RoomCalc
Drywall tools

Paint calculator

How much paint for a room, using the coverage printed on your can.

What are you painting?

Paint for the room from the coverage figure on your own can rather than an average, with primer included when the surface needs it and the trim allowance added.

Worked example: 12 × 12 ft, two coats, bare drywall

Walls and ceiling with openings deducted477 sq ft
Two coats of finish954 sq ft of paint
At 350 sq ft per gallon3 gallons
Primer for bare drywall2 gallon
Buy5 gallons total

Primer is not optional on new drywall

Fresh joint compound and bare paper are extremely porous. Paint applied straight over them soaks in unevenly: the patches flash through as dull spots, and the finish coat that was calculated to cover the room disappears into the wall. A primer coat seals the surface and brings the whole wall to the same porosity, which is what makes the calculated quantity of finish paint actually enough.

The exception is a repaint over an existing painted surface in a similar colour, where no primer is needed. Going from a dark colour to a light one is the case where a tinted primer saves a coat of finish.

Why coverage figures vary so much

The number printed on the can assumes a smooth, sealed surface. A few things change it substantially:

  • Texture. A heavy knockdown or popcorn ceiling has far more surface area than its footprint suggests, and can take 30 to 50% more paint.
  • Colour change. Going from a saturated colour to white in one coat is not realistic; budget three coats or use a tinted primer.
  • Surface condition. Chalky or glossy surfaces take paint unevenly. Sanding and a bonding primer do more for the result than an extra gallon.

Buying and storing

Buy all the paint for one room in a single order. Colour matching between batches is close but not identical, and a slight difference shows across a large wall where the two batches meet. Keep a labelled quarter-pint of each colour for touch-ups: the tin you used six months ago will have skinned over, and the exact tint may no longer be in stock.

Frequently asked questions

How much paint for a 12×12 room?
3 gallons for two coats on walls and ceiling at 350 sq ft per gallon, plus a gallon of primer if the drywall is bare.
How much paint do I need to paint a room?
Walls and ceiling area with openings taken off, times the number of coats, divided by the coverage on the can. A 10 × 10 ft bedroom with 8 ft ceilings, a door and a window is 384 sq ft, so two coats at 350 sq ft per gallon come to 3 gallons. Add a gallon of primer if the walls are bare drywall or you are covering a dark colour.
How much paint should I buy?
Round the calculated gallons up to whole cans and buy them in one order, so they come from one tint batch. If the figure lands just over a whole number, a gallon plus a quart is cheaper than two gallons, but only when you are sure the second coat will not need more: a textured wall or a big colour change usually does.
Does a ceiling need a separate paint?
Not strictly, but ceiling white is formulated flatter to reduce glare and reflections. It also takes more paint per square foot than a wall because it is usually applied thicker.
How many square feet does a gallon of paint cover?
Typically 350 to 400 sq ft on a smooth, primed surface. Check the can: the figure assumes ideal conditions and textured surfaces take considerably more.
Is primer really necessary?
On bare drywall, bare wood and patched surfaces, yes. On a previously painted wall in a similar colour, no. On a dark wall going light, a tinted primer saves a finish coat.

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