A worked example: a 20ft × 10ft wall measured on site
Many site tape measures read in feet, so a wall measured at 20ft long by 10ft high first gives a square footage of 200 sq ft (20 × 10), which is 18.5806 m² once converted — the exact area a metric plaster calculation needs.
The formula: converting feet to meters keeps the area exact
Converting each dimension individually (1 ft = 0.3048m) — rather than just converting the final area — keeps the plaster calculator's separate length and height inputs correct: 20ft becomes 6.096m and 10ft becomes 3.048m, and 6.096 × 3.048 = 18.5806 m², matching the square-footage calculator's own area-in-meters output exactly. Feeding those converted dimensions into the plaster formula — length × height × thickness, at 12mm with a 1:4 mix and 5% wastage — gives a plaster volume of 0.223 m³, 1.8 cement bags, and 0.2491 m³ of sand for that exact 200 sq ft wall.
A second wall: 30ft × 9ft, thicker plaster, leaner mix
A larger 30ft by 9ft wall comes to 270 sq ft, or 25.0838 m² (converted dimensions 9.144m by 2.7432m, which multiply back out to that same 25.0838 m²). Plastered 15mm thick with a 1:6 mix and 10% wastage, that wall needs 0.376 m³ of plaster volume — 2.3 cement bags and 0.4718 m³ of sand.
Why chaining the two calculators avoids a manual conversion error
Plaster volume depends on length and height separately, not just on their product, so a shortcut that converts only the final square-footage figure back into a single "area" and guesses new length/height numbers can quietly introduce rounding drift. Converting each real dimension in feet to meters first, then handing those two exact numbers to the plaster calculator, keeps the wall's true area identical between the two tools — the same approach applies to any material calculator, like paint or tiling, that starts from a feet-measured room but needs metric dimensions to work from.