How to calculate how much concrete you need
Concrete is ordered by volume, in cubic metres. Get the volume right and everything else follows — how many bags to buy, how much cement, sand and gravel to mix, or what to order from a ready-mix supplier. This concrete calculator works it out for slabs, bases, footings and post holes, and adds a wastage allowance so you do not run out mid-pour.
Slabs, bases and footings
Volume = length × width × depth, all in metres. A 3m × 3m base 100mm deep is 3 × 3 × 0.1 = 0.9 m³. Remember to convert millimetres: 100mm is 0.1m and 150mm is 0.15m.
Post holes
Each hole is a cylinder: π × radius² × depth. Four holes 300mm across and 600mm deep hold about 0.17 m³ between them. Enter the number of holes and the calculator multiplies for you.
Add wastage
Ground is never perfectly level and some concrete is always lost. Add 5–10%. Running short part way through leaves a weak joint, while a couple of spare bags will keep.
How much concrete for common jobs
With 10% wastage added, using 25kg bags of pre-mixed concrete.
| Job | Size | Order | Weight | Pre-mix bags |
|---|---|---|---|---|
| Shed base | 2.4m × 1.8m × 100mm | 0.48 m³ | 1.14 t | 42 |
| Garden base | 3m × 3m × 100mm | 0.99 m³ | 2.38 t | 88 |
| Four fence post holes | 300mm × 600mm deep | 0.19 m³ | 0.45 t | 17 |
| Driveway section | 6m × 3m × 150mm | 2.97 m³ | 7.13 t | 262 |
Bag yields vary by brand. The figures assume about 2,200kg of dry mix per cubic metre — roughly 88 bags of 25kg or 110 bags of 20kg.
Mixing your own: cement, sand and gravel
The 1:2:4 mix
One part cement, two parts sharp sand and four parts 20mm gravel is a general-purpose mix for bases and footings. Dry materials settle and fill each other’s gaps when water is added, so you need about 1.54 times the finished volume in dry materials.
Worked example: 3m × 3m base
Ordering 0.99 m³ means about 314kg of cement (13 × 25kg bags), 0.70 tonnes of sharp sand and 1.31 tonnes of gravel. Sand and gravel are usually sold in bulk bags of about a tonne, so this job needs one bag of sand and two of gravel.
Bags, mixing or ready-mix?
Bagged pre-mix
Easiest for post holes and small repairs — just add water. It becomes expensive and very heavy on larger jobs: the 88 bags for a 3m × 3m base weigh 2.2 tonnes before any water goes in.
Mixing your own
Cheaper per cubic metre and flexible for medium jobs, especially with a hired mixer. It takes time, and the mix needs to stay consistent so the slab cures evenly.
Ready-mix delivery
Best for larger pours like driveways. It arrives mixed and is poured in one go, but you need lorry access, enough people to spread it quickly, and to check any minimum order. Enter the supplier’s price per m³ above to see the cost.
Depth, weight and planning
How thick?
Around 100mm suits shed bases and paths. Driveways and garage floors carrying vehicles are usually 150mm or more, often with reinforcement and a compacted sub-base. Anything structural should be specified by an engineer or to Building Regulations.
Plan the whole job
Concrete weighs about 2.4 tonnes per cubic metre, so plan how it will be moved from the mixer or lorry. Measure the area first with the square meter calculator, and if you are tiling the base afterwards, price it with the tile calculator. For keeping the wider project on budget, read budgeting a renovation.