For 20 m², adhesive consumption of 4.5 kg/m², 25 kg bags and 5% extra gives 94.5 kg, so four full bags. With 600 × 600 mm tiles and a 3 × 10 mm joint, theoretical grout need is 3.36 kg at 1.6 kg/L density.
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Common mistakes
Using a generic rate without reading the selected adhesive data sheet.
Ignoring back-buttering or poor substrate flatness.
Using tile thickness as joint depth without checking the actual filled depth.
Confusing calculated quantity with technical suitability.