How much concrete do I need?

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.

Worked example: a 4 m × 3 m slab

A 4 m × 3 m slab at 100 mm deep in a 1:2:4 mix — 1.200 m³ of wet concrete.

How we got there

1. 4 m × 3 m × 100 mm = 1.200 m³ — Length × width × depth gives the wet volume: 1.200 m³.
2. Dry material: 1.848 m³ — A 1:2:4 mix needs 1.848 m³ of dry material to make 1.200 m³ of wet concrete.
3. 380 kg cement = 16 bags — One part of cement in seven: 380 kg, which is 16 × 25 kg bags.
4. 34 bags of sharp sand, 64 bags of ballast — Two parts sand and four parts aggregate — 34 and 64 bags of 25 kg.
5. Water: 190 litres — Mixing water is around half the cement weight by volume.
6. Ready-mix alternative: 1.26 m³ — Ordered as ready-mix, allow 1.26 m³ — roughly 3.02 tonnes.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Ready-mix concrete (1:2:4) — 1.26 m³ (One line if you are ordering a wagon — includes 5% over-order.)
• Cement (25 kg bags) — 16 bags (For mixing on site.)
• Sharp sand (25 kg bags) — 34 bags (≈ 845 kg.)
• Ballast / 20 mm aggregate (25 kg bags) — 64 bags (≈ 1,584 kg, or 2 bulk bags.)
• Water — 190 litres (Add gradually — too much water weakens the concrete.)
• Steel reinforcement mesh A142 (4.8 × 2.4 m sheet) — 2 sheets (Rule of thumb: a 4.8 × 2.4 m sheet covers about 11.5 m², and the sheets lap one full square, so 1.12× the area is ordered. Mesh is only needed where the slab is meant to carry it — a plain garden-shed slab on firm ground may need none, and a footing is designed its own way. Confirm it against the design, not against this line.)
• Mesh spacers / chairs — 24 spacers (Rule of thumb: 2 per m² to hold the mesh in the middle of the slab. Mesh lying on the ground or on the membrane reinforces nothing — it has to sit in the concrete.)
• Tying wire (1.6 mm coil) — 1 kg (Rule of thumb: about 1 kg per 20 m² of mesh, to tie the laps. Soft annealed tying wire, not a heavier gauge.)
• Expansion / perimeter joint filler board — 14 m (Rule of thumb: the slab perimeter, 14.0 m here, standing against walls, kerbs, a manhole or an existing slab. A slab pinned all the way round cracks somewhere else instead.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

Concrete is ordered as a volume, so the allowance is for spillage and an uneven sub-base rather than offcuts. Ready-mix is batched in part-cubic-metre steps and generally has a minimum load, so check the supplier before ordering an exact figure.

Regs & planning for this calculator

The volume is arithmetic. The mix is a specification, and the structure is a design.

• BS 8500 and BS EN 206 — mix specification The regulations point at BS 8500 for the concrete mix: Approved Document C specifies mixes ST 1, ST2 and ST4 by name. Read the requirement.
• Approved Document A — structure Loading, ground movement and the Eurocodes that structural design is carried out to. Read the requirement.
• Planning and building regulations exemptions Whether the structure needs planning permission at all, and the Schedule 2 exemptions from building regulations. Read the requirement.