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How the concrete calculator works
First it works out the volume you are filling, in cubic metres. For a rectangle that is length × width × depth; for a round post hole it is π × radius² × depth. Multiply by how many identical footings you are pouring and you have the wet volume — the amount of finished concrete you need in the hole. A 4 m × 3 m slab at 100 mm is 1.2 m³.
Then comes the part that catches people out. Cement, sand and aggregate are measured dry, and the dry materials occupy roughly 1.54 times the volume of the finished concrete. That is the standard bulking allowance, and without it every mix comes up short. So 1.2 m³ of concrete needs 1.85 m³ of dry material.
That dry volume is then split by the mix ratio. For 1 : 2 : 4 there are seven parts, so cement is 1/7 of the dry volume, sand 2/7 and aggregate 4/7. Converting to weight at typical UK bulk densities — 1,440 kg/m³ for cement, 1,600 kg/m³ for sharp sand, 1,500 kg/m³ for ballast — gives 380 kg of cement, 845 kg of sand and 1,584 kg of aggregate: 16, 34 and 64 bags of 25 kg. Water is estimated at a water-to-cement ratio of 0.5, which is about 190 litres for that slab.
Which mix should I use?
C20 (1 : 2 : 4) is the sensible default for shed bases, patio slabs, paths and general footings. C25 and C30 are stronger, more cement-hungry and used where structural strength matters — always follow an engineer's specification if you have one. Blinding and mass fill (1 : 4 : 8, 1 : 3 : 6) are lean mixes for levelling under a slab or filling deep trenches where strength is not the point.
Ready-mix or mix it yourself?
As a rough rule, under about 1 m³ is easier with bags and a mixer; over that, a ready-mix wagon usually wins on both time and money, though suppliers have minimum loads and charge for part loads. The calculator shows both routes with a 5% over-order on the ready-mix figure, because running out mid-pour is the expensive mistake.
Depth, curing and reinforcement
A garden path is typically 75–100 mm, a patio or shed base 100 mm, a driveway 150 mm with mesh, and strip footings usually 600–900 mm deep below ground level onto firm subsoil. Concrete gains most of its strength in the first week, so keep it damp — cover it and water it — for at least seven days in warm weather. Do not let it dry out too fast, and do not work it before it has cured.
Concrete FAQs
How much concrete do I need for a 4 m × 3 m slab at 100 mm?
1.2 m³ of wet concrete, or about 1.26 m³ of ready-mix once the 5% over-order is included — roughly 3 tonnes. Mixed on site at 1 : 2 : 4 that is 16 bags of cement, 34 bags of sharp sand and 64 bags of 20 mm aggregate, plus about 190 litres of water.
How many bags of cement are in a cubic metre of concrete?
At a 1 : 2 : 4 mix, roughly 13–14 bags of 25 kg cement per cubic metre of finished concrete (about 317 kg of cement). Leaner mixes use less cement; stronger mixes use more. The calculator gives you the exact figure for your mix and volume.
Should I use ballast or separate sand and aggregate?
Ballast (also called all-in aggregate) is a pre-blended sand and gravel mix, and it is what most DIYers should use — fewer bags, fewer decisions. It is usually supplied at a 1 : 2 sand-to-gravel ratio, which suits a 1 : 2 : 4 type mix reasonably well when combined with cement. Separate sharp sand and 20 mm aggregate gives you exact control over the mix, which matters on structural work.
How deep should footings be?
For garden structures, footings typically go 600–900 mm below ground level, and they must be on firm, undisturbed subsoil rather than made ground. Depth depends on the soil and on what is nearby — trees, drains, existing foundations. If it is attached to a building or anything structural, get it specified properly by an engineer or building control.
How long before I can walk or build on it?
You can usually walk on a slab after 24–48 hours and drill or fix into it after about a week, but it continues to gain strength for weeks. Keep it damp and covered for the first seven days, and give it a full 28 days before loading it heavily or building on it.