Paint Calculator
Work out how much paint a room needs: enter each room as length × width × height, deduct the doors and windows, and get the paintable wall area, the gallons for your coats and the cans to buy — plus primer for the first coat.
Cans round up, so you finish with leftovers — keep them for touch-ups. Bare or porous drywall soaks up 25–30% more primer than this one-coat figure.
Formula and calculation
Worked example
Two rooms, 12 × 14 × 8 ft and 10 × 12 × 8 ft, with 3 doors and 4 windows: the walls add up to 768 sq ft and the openings take off 120 sq ft, so 648 sq ft actually gets painted. Two coats need 3.7 gallons of paint — buy 4 cans. One coat of primer over the same area is 1.85 gallons, so buy 2.
Doors, windows and trim
Taking 20 sq ft off per door and 15 off per window keeps the estimate honest. If you are painting the doors and trim as well, add them back: a door slab is about 20 sq ft per side, and the trim around an average window is roughly 7 sq ft. Ceilings go quickly because they are rolled or sprayed in one direction with a thicker nap, and a flat white ceiling paint covers an existing ceiling in one coat.
One coat or two?
Two coats is the standard and the reason a color change looks solid — a single coat over a darker or saturated color shows through, especially in reds and yellows. One coat is fine when you are recoating the same color from the same can. Primer is a different product: on bare drywall and patched spots it seals the surface so the finish coat covers evenly instead of flashing.
What the results mean
- Paintable wall area
- The paintable wall area in square feet: the walls of every room you entered, less 20 sq ft for each door and 15 sq ft for each window.
- Metric
- The same wall area in square meters, for metric paint labels and ordering.
- Ceiling
- The ceiling area in square feet (the floor area of the rooms). It is 0 unless the ceiling box is ticked.
- Paint needed
- How many gallons of paint the chosen number of coats takes, at the coverage you entered. This is what the cans are rounded up from.
- Paint to buy
- Gallons rounded up to whole 1 gallon cans — the number to carry out of the store.
- Primer, 1 coat
- One coat of primer over the same paintable area, walls and ceiling together, at the same coverage rate.
- Primer to buy
- Primer rounded up to whole 1 gallon cans.
Frequently asked questions
How much paint do I need for a 12 x 12 room?
A 12 ft × 12 ft room with 8 ft ceilings has 384 sq ft of wall. Take off one door and two windows and 334 sq ft is left, so two coats need about 1.9 gallons — buy 2. Ceiling and trim are extra.
Should I deduct windows and doors when buying paint?
Most estimating guides do: 20 sq ft for a door and 15 sq ft for an average window. On a whole-house job it adds up to gallons, but in a single small room it is a fraction of a can, and many painters skip the deduction and keep the extra for touch-ups.
What does one gallon of paint cover?
About 350 sq ft per coat on a smooth, previously painted wall, and 250–300 sq ft on rough, textured or bare surfaces because they absorb more. Check the can — premium paints often quote 400 sq ft, assuming a thin, even coat.
How much primer do I need?
One coat over the same area, at roughly the same coverage as paint: a 648 sq ft wall area takes about 1.85 gallons, so buy 2. New drywall and fresh patches soak up more, so add 25–30% if the walls have never been painted.