Paint Calculator – How Much Paint You Need for a Room or Wall

Where the paint goes

Two pictures of the same job. The first bar is your wall: the solid part is what actually gets painted, the hatched part is the doors and windows you correctly leave out. The second bar is the tin you buy: filled up to the paint your coats will drink, with the spare paint for touch-ups hatched at the end.

Keep exploring

Painting is mostly measuring. These neighbours handle the areas and material quantities that come up on the same weekend jobs.

How it works in three steps

The opening room, worked through

Going deeper

Three rooms, three answers — coats change everything

The default room on this page is a 12 by 10 ft room with 8 ft ceilings, one door and two windows, and its arithmetic is the whole method in miniature. The gross wall area is 352 sq ft — two walls of 12 ft plus two walls of 10 ft, a perimeter of 44 ft, times 8 ft of height. The openings subtract 51 sq ft: one door at 21 sq ft and two windows at 15 sq ft each, which is 30 sq ft. The wall area to paint is therefore 301 sq ft. At the usual coverage of 350 sq ft per gallon, the paint needed depends entirely on coats, and the table below is the calculator’s own output for that room at each coat setting.

Read the middle row first, because it is the room most readers actually have. Two coats over 301 sq ft is 602 sq ft of coating, and 602 divided by 350 is 1.72 gallons — so you buy 2 gallons, leaving about 0.28 gallons in the tin for touch-ups. The touch-up remainder is not waste: walls get scuffed, and a small reserve of the exact paint, from the same tin, is worth more than the few dollars the leftover represents. The quarts column shows the alternative the calculator offers for small jobs: 7 quarts is 1.75 gallons, closer to the exact 1.72 than 2 gallons is, but quarts cost more per gallon in most shops and leave less reserve. The choice between gallons and quarts is therefore a choice between reserve and precision, and the calculator shows both so the trade is visible.

Now stretch the room: a 15 ft length with the same 10 ft width, 8 ft ceilings and the same openings. The gross wall area becomes 400 sq ft — a perimeter of 50 ft times 8 ft — minus the same 51 sq ft of openings, giving 349 sq ft to paint. Two coats over 349 sq ft is 698 sq ft of coating, and 698 divided by 350 is 1.99 gallons — so you still buy 2 gallons, but now with only about 0.01 gallons left for touch-ups. The same purchase covers a room nearly a sixth larger, with almost no reserve. That is the practical lesson of the two rooms together: paint buying is a step function, not a smooth one. A room that needs 1.72 gallons and a room that needs 1.99 gallons both buy 2 gallons, but the second room leaves nothing for the scuff that will certainly happen. When the exact need lands within a tenth of a gallon of the next whole gallon, buying the extra gallon is not overbuying — it is the touch-up reserve, and it is cheaper than a second trip.

Primer follows the same arithmetic, with the page’s own rule for when it applies: add 1 gallon of primer if the walls are new, bare or stained, as the page’s FAQ states. On the default room, primer at the same 350 sq ft per gallon coverage needs 301 divided by 350, which is 0.86 gallons for one coat — so 1 gallon of primer, matching the FAQ’s figure. Primer is a single coat under the paint, not a coat of paint itself, and it is not counted in the paint gallons above: a new-wall job on the default room buys 2 gallons of paint plus 1 gallon of primer, not 3 gallons of paint. Walls that are already painted, sealed and in good condition do not need it, which is why the calculator’s primer toggle exists rather than assuming it always.

The tin’s coverage is a best case — and the ceiling is extra

The 350 sq ft per gallon on the tin is measured on smooth, sealed, already-painted walls — the surface paint wishes every wall was, as the page’s own deep dive puts it. Real walls are often rougher, more absorbent, or previously unpainted, and every one of those conditions lowers the coverage the gallon actually delivers. The calculator’s coverage field is editable for exactly that reason: the default is 350, but a rough or thirsty wall should be entered at a lower figure, and the gallons will rise accordingly. The direction is the only safe general statement — rougher means more paint — because the page gives no texture table, and this guide invents none: the audit note suggesting texture presets (smooth, textured, wood, brick, stucco figures from another tool) is not this page’s data, and it is not repeated here. Enter the coverage your paint’s own data sheet gives for your wall type, or, if you have painted this wall before, the coverage you actually got last time. A gallon figure computed at 350 for a wall that truly covers 250 is short by nearly a third, and the shortfall is discovered mid-wall, which is the worst time to discover anything.

Openings deserve the same care, because they are the second place estimates go wrong. The default subtracts one door at 21 sq ft and two windows at 15 sq ft each, but real doors and windows vary, and the calculator’s per-opening area fields exist so the actual sizes can be entered. A tall door, a wide window, a built-in wardrobe wall, or a fireplace breast all change the subtracted area, and each square foot subtracted is a square foot not painted — at two coats, each 10 sq ft of opening wrongly counted as wall adds 20 sq ft of coating, or about 0.06 gallons. Small on its own, but openings are where rooms differ most, and three wrong openings compound. Measure the openings you actually have, enter their real areas, and let the subtraction do its work. The page’s own FAQ asks “should I subtract doors and windows” and answers yes, with the default figures — this guide adds only that the defaults are a starting point, not a measurement.

Finally, the ceiling. The whole-room mode prices walls: the 352 sq ft gross figure is perimeter times height, with no ceiling in it. A ceiling, if you paint it, is extra area, and the honest way to price it on this page is the “Area I know” mode: a 12 by 10 ft ceiling is 120 sq ft, two coats over it is 240 sq ft of coating, and at 350 sq ft per gallon that is 0.69 gallons — so 1 extra gallon, bought alongside the wall paint. The same applies to any surface the whole-room mode does not count: an accent wall in a different colour is a separate area, a hallway is a separate room, and an L-shaped room is two rectangles, not one. The page’s modes exist for that reason — whole room for four walls, one wall for a single surface, area for anything measured directly — and the deep dive on odd rooms, L-shapes, vaulted ceilings and hallways makes the same point from the other side: measure the shape you have, not the shape the form assumes. Paint is bought by area, and area is the one input no calculator can guess for you.

One wall, measured twice

Not every job is a whole room. The page’s one-wall mode exists for the accent wall, the single repainted wall, and the wall behind the bed — and its arithmetic is the same method with one wall instead of four. Take one 12 ft wall with 8 ft ceilings: the gross area is 96 sq ft, twelve times eight. If that wall has one window of 15 sq ft, the area to paint is 81 sq ft. Two coats over 81 sq ft is 162 sq ft of coating, and 162 divided by 350 is 0.46 gallons — so you buy 1 gallon, leaving about 0.54 gallons for touch-ups. The remainder here is generous, because a single wall rarely lands near a whole gallon, and the touch-up reserve from one wall’s gallon is often enough for the whole room’s later scuffs, provided the colour matches — which it does, because it is the same tin. An accent wall in a different colour is a separate purchase, not a share of the main paint: its own area, its own coats, its own gallon, even if the exact need is small, because colours do not mix across tins.

The one-wall mode also corrects the most common whole-room error, which is measuring the wrong wall. A room’s walls are not all the same length, and a room with a chimney breast, a built-in wardrobe, or a sloped ceiling has walls whose areas differ by more than the openings account for. Measuring each wall separately — length times height, minus that wall’s openings — and adding the four figures gives the true wall area, and the “Area I know” mode takes that total directly. The whole-room mode assumes four straight walls at one height; when the room breaks that assumption, the separate measurement is the honest one, and the difference is usually larger than the touch-up reserve. Measure twice, in the carpenter’s sense: once for the gross area, once for the openings, and let the subtraction, not the guess, set the gallons.

Frequently asked questions

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