Common paints on the market can be bought per gallon or 4 liters of paint. For instance, 1 gallon of Semi-Gloss Latex Paint covers 30 to 40 square meters surface area depends on the manufacturer.
To simplify, let us take as an example a concrete wall that measures 20 meters in length and 10 meters in height. The wall has a course to rough surface and we want to paint it with a Semi-Gloss Latex Paint.