Advanced options
Typical slab depths: 10cm for paths and shed bases, 15cm for driveways, 20cm plus for heavy vehicle areas. Above about half a cubic metre, ready-mixed delivery usually beats mixing bags by hand on both cost and effort.
What The Concrete Calculator Works Out
- Volume: length x width x depth with the depth converted to metres, in cubic metres. A 2 x 2m shed base at 10cm deep is 0.4 m3. Always order 5% to 10% over, because sub-bases are never perfectly level and running short mid-pour creates a weak joint you cannot fix.
- Approximate weight: volume x 2,400kg per m3, the density of set concrete. The example base is nearly a tonne, which is why the bags-versus-delivery decision matters.
- 25kg bags: the weight divided by 25kg, rounded up - roughly a hundred bags per cubic metre. Up to about half a cubic metre, bags of ready-to-mix concrete (just add water) are practical; beyond that a delivery is less work and usually less money.
- Depth guide: 10cm for paths and shed bases, 15cm for car driveways over a compacted hardcore sub-base, 20cm or more for anything heavier. Mention the intended use when ordering ready-mix so the plant supplies the right strength grade.
- Shape, pours and allowance (advanced): switch to a circular pad (the length box becomes the diameter), multiply up for several identical footings or pads, and add an over-order percentage so the volume shown is what you actually order.
- Mixing your own (advanced): converts the volume into 25kg bags of cement plus tonnes of sharp sand and gravel for your chosen mix. 1:2:4 (about C20) suits most garden slabs, 1:1.5:3 (C25 to C30) suits driveways and structural work, and 1:3:6 is a lean mix for blinding. Dry materials bulk to about 1.54 x the finished volume, which is where the quantities come from; the all-in ballast figure is the sand and gravel combined.
- Ready-mix delivered (advanced): the volume rounded up to the nearest quarter cubic metre, with a 1 m3 minimum, x your delivered price. UK ready-mix typically costs GBP 100 to GBP 160 per m3 plus a part-load charge for small quantities; volumetric trucks mix on site and charge for what you use, which suits awkward amounts.
- Bagged concrete and cheaper option (advanced): bags x your bag price, then a straight comparison with ready-mix. The sub-line reminds you how many tonnes you would be mixing by hand if bags win on price.