One coat or two?

When one coat is legitimate

Same color (or very close) over sound, clean, previously painted walls in a quality paint. That's the whole list. A landlord white over a landlord white, a five-year-old greige getting freshened before a sale: one careful coat with a decent line covers, because the old paint is doing half the hiding.

One-coat success also assumes you're not changing sheen. Eggshell over flat in "the same white" reads as a different color at every angle until the second coat evens it out.

When two coats are mandatory

Any real color change. Patched or skim-coated spots, which flash (dry to a different sheen) under a single coat no matter what the can promises. Bare drywall, after its primer. New paint over builder flat, which is porous enough to gulp the first coat unevenly. Two coats is not a quality upgrade in these cases; it is the spec.

What about paint-and-primer-in-one? It's thicker paint with better adhesion, and it's real, but it is not a coat of primer plus a coat of paint in one pass. Over a color change it behaves like what it is: one coat of paint.

When the wall wants three

Deep reds, oranges, navies, and near-blacks over light walls. Deep tint bases carry less titanium dioxide, the white pigment that does the hiding, so coverage per coat drops exactly when the color change is biggest. The pro move is a gray-tinted primer under the color (ask the desk; they have a chart matching primer grays to colors), which usually turns four color coats into primer plus two.

The cost of guessing wrong

Thin coverage shows up as lap marks and ghost stripes in raking light, usually at sunset, usually after the rollers are washed. Budget the second coat up front: the room calculator defaults to two, and its one-coat setting is there for the repaints that have earned it.