Do you need a primer? Here's when.
Not a finish coat, and not something to skip without thinking about it either.
Primer and finish coats do different jobs
A primer (or undercoat) prepares the surface: it evens out absorption, improves how well the finish adheres, and on some surfaces blocks stains or tannin bleed-through. Finish coats bring the colour and final look. The calculator never mixes the two up: when a primer is recommended, it's shown separately from the number of finish coats — never silently counted as one of them.
When a primer is usually recommended
- New surfaces (plaster, drywall, fresh render): highly absorbent, so the primer evens out what the finish coat would otherwise soak up unevenly.
- Porous or patchy surfaces: several materials or local repairs on the same wall absorb differently; a primer evens things out before the finish goes on.
- Repaired surfaces: patched or filled areas (filler, plaster) have different porosity from the rest of the wall.
- A big colour shift, especially dark to light: an opacifying primer cuts down the number of finish coats you'll need afterwards.
What the calculator does
The calculator flags a primer for every wall and paint combination. That isn't a claim that one is always essential: it knows nothing about your wall's condition or history, and no public numeric rule lets it rule a primer out reliably across all paints. If the data sheet of the product you chose explicitly allows direct application on your surface — a sound old coat of the same paint family, say — you can skip it: the data sheet is what counts.
Where the primer shows up in the calculator
In the calculator, the primer appears next to the number of finish coats, in the “Coverage & number of coats” step, as “2 (+ primer)”. It is never added to the finish-paint quantity, and its own quantity isn't worked out: use the coverage rate on the data sheet of the primer you choose.