Paint planning
When Do You Need Primer and Extra Paint?
Use a separate primer on bare drywall, patches, stains, raw wood, glossy surfaces, and big color changes. Plan on roughly 200 to 300 square feet per gallon of primer on porous surfaces, and add about 10 percent extra finish paint for roller loss and touch-ups.
Open the paint calculatorWhen primer is worth it
Primer does a different job from paint. It seals porous surfaces, blocks stains, and gives the finish coat something even to grip. Skipping it on the wrong surface usually costs more in extra finish coats than the primer would have.
| Surface or situation | Primer? | Why |
|---|---|---|
| New or bare drywall | Yes | Paper and joint compound absorb paint unevenly |
| Patched or repaired areas | Spot prime | Patches flash through the finish coat |
| Water, smoke, or marker stains | Yes, stain-blocking | Stains bleed through regular paint |
| Raw wood or MDF | Yes | Seals the grain and stops tannin bleed |
| Glossy or oil-based old paint | Yes, bonding primer | New paint will not stick well to gloss |
| Dark color to light color | Usually | Cheaper than a third finish coat |
| Repainting a similar color in good condition | Usually not | Two finish coats are enough |
How much primer to buy
Primer coverage is lower than finish paint on porous surfaces. New drywall commonly gets 200 to 300 square feet per gallon, while previously painted walls can reach 300 to 400. One coat is usually enough.
To size it, use the same wall area as your paint estimate, count one coat, and divide by the primer coverage on the label. A 366 square foot room at 250 square feet per gallon needs 1.5 gallons, so buy 2.
How much extra paint is reasonable
A 10 percent buffer covers what stays in the roller and tray, small measuring errors, and a little left for touch-ups. Raise it to 15 percent for heavily textured walls, rooms cut up by many windows and corners, or if you are spraying.
More than that is usually waste. Leftover paint has a shelf life, and a quart for future touch-ups is plenty for most rooms.
About paint-and-primer-in-one
Paint-and-primer products are thicker finish paints. They work well when repainting sound, previously painted walls. They do not replace a dedicated primer on bare drywall, stains, or glossy surfaces, where sealing and adhesion are the real problem.
- Prime bare, patched, stained, glossy, or drastically recolored surfaces.
- Expect 200 to 300 square feet per gallon of primer on new drywall.
- Ten percent extra finish paint is enough for most rooms.
Run the numbers
The Paint Calculator applies this math to your own measurements and shows each step. Best for a quick store run before painting one room or several similar rooms.