
- Ceilings and trim are not included — measure them separately.
- Always confirm coverage and yield with the manufacturer's label — products differ.
FAQ
Should I count one coat or two?
Two coats is the common choice for repainting, and it is the default here. If your product label says one coat covers, you can change it.
Why round up?
Paint is sold in whole cans, and buying the same batch at once keeps the color consistent. A partly used gallon is better than a second trip.