A floor plan area is rarely the same as the area to be plastered. For this calculator, each schedule row represents a surface: its measured length, its measured width and the number of identical surfaces.
A wall face may therefore use width as its height. Naming the row “Bedroom north wall” is clearer than leaving it as “Room 1” when you return to the worksheet later.
Build the surface schedule first
Use one row for a rectangular surface, or split an irregular face into measured rectangles. All schedule dimensions follow the main unit selection. Plaster thickness stays in millimetres.
The deduction is square feet for one set of faces. The calculator subtracts it from the gross schedule area and then multiplies the remaining area by the number of faces selected.
That order matters. If opposite wall faces have different openings or extents, calculate them separately with faces set to 1. Selecting two faces simply doubles the same net area.
A small surface that needs one bag
For the checked example, enter one 12 × 10 ft surface, a 21 sq ft deduction, one face and 12 mm thickness. Use sand parts 4, dry factor 1.33, a 50 kg bag, cement density 1,440 kg/m³ and 5% allowance.
The net application area is 99 sq ft. The measured plaster volume is about 0.11 m³. After dry conversion and allowance, the requirement is 44.39 kg of cement and 4.35 cft of loose sand. The purchase quantity is one 50 kg bag.
A whole bag is a purchase result, not an instruction to use every kilogram on that small surface. Keep the calculated mass separate when combining several areas into one order.
Thickness is a real quantity decision
The estimate changes directly with thickness. A rough surface that needs local build up cannot be represented accurately by an arbitrary thin coating everywhere. Use the thickness called for by the work and measure unusual areas separately.
The dry factor and cement to sand proportions are adjustable assumptions. The tool does not assess adhesion, prepare the substrate or select a suitable render system.
Finish the worksheet before pricing it
The surface table shows gross areas. The headline result includes the deduction and selected faces, so those two totals can differ without an error.
Enter bag and sand quotes in their displayed units. Scaffolding, labour, surface preparation and subsequent finishes are outside that material cost. If putty will follow, estimate its product consumption separately with the Wall Putty Calculator rather than reusing the plaster mix quantities.