Tools in this category
From cubic metres to bags of cement
The concrete calculator takes the dimensions of a slab, a strip footing or a post, gives the volume, then converts it into bags of cement and quantities of sand and gravel for the mix you choose. For a loose delivery, the cubic metres to tonnes converter gives the weight to order. The block calculator counts the blocks and mortar for a wall. The screed calculator gives the screed or self-levelling compound for a given depth. They all start from the same thing: a volume worked out properly.
Volume of a slab by thickness
Volume is area multiplied by thickness, both in metres. A couple of extra centimetres across the whole area add more to the order than you might expect: on 40 m², going from 100 to 120 mm adds 0.8 m³. The table is pure geometry and holds for any concrete.
| Thickness | Per m² | Litres per m² | 10 m² | 25 m² | 40 m² |
|---|---|---|---|---|---|
| 100 mm | 0.10 m³ | 100 L | 1 m³ | 2.5 m³ | 4 m³ |
| 120 mm | 0.12 m³ | 120 L | 1.2 m³ | 3 m³ | 4.8 m³ |
| 150 mm | 0.15 m³ | 150 L | 1.5 m³ | 3.75 m³ | 6 m³ |
| 200 mm | 0.20 m³ | 200 L | 2 m³ | 5 m³ | 8 m³ |
For a strip footing, multiply width by depth: a 400 × 200 mm footing takes 0.08 m³ per metre run, a 500 × 200 mm one 0.1 m³. These are worked examples, not recommended sizes.
Mixes: the manufacturer’s and the designer’s
Quantities per bag vary from one manufacturer to another, and with the use: concrete for a garden path is not mixed like structural concrete. The calculator states which table it applies and where it comes from. In the UK, concrete is specified to BS 8500, which works alongside BS EN 206, and foundations fall under Approved Document A in England. For anything that carries a load, the mix and the reinforcement are set by a structural engineer and checked by Building Control, not by a calculator.
Common mistakes
Ordering gravel in cubic metres when the merchant charges by the tonne, or the other way round: the weight of a loose aggregate depends on how wet it is and on its grading, so use the supplier’s figure for the conversion if they give one. Working from the design thickness on ground that is not level: measure in several places and take the average. Forgetting the bag size: the number of bags depends on the weight of each one, and not every merchant sells the same size.