Calculating concrete correctly: volume, weight and bag count
When ready-mix pays off and which concrete grade is right
Calculating the required amount of concrete starts with the volume: V = area × thickness. Standard concrete has a density of about 2,400 kg/m³ – so 1 m³ of concrete weighs 2.4 tonnes. From the weight you can work out the number of bags: for 25 kg bags, that is exactly 96 bags per m³. The high bag count makes clear that dry concrete mix from the hardware store is only economical for small quantities under 1 m³.
Ready-mix vs. dry concrete mix: The break-even point is around 1.5–2 m³. One cubic metre of ready-mix concrete grade C25/30 costs about €80–120 ex works, plus delivery and pumping. Dry concrete mix from the hardware store costs about €4–6 per 25 kg bag – for 1 m³ that would be 96 bags × €5 = €480 in material alone, plus mixing costs and labour. Ready-mix concrete also has the advantage of defined grade, consistent quality and good workability through controlled water-cement ratios.
Concrete mix ratio: For hand-mixed concrete, the rule of thumb is a volume ratio of 1 : 2 : 3 (cement : sand : gravel). One 25 kg bag of cement, together with aggregates, yields about 0.1 m³ of fresh concrete. For 1 m³ of concrete you therefore need about 250–300 kg of cement, 600 kg of sand (0/4 mm) and 900 kg of gravel (4/16 mm). The water-cement ratio (w/c) should be 0.45–0.55; too much water weakens the concrete.
Frost and de-icing salt resistance: For outdoor surfaces such as driveways, patios and foundations, the exposure class per DIN EN 206 is decisive. XF1 (moderate water saturation, no de-icing salt) requires grade C25/30, XF4 (high water saturation with de-icing salt) requires at least C30/37 with an air-entraining admixture (AEA). Anyone who grits with salt in winter should always use XF4-rated concrete – standard dry concrete mix does not meet this requirement.