Screed, skim and texture quantities from the wall areas you identify, at the thickness you set.
Painting, tiling and finishes. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Wall Finish Estimator tool.
A wall finish is a thickness applied to an area, and the estimator measures both. Plaster, render, screed and the bagged alternatives all have a stated yield per millimetre, so the area alone does not tell you what to order until the thickness is fixed. The estimator separates the surfaces by type and treats ceilings as their own surface, since a ceiling is a different mix, a different technique and often a different trade. Openings are deducted, beads and reveals are counted, and the preparation the finish requires is priced where you specify it. Where a thickness is left blank, the tool reports it rather than applying a standard coat.
A room with forty two square metres of wall and fourteen of ceiling, walls at twelve millimetres and the ceiling at three, gives half a cubic metre of wall mortar and a small quantity of a bagged ceiling finish. Deduct a door and two windows from the wall area before the volume is calculated, and add twelve metres of bead at the external corner and the reveals. The two surfaces produced two different orders at two different yields, which is the reason they are measured apart.
Plaster and render by area and thickness, ceilings on their own line, beads and reveals by length, and the preparation products you specify. Screed and the bagged alternatives are handled at their own stated yields. What it does not do is decide which finish a wall needs, because that follows from the substrate, the room and the standard required.
Because material follows volume, and volume is area multiplied by thickness. Doubling the thickness doubles the sand and cement on the same wall, and the difference between a ten and an eighteen millimetre render is a lorry load rather than a rounding. Thickness also decides whether a single coat will do, which changes the labour as well as the material, and it is the first thing a specification changes when a standard is raised.
Yes, as their own surface with their own finish. A ceiling is usually a different mix from a wall, applied by a different technique, and it may be a different trade. Measuring it separately means its quantity can be ordered and priced on its own, and it also means a change to the ceiling specification does not disturb the walls.
Yes, by dimension, before the volume is calculated. A door or a window is an area removed from the wall, and the deduction happens at the area stage so the thickness is applied to what is actually covered. What is not deducted is the bead or reveal around the opening, which is counted as a length instead, since that is the detail the opening creates.
Corner beads and reveals are counted by length from the details you list, and priced as their own line. They are the part of a plastering estimate most often left out, because they are small items that follow from the geometry rather than from the area. A room with eight external corners needs eight times the bead of a room with one, and the area does not show it.