Room by room painting quantities from real product data, with warnings instead of hidden percentages.
Painting, tiling and finishes. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Painting Estimator tool.
The paint calculator answers how much paint a room needs. The painting estimator answers what the job costs, which is a different question with more lines: preparation, ceilings, trim, the difference between theoretical coverage on paper and what a painter actually gets out of a tin. That difference is why the page shows two quantities. Theoretical coverage is the product's stated spread rate multiplied by the area and the coats. Practical quantity adds the losses that real work has: roller retention, cutting in, porous surfaces and the second pass over a patch that did not cover. Showing both is what stops a job being ordered at the theoretical figure and running out two thirds of the way through.
A flat with a hundred and eighty square metres of wall and sixty of ceiling, in two coats at twelve square metres per litre, is theoretically thirty litres of wall paint and ten of ceiling paint. Add a practical loss of ten percent and the order becomes thirty three and eleven, which is the difference between finishing with a part tin and going back for more. Trim at forty metres of skirting and twelve doors is priced by length and unit, not by area, and two of the doors need a primer coat because they are bare timber.
Scope and cost rather than a single quantity. The paint calculator gives litres for a room. The estimator covers walls, ceilings, trim, doors and preparation separately, each measured by its own rule, and shows theoretical against practical quantities so the losses that site work produces are visible. It is the same arithmetic underneath, applied to a whole job instead of one surface.
Because theoretical coverage assumes a perfect substrate and a perfect applicator. The product states a spread rate on a prepared, sealed surface, and a real wall takes more: porous plaster absorbs, cutting in uses paint that never reaches the wall, and a roller holds a litre that never leaves it. Practical quantity applies the loss percentage you set to the theoretical figure, and showing both means the difference can be argued about rather than discovered.
As its own surface, with its own area and often its own product. A ceiling takes a matt emulsion in most specifications, and it may need a different preparation from the walls. Measuring it separately means the ceiling quantity can be ordered and priced on its own, which matters on a job where ceilings and walls are being decorated at different times or by different people.
Colour does not change the quantity in the arithmetic, and surface condition does, through the coats you specify and the preparation you list. A sharp colour change usually needs an extra coat, which is a decision you make and enter. A porous or patched wall needs sealer or primer, which is a separate product with its own coverage. The tool does not decide either; it prices what you decide.
From the product record for the paint you select, not from an assumed spread rate. Where a product has no stated coverage, the page asks for the figure rather than substituting a typical one, because coverage varies widely between a premium emulsion and a contractor grade product in the same tin size. That is also why the estimate is honest about the gap between the stated coverage and the practical one.