The arithmetic for paint is simple. The inputs are harder, which is why so many people end up with either a second trip or a shelf of half-used cans.
Everything on this page comes back to one formula:
Gallons = (wall area × number of coats) ÷ coverage per gallon
Each of the three numbers has a trap in it.
Wall area: measure the walls, not the floor
The most common mistake is using floor area. A 12 × 16 foot room has 192 square feet of floor and, with an 8-foot ceiling, 448 square feet of wall, more than twice as much. Paint goes on the walls.
The calculation is perimeter × height:
- Add up the length of every wall. For a 12 × 16 room: 12 + 16 + 12 + 16 = 56 linear feet.
- Multiply by ceiling height: 56 × 8 = 448 square feet.
For one or two doors and windows, don’t subtract them. Leave them in as a buffer against touch-ups and mis-estimates. For a room with a wall of glass or three doorways, subtract them: a standard door is about 21 square feet and a typical window about 15. Start subtracting once openings add up to more than about 10% of your total.
Ceilings are floor area, calculated separately, and usually need their own can because ceiling paint is a different product.
Coverage: the number that varies
This is where estimates go wrong. Coverage is quoted as a single tidy figure, and the real number moves a lot.
A gallon of interior latex covers roughly 350 square feet on a smooth, primed, previously painted wall. PlankMath uses that as its default, and most labels print it. Labels say less about how far the real figure moves:
| Surface | Realistic coverage per gallon |
|---|---|
| Smooth, primed drywall | 350-400 sq ft |
| Previously painted, similar colour | 350-450 sq ft |
| Fresh unprimed drywall | 200-250 sq ft |
| Light texture (orange peel) | 300-350 sq ft |
| Heavy texture (knockdown, popcorn) | 200-275 sq ft |
| Bare or rough wood | 200-300 sq ft |
| Smooth block or masonry | 200-250 sq ft |
| Rough block, brick | 100-200 sq ft |
Texture is surface area. A knockdown-textured wall has considerably more actual surface than its flat dimensions suggest, and every extra square inch takes paint. Porosity has the same effect by a different route: unsealed drywall and bare wood drink the first coat instead of holding it on the surface.
Use the number on your can. It is on the back, it is specific to that product, and it beats any default. Where it disagrees with the calculator, change the calculator.
Coats: two, with exceptions in both directions
Two coats is the working assumption and the right answer for most jobs. One coat looks fine while wet and patchy once dry, particularly on a wall lit from the side.
You may get away with one when repainting the same colour in the same sheen on a wall in good condition, or when using a genuinely high-build paint on a surface that is already close to the target colour.
You will need three when going dramatically lighter over a dark colour, when using a saturated red, orange or yellow (those pigments hide poorly and need the extra build), or when painting over a patchy repair.
Primer changes the maths. Over fresh drywall, a stain, or a colour change, primer is a separate product with its own coverage rate, calculated separately. It doesn’t count as a coat of paint. It often turns a three-coat job into a two-coat one, though, which usually makes the job cheaper as well as better.
Turning gallons into a buy list
Most calculators skip this step. Paint is sold in gallons and quarts, and 2.7 gallons is not something you can buy.
Worked example, our 12 × 16 room at 8 feet:
- Wall area: 448 sq ft
- Two coats: 448 × 2 = 896 sq ft of coverage needed
- At 350 sq ft per gallon: 896 ÷ 350 = 2.56 gallons
That is two gallons and 0.56 of a gallon, which rounds up to three quarts. The buy list is 2 gallons + 3 quarts, and that is what the paint calculator returns instead of “2.56”.
The arithmetic misses one cost point: quarts are poor value. A quart typically costs 40-50% of a gallon while holding a quarter as much. If your remainder is two quarts or more, buy the extra gallon. It usually costs about the same and leaves you touch-up paint that matches, which you will want in a year.
What to do with the leftovers
Buy a little extra on purpose. A wall gets scuffed, a picture hook gets moved, and a colour matched two years later from a chip is never quite right. Half a quart in a sealed can, labelled with the room and the date, is the cheapest insurance in the house.
Store it somewhere that doesn’t freeze, upside down if the lid seals well. The paint forms the seal itself and skins over far less.
The checklist
- Walls: perimeter × ceiling height. Floor area is for the ceiling only.
- Subtract openings only when they exceed roughly 10% of the wall area.
- Read the coverage figure off your can, and adjust it down hard for texture, bare drywall or masonry.
- Assume two coats, and three for a big colour change or a saturated hue.
- Prime separately where the surface calls for it, with its own coverage.
- Round up to what the store sells, and take the gallon over three quarts.
The paint calculator does steps one through six and prints which coverage rate it used on the result, so you can change it to match your label. You still have to measure the room correctly: the measuring guide covers that part.