When you need primer
Prime new drywall and joint compound, bare wood, patched areas, water and smoke stains, and walls where you are making a big colour change, especially dark to light. Primer seals the surface so the paint goes on evenly and covers in fewer coats.
How much primer covers
Primer typically covers 200 to 300 square feet per gallon, less than finish paint, because its job is to soak into and seal the surface. The calculator uses 275 for new drywall, 300 for colour changes and stains, 250 for bare wood and 200 for brick, block and plaster, plus 10% for waste.
One coat or two?
One coat is enough for most drywall and colour changes. Use two on stains that bleed through, on very porous masonry, and when covering a strong colour such as red or navy with a light one. A primer tinted towards your final colour can save a coat of paint.
Can paint-and-primer products replace primer?
On walls that were painted before and are in good condition, often yes. On new drywall, bare wood and stains, a dedicated primer still does the job better and usually costs less per gallon.
Results are estimates. Coverage depends on the product, the tools and the wall; the rate printed on your can wins.