About this calculator
This calculator works out how much putty to buy for walls, a ceiling, or both: the starting (filler) coat that fills and levels the surface, the finishing coat that gives the smooth surface for paint, and — for a plasterboard surface — the joint treatment: joint tape and joint filler.
Enter the total length of the walls (a room's perimeter, or a single wall), their height and the openings, and/or the ceiling's length and width. Each coat is worked out from its own thickness and the product's own consumption in kg per m² per 1 mm of layer, then rounded up to whole bags or pails. For powder putties the calculator also gives the mixing water.
Formula
wall area = wall length × wall height − Σ openings
area to putty = wall area + ceiling length × ceiling width
coat (kg) = area × consumption (kg/m² per mm) × thickness (mm)
with waste = coat × (1 + waste %)
bags = ceil(with waste / bag size)
water (l) = with waste × water per bag / bag size
joint filler (kg) = area × filler consumption (kg/m²) × (1 + waste %)
joint length = walls: (ceil(L / board width) − 1) × H
+ (ceil(H / board length) − 1) × L
+ ceiling: (ceil(D / board width) − 1) × C
+ (ceil(C / board length) − 1) × D
tape rolls = ceil(joint length / roll length)
Worked example
A room with walls 4 + 3 + 4 + 3 = 14 m long and 2.6 m high, one door 900 × 2100 mm and one window 1200 × 1400 mm, and a 4 × 3 m ceiling, all plasterboard (1200 × 2500 mm boards), 10 % waste:
walls = 14 × 2.6 − 1.89 − 1.68 = 32.83 m²
ceiling = 4 × 3 = 12.00 m²
area = 44.83 m²
starting coat, 2 mm @ 1.1 kg/m²/mm = 98.626 kg; +10 % = 108.49 kg
→ ceil(108.49 / 25) = 5 bags; water 108.49 × 18 / 25 = 78.1 l
finishing coat, 1 mm @ 1.0 kg/m²/mm = 44.83 kg; +10 % = 49.31 kg
→ ceil(49.31 / 20) = 3 pails (ready-mixed, no water)
joints walls: 11 × 2.6 m + 1 × 14 m = 42.6 m
ceiling: 2 × 4 m + 1 × 3 m = 11.0 m
total 53.6 m → ceil(53.6 / 50) = 2 rolls of tape
filler 44.83 × 0.3 = 13.45 kg; +10 % = 14.79 kg → 1 bag; water 7.1 l
FAQ
Where do the default consumption figures come from? From Knauf data sheets: Multi-Finish (about 1.1 kg/m² per 1 mm, 25 kg bag, about 18 l of water per bag) for the starting coat; Fill & Finish Light (a ready-mixed paste, 1.0 kg/m² per 1 mm, 20 kg pail, up to 2 mm per layer) for the finishing coat; Uniflott (0.3 kg/m² on a 12.5 mm board ceiling, 0.25 kg/m² on a one-side wall lining, 5 kg to about 2.4 l of water) for the joints. Other products differ — a cement-based putty can be close to 1.8 kg/m² per 1 mm — so enter your own product's figures.
What thickness should each coat be? The product's data sheet gives the maximum layer per pass. The defaults (2 mm starting, 1 mm finishing) are typical; a very uneven wall needs more, and a thick layer is better built up in several passes.
Why is joint filler given per m² and not per metre of joint? Because that is how the manufacturer states it: per square metre of boarded surface, for a given board type. The joint length is still worked out, for the tape.
Does the calculator count corner beads or mesh? No. Corner beads, inside-corner tape and reinforcing mesh in the starting coat are not counted.
Which standard applies? None is cited. EN 13963 and EN 13279 are product standards for jointing materials and gypsum plasters; neither gives a consumption method. The quantity is area × thickness × the product's own consumption.
Assumptions and limits
No standard is cited: the take-off is plain geometry; consumption, bag sizes and water are product figures.
Defaults to replace with your product's figures: the coat consumptions, bag sizes and water per bag (Knauf data sheets, see the FAQ), the 1200 × 2500 mm board, the 50 m tape roll (a placeholder) and the 10 % waste.
Area: the wall length is the total length of the walls; openings are deducted at their full size, reveals are not added. In the drawing the walls are shown as one run with the openings spaced evenly — their real position is not an input. The ceiling is a rectangle.
Joints: boards stand vertically on the walls, with a horizontal joint every board length when the wall is taller than a board; on the ceiling the long edges run along its length. Staggered end joints are drawn in line. Joint length is not reduced at openings, and inside corners are not counted separately, so the tape figure is slightly on the generous side.