How the paint calculator works
The formula
Wall area = perimeter × height = 2 × (length + width) × height. Subtract ~20 sq ft per door and ~15 sq ft per window. Divide by the industry-standard coverage of 350 sq ft per gallon, then multiply by coats. The calculator rounds up — paint is mixed by batch, and a second can from a different lot can differ noticeably in color.
When reality drinks more than 350 sq ft/gallon
- Rough or porous surfaces (unprimed drywall, brick, heavy texture): expect 250–300 sq ft/gal.
- Dark-over-light or dramatic color changes: a tinted primer costs less than a third coat.
- Deep, saturated colors (reds, some blues) have weak hide even in premium paints — plan the primer.
Two coats is the standard for a reason
One coat rarely gives the color on the chip; two coats give even color and durability. Save one-coat ambitions for same-color refreshes. And buy the full quantity at once — dye lots vary between batches, so a third can bought later may not match perfectly.