Calculating Concrete and Screed: Volume, Bags, Classes
How much concrete or screed do I need? Volume for slabs, footings and columns, bags from the manufacturer's yield and what the class on the bag means.
Building & Living2 linked calculatorsReading time: 6 min.
Whether it is a shed footing, a patio slab or a new floor: first calculate the volume, then the number of bags or the amount of ready-mix concrete. This guide shows both steps and explains the designations on the bag and delivery note.
Key takeaways
- Volume is area times thickness – convert the thickness to metres first (15 cm = 0.15 m).
- The number of bags follows from the yield in litres per bag printed on the packaging; weight is only an approximation.
- In Germany, concrete is made to DIN EN 206 together with DIN 1045-2; the building authorities have adopted these rules.
- The class on the bag, such as C25/30, is a compressive strength class after 28 days; which one you need depends on the component's environment.
- Floating screed rests on impact sound insulation, with edge insulation strips and a separating layer.
Volume: area times thickness
Three formulas cover the usual components. Convert all dimensions to metres first; the result is then in cubic metres (m³).
- Slab or floor slab: area × thickness
- Strip or pad footing: length × width × depth (× number)
- Round column or round footing: π × (diameter ÷ 2)² × height
Example: a slab of 12 m² and 15 cm thickness needs 1.80 m³ of concrete. 4 pad footings of 40 × 40 cm each and 80 cm deep need 0.512 m³ in total.
How many bags?
Whether to mix it yourself or order ready-mix usually depends on the quantity. The number of bags depends on the yield, i.e. the litres of fresh concrete per bag printed on the packaging. Without it, you can estimate via weight. The concrete calculator assumes 2,400 kg/m³ – an assumption you can adjust to the manufacturer's figure.
Example via weight: 1.80 m³ × 2,400 kg/m³ = 4,320 kg, so about 173 bags of 25 kg. Since part of the mass is water added only when mixing, the real number differs – the yield on the bag is more accurate.
When to order ready-mix
Bagged concrete is dry concrete: the ingredients are pre-mixed, only the stated amount of water is added. The mixing ratio is therefore printed on the bag, not given by a general formula. For larger quantities, ready-mix concrete from a plant is often simpler – it is ordered by the cubic metre.
Calculate concrete volume and bags
Choose slab, strip footing, pad footing or column, enter the dimensions – using the yield printed on the bag.
Compressive strength class and environment
In Germany, concrete is produced according to DIN EN 206 together with DIN 1045-2; the German Institute for Building Technology lists DIN 1045-2 (2023-08 edition) as the governing rule for concrete in the Model Administrative Provisions on Technical Building Rules. The compressive strength class appears as C with two numbers on the bag or delivery note, for example C25/30, and refers to the strength after 28 days.
Which class is needed at minimum depends on the environment: Annex F of DIN 1045-2 assigns each exposure class – codes such as XD, XF, XS or XA – a minimum compressive strength class and a maximum water-cement ratio (w/c). For some components the building authorities require more, for example C30/37 for certain bridge components.
Load-bearing components
For load-bearing components such as floor slabs, columns or foundations, the structural engineer specifies the strength class and reinforcement. The quantity calculation here does not replace structural design.
Screed: construction and quantity
Screed is the levelling layer between the structural floor and the floor covering. Screed material is a construction product standardised in Europe under EN 13813; type, class and instructions are printed on the bag.
It is either bonded directly to the slab or laid as floating screed: it then rests on impact sound insulation, separated from it by a separating layer, and edge insulation strips separate it from the walls. How thick it must be depends on the screed type, the load and any underfloor heating – planning and manufacturer's instructions are what count.
Example: 20 m² with 5 cm of screed gives 1.00 m³. With the screed calculator's default of 2,000 kg/m³ this is 2,000 kg, i.e. 50 bags of 40 kg. Both values are assumptions – the figures on the bag are what count.
Calculate screed
Enter area and thickness, take density and bag weight from the manufacturer's information.
Frequently asked questions
How much concrete do I need per square metre?
What ratio should I use to mix concrete?
Which concrete class do I need?
How many bags of screed do I need?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- DIBt – Model Administrative Provisions on Technical Building Rules (MVV TB 2025/1) (accessed September 30, 2026)
- DIBt – technical building rules (accessed September 30, 2026)
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