Paint Coverage Calculator (Square Feet)

For a 12 ft by 12 ft room with 8 ft walls, gross wall area is 2 × (12+12) × 8 = 384 sq ft. After typical door/window deductions (around 50 sq ft), net area is roughly 334 sq ft. At 400 sq ft per gallon per coat, that is about 0.84 gallons per coat, or around 1.8 gallons for 2 coats with a 10% buffer, usually 2 gallon cans.

Most interior paints cover approximately 350 to 400 square feet per gallon per coat on a smooth, previously painted surface. Textured, masonry, or unprimed surfaces reduce coverage, often to 250-300 square feet per gallon, because more paint is absorbed or needed to fill texture.

Multiply the room's perimeter by the wall height to get gross wall area, subtract an allowance for doors (about 20 sq ft each) and windows (about 15 sq ft each) to get net paintable area, then divide by your paint's coverage rate per gallon and multiply by the number of coats.

Two coats is the standard recommendation for full, even colour coverage, especially over a different existing colour or bare surface. A single coat may be enough for a very close colour match on an already-painted wall in good condition, but two coats gives a more reliable, professional finish.

At a typical 400 sq ft per gallon coverage rate, one square foot needs about 1/400 of a gallon, or roughly 0.0025 gallons, per coat. For 2 coats that is about 0.005 gallons per square foot.

Yes. Textured walls, masonry, and unprimed drywall or plaster all absorb more paint and have more surface area to cover than a smooth, previously painted wall. Expect coverage closer to 250-300 sq ft per gallon rather than 400 sq ft per gallon on these surfaces.

Yes, if you are not painting them the same colour as the wall. A standard allowance of 20 sq ft per door and 15 sq ft per window is commonly used instead of measuring each opening individually. If you plan to paint trim, doors, or window frames the same colour, you may not need to subtract them.

A common rule of thumb is to buy about 10% more paint than your raw calculation suggests. This covers cutting in around edges, roller inefficiency, and future touch-ups without running short mid-job.

Often, yes. Once a job needs more than around 3-4 gallons, a 5-gallon bucket is usually more cost-efficient per gallon than buying several separate 1-gallon cans, and avoids the risk of a slight colour-batch mismatch between individual cans.

Measure the ceiling's length by width to get its area (ceilings have no openings to subtract), then divide by your paint's coverage rate the same way as a wall. Ceiling paint is often a flatter finish with a slightly different coverage rate, so check the specific tin.

Primer typically covers a smaller area per gallon than finish paint, often around 200-300 sq ft per gallon, because it is designed to soak into and seal a porous or new surface. If your walls are new drywall, plaster, bare wood, or a big colour change, budget primer separately from your topcoat paint.

It gives a solid planning estimate using published manufacturer coverage ranges, but actual usage depends on your exact paint brand, colour depth, sheen, application tool, and how experienced the painter is. Always check the coverage rate printed on your specific paint tin and buy the recommended extra buffer.

The same formula works for exterior walls, but exterior surfaces such as brick, render, pebbledash, or previously weathered paint are usually rougher and more absorbent than interior walls, so choose the textured, masonry, or rough surface option for a more realistic estimate.

At a typical 400 sq ft per gallon coverage rate for one coat, 1 gallon covers roughly a 10 ft by 10 ft room's walls at 8 ft high for a single coat, about 320-350 sq ft after door and window deductions. For 2 coats, budget closer to 2 gallons for a room that size.

Real-world paint use varies with roller nap thickness, number of coats actually applied, wall porosity, spillage, and how much is used for cutting in with a brush versus rolling. Treat this calculator's result as a shopping guide and keep any leftover paint for future touch-ups.