This paint calculator estimates how many gallons of paint a room needs. Enter the room size, subtract doors and windows, choose coats — and get gallons, quarts and an approximate cost before you go to the store.
What Is a Paint Calculator?
A paint calculator turns room dimensions into a paint quantity. Paint is one of the cheapest ways to transform a room, but running out mid-wall (with a dried edge) or buying three gallons too many are both classic mistakes. This calculator counts the walls, subtracts the openings you will not paint, applies your coats, and converts area into gallons at realistic coverage rates. It is part of our Construction Calculators collection.
Worked Example (Verified)
A 12 ft × 10 ft room with 8 ft walls, 2 doors and 2 windows, 2 coats:
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft
- Minus openings: 2 × 21 + 2 × 12 = 66 sq ft → 286 sq ft paintable
- Two coats: 572 sq ft of coverage needed
- At 350 sq ft per gallon: 1.63 gallons → buy 2 gallons
- At $45/gallon: about $90
Add the ceiling (another 120 sq ft in this room) and the same job needs about 2.32 gallons for two coats everywhere. For floor and room areas generally, our Square Footage Calculator helps, and the Concrete Calculator handles the patio outside.
Coverage Rules of Thumb
- Interior latex: 350–400 sq ft per gallon on smooth, primed walls (this calculator defaults to 350, the safe end).
- Primer: 200–300 sq ft per gallon — it soaks in more, especially on new drywall.
- Rough or textured surfaces: drop coverage by 15–20%.
- Dramatic color changes (dark over light or vice versa) often need a tinted primer plus 2 finish coats.
- One gallon = 4 quarts. For small remainders, a quart is cheaper than a second gallon.
Painting Tips That Save a Second Trip
- Buy all the paint for a room at once and “box” it (mix the cans together) so color is uniform.
- Keep a labeled touch-up jar from the same batch.
- Two thin coats beat one thick coat for coverage and finish.
- Do not subtract closets you are painting, built-ins, or wainscoting from the area unless you truly will not paint them.
Related Tools
- Square Footage Calculator — room and wall areas
- Concrete Calculator — yards and bags for outdoor projects
- Construction Calculators — the full collection
Frequently Asked Questions
How much paint do I need for a 12×12 room?
With 8 ft walls, 1 door and 1 window: walls are 384 sq ft minus 33 = 351 sq ft; two coats at 350 sq ft/gal is almost exactly 2 gallons. Enter your exact openings above.
How many square feet does a gallon of paint cover?
Typically 350–400 sq ft for one coat on primed, smooth drywall — check the can, and use 350 for planning.
Do I need primer?
On new drywall, bare wood, stains, or big color changes, yes. On previously painted walls in good condition, quality paint-and-primer products usually suffice for the finish coats.
Should I paint the ceiling with wall paint?
Ceiling paint is flatter and spatters less — but the amount is calculated the same way; just tick “include ceiling” and buy the ceiling paint separately.
How much extra should I buy?
About 10% beyond the calculation for touch-ups and mistakes — this calculator’s rounding-up in the “buy” figure usually covers it.
Is this paint calculator free?
Yes — free, instant, no sign-up.