Paint Calculator

Enter the room's length, width and wall height, subtract the area taken up by doors and windows, and set how many coats you plan to apply. The calculator returns the litres of paint to buy and the bare wall area it covers, updating as you type. It sizes the four walls only, so ceilings are handled separately, and it works in metric coverage figures given as square metres per litre.

How the estimate works

The paintable wall area is the room perimeter times the wall height, less the openings you will not paint over. The perimeter of a rectangular room is twice the length plus the width, so multiplying that loop by the wall height gives the surface of all four walls in one step:

wall = 2 × (length + width) × height − openings

Doors, windows and anything you cut around come off the top, because paint aimed there is wasted. Volume then follows directly. Multiply the net wall area by the number of coats, since each coat repaints the whole surface, and divide by the coverage figure that says how far one litre reaches:

litres = wall × coats ÷ coverage

Worked example

Take the defaults: a 5.2 by 4.1 metre room with 2.4 metre walls. The perimeter is 2 × (5.2 + 4.1) = 18.6 m, and at 2.4 m high the four walls gross 44.64 m². Knock off 4 m² for a door and a couple of windows and 40.64 m² of wall is left to paint. Two coats at 11 m² per litre works out to 40.64 × 2 ÷ 11 = 7.4 litres. UK and European tins come in 2.5 and 5 litre sizes, so one 5 L plus one 2.5 L (7.5 L) covers the job with a small margin for drips and second-guessing. The calculator carries the unrounded figure, 7.389 litres, and shows it to one decimal place.

From ground pigment to a canned product

People have mixed paint far longer than they have bought it. The oldest known paint-making kit, two abalone shells holding a liquefied ochre-rich mixture, was excavated in 2008 at Blombos Cave on South Africa's southern Cape coast and dates to roughly 100,000 years ago. Ochre, bone, charcoal, grindstones and hammerstones made up the rest of the toolkit. Christopher Henshilwood and colleagues published the find in Science in October 2011, describing it as evidence that people could already source, combine and store a compound; what the mixture was actually used for, decoration or skin protection among the possibilities, is still unknown. For most of the centuries that followed, paint stayed a made-to-order material. A painter or householder ground dry pigment, often white lead, into a binder such as linseed oil, then thinned the paste with more oil, solvent and driers on the spot. Colour, body and quantity were judged by eye, and there was no can to reseal and no coverage figure on a label.

Ready-mixed paint changed that. An early US patent for a premixed product went to D. R. Averill of Ohio in 1867, but the format only caught on once it could be stored between jobs. Sherwin-Williams, founded in Cleveland in 1866, dates its own first ready-mixed oil paint to 1873, patented a reclosable paint can in 1877 and around 1880 put out the SWP line, the first ready-mixed paint to win broad public acceptance, sold under a guarantee. The trade shifted from grinding batches toward buying by the tin. The next leap was the binder. Chemists at Rohm and Haas developed water-based acrylic emulsion technology through the 1940s, first for leather and textiles, and in 1953 released the acrylic binder Rhoplex AC-33 for house paint, work the American Chemical Society later marked as a National Historic Chemical Landmark. These water-thinned emulsions, sold as latex or emulsion paint, dried quickly, washed up with water and largely displaced solvent-based coatings on interior walls. Only once paint came in a standard tin with a stated spreading rate did estimating the litres for a room become arithmetic rather than guesswork.

Where the coverage number comes from

The coverage input, the square metres one litre covers in a single coat, is the value that makes or breaks the estimate, and it rests on simple geometry. Spread one litre of liquid evenly at a thickness of one millimetre and it covers exactly one square metre, because a litre is one thousand cubic centimetres and a square metre laid one millimetre deep holds that same volume. Real paint goes on far thinner, and as it dries only the solid part stays on the wall while the water or solvent evaporates. Manufacturers turn this into a theoretical spreading rate from two numbers: the volume solids, meaning the fraction of the tin that remains as dry film, and the target dry film thickness in microns. Coverage in square metres per litre equals ten times the percentage volume solids divided by the dry film thickness, so a paint at forty percent solids laid down at forty microns dry covers about ten square metres per litre.

