Concrete grades (C20, C25, C30) correspond to standard mix ratios by volume — cement : sand : aggregate, 1:2:4 for C20 through 1:1.5:3 for C30. Dry, loose materials take up more space than the finished wet concrete they become, because mixing and water fill the gaps between particles — the standard correction is a 1.54× dry-volume factor: to make 1 m³ of finished concrete you need to start with 1.54 m³ of combined dry ingredients. Cement weight comes from its dry-volume share at a standard 1440 kg/m³ bulk density; sand and aggregate then scale directly off the cement weight by the mix ratio, and water is added at roughly half the cement's weight (a 0.5 water-cement ratio, typical for general-purpose mixes).
A 0.475 m³ shed base at C20 (1:2:4) needs about 150 kg of cement (6.0 bags of 25kg), 301 kg of sand, and 602 kg of aggregate — plus around 75 kg (litres) of water at a 0.5 water-cement ratio.
Loose sand and aggregate have air gaps between the particles — when cement paste and water fill those gaps during mixing, the total volume shrinks. The 1.54× factor is the standard correction for that settling, verified against multiple published concrete-calculator references.
C20 handles the vast majority of domestic jobs — shed bases, patios, footings — and is the easiest to batch by hand. Step up to C25 for anywhere vehicles will drive or park, and C30 only for structural elements like beams or columns, where hand-mixing consistency becomes harder to guarantee.
Yes — ballast is pre-blended sand and aggregate, and simplifies ordering. Use a 1:4 cement-to-ballast ratio for C20 or 1:3 for C25 in place of the separate sand/aggregate figures this tool gives.
Not really — consistent batching by hand gets difficult to guarantee above C30, and most suppliers recommend ready-mix delivery for anything stronger, where mixing precision and quality control are handled at the plant rather than on site.