Divide the floor by required thickness
A floor rarely needs the same correction everywhere. Record the finished level and survey the substrate before choosing zones. In the self-leveling compound calculator, use a separate zone wherever the assumed thickness changes.
Keep coverage and depth together
“Square metres per bag” is incomplete without the thickness at which that coverage was measured. Coverage of 5 m² at 5 mm does not mean the same bag covers 5 m² at 10 mm. The model scales mass with volume; only use it where that relationship is appropriate for the selected product.
Alternatively, enter the product’s consumption per area and thickness. A value of 1.6 kg/m²/mm means 1.6 kg for one square metre at one millimetre. It is not a universal compound density or a recommendation for a particular floor.
Interpret the average carefully
For 12 m² at 5 mm and 8 m² at 10 mm, total volume is 0.14 m³ over 20 m². The weighted average depth is 7 mm. A simple average of the two depths would be wrong because the areas differ.
Before ordering, check substrate preparation, primers, thickness limits and working conditions against the product instructions. Those decisions are outside the bag calculation.