That figure is a best case, measured on a flat, sealed panel. On a real wall you lose paint to the roller, the tray and the cut-in edges, and a porous or textured surface pulls the first coat in rather than letting it sit on the face. Contractors typically allow ten to twenty percent for that loss, which is why the default here is a cautious 11 m² per litre rather than the higher number often printed on the tin.

Coats, colour changes and other assumptions

The tool assumes each coat covers the whole wall, so a value of two simply doubles the paint. Two coats is the norm for an even, durable finish. A strong colour change needs more. A dark shade over white, or a vivid colour over a pale one, usually takes three coats to stop the old colour ghosting through, so raise the coat count rather than trusting the coverage figure to hide it. Bare new plaster behaves differently again, drinking a thinned first coat, sometimes called a mist coat, that acts as a primer and does not count as full colour. Sheen matters less than people expect: manufacturers quote broadly similar spreading rates for matt and silk emulsions, and it is the surface underneath rather than the finish on top that moves the number most. Application method matters too, since an airless sprayer loses far more paint to overspray than a roller does. Because coverage swings with surface, porosity and method, the sensible move is to round the result up to whole tins and keep the offcut for touch-ups.

Ceilings, and buying in gallons

This tool covers the walls, not the ceiling. For a flat ceiling, paint an area equal to length times width, which is 5.2 × 4.1 = 21.32 m² for the example room, and add its own coats. If you are shopping in the US, paint is sold by the gallon and the quart. One US gallon, 3.785 litres, covers roughly 350 to 400 ft², about 33 to 37 m², in a single coat, so the same 40.64 m² room is around 437 ft², or a gallon and a bit per coat. Canadian tins keep those gallon-derived volumes but carry metric labels, typically 3.78 L and 946 mL. UK and Irish tins are filled to 2.5 and 5 litres, while Australian ranges run to 1, 2, 4, 10 and 15 litres. Metric labelling is why the coverage input here is in square metres per litre, and converting a US figure is a matter of dividing square feet per gallon by 40.75 to get square metres per litre.

Paint estimates depend on surface, method and colour change, so round up and keep a spare tin for touch-ups. See the site disclaimer.

Frequently asked questions

How much paint do I need for a room?

Work out the wall area, multiply by the number of coats, and divide by how far a litre stretches. The default 5.2 by 4.1 metre room with 2.4 metre walls has 40.64 m² of wall after taking off 4 m² for a door and windows, so two coats at 11 m² per litre need 7.4 litres. Rounding up to one 5 L tin plus one 2.5 L tin leaves a little spare for touch-ups.

How many square metres does a litre of paint cover?

Between 9 and 13 m² for a single coat, depending on the paint and the surface. A smooth, sealed, previously painted wall sits at the top of that range, while bare plaster, textured render or a porous filler drinks more and drops you toward 9 m². The figure printed on the tin is a best case, so 11 m² per litre is a safer planning number.

Does this include the ceiling?

No, it sizes the four walls only. For the ceiling, paint an area equal to length times width, which is 5.2 × 4.1 = 21.32 m² for the default room, and add its own coats separately. Ceilings are usually a flat matte paint rather than the wall finish, which is why they are costed on their own.

How many coats of paint do I need?

Two coats is standard for a durable, even finish and is the default here. A big colour change, such as a dark shade over white or white over a strong colour, often needs three coats to stop the old colour ghosting through. Fresh bare plaster also wants a thinned mist coat first, which acts as a primer.

How much does a gallon of paint cover in the US?

One US gallon covers roughly 350 to 400 square feet, about 33 to 37 m², in a single coat. The default room's 40.64 m² is around 437 ft², so budget a gallon and a bit per coat, or two gallons for two coats. Paint in the US and Canada is sold by the gallon and quart rather than the litre.