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Concrete calculator
Enter the size of your slab, footing or post holes. You get the volume in m³, what to order as ready-mix, and how many bags of cement and tonnes of ballast to mix it yourself.
- Concrete needed (incl. waste)
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- Ready-mix to order
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- or 20 kg pre-mix bags
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- or mix on site: 25 kg cement
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- plus all-in ballast
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How the calculation works
Volume is length × width × depth, all in metres. A 100 mm slab is 0.1 m deep, so a 4 m × 3 m slab is 4 × 3 × 0.1 = 1.2 m³. For round holes it is π × radius² × depth for each hole. The calculator then adds your waste allowance, because ground is never perfectly level and some concrete always stays in the mixer or barrow.
To mix on site you need more dry material than the finished volume, because sand and cement fill the gaps between the stones. The usual rule is 1.54 m³ of dry materials for every 1 m³ of concrete. That dry volume is split by the mix ratio: a 1:5 mix is 1 part cement to 5 parts all-in ballast.
Worked example: a 4 × 3 m shed base
- Volume: 4 × 3 × 0.1 = 1.2 m³. With 10% waste: 1.32 m³.
- Ready-mix: order 1.5 m³ (most suppliers deliver in 0.25 or 0.5 m³ steps, and many have a 1 m³ minimum).
- Mixing a 1:5 mix yourself: about 20 bags of cement (25 kg) and 2.7 tonnes of ballast, which is roughly 4 bulk bags.
- Bagged pre-mix: about 146 × 20 kg bags. Fine for a few post holes, but at this size ready-mix or a mixer is far less work.
Which mix ratio should I use?
| Job | Typical mix | Rough strength |
|---|---|---|
| Fence posts, blinding, bedding | 1:6 | Low (non-structural) |
| Foundations and strip footings | 1:5 | About C20 |
| Floor slabs, shed bases, paths, patios | 1:4 | About C25 |
| Driveways, garage floors, heavy use | 1:3 | About C30 |
If your drawings, engineer or Building Control give a concrete grade (for example GEN1, RC30 or C25/30), order that grade as ready-mix. Site-mixed ratios are approximate and cannot be certified.
Ready-mix, site-mixed or bagged?
- Ready-mix is the best value above about 1 m³. It arrives consistent and you can place it in one go. Check access for the lorry, or book a pump or a "mix on site" (volumetric) truck that only charges for what it pours.
- Mixing yourself with cement and ballast is cheapest for small jobs between about 0.2 and 1 m³, if you have a mixer and labour.
- Bagged pre-mix (just add water) costs the most per m³ but suits a handful of post holes or small repairs.
Tips to avoid running short
- Measure the depth in several places. On an uneven base, 10 mm extra over the whole area adds 10% more concrete.
- Strip footings are rarely the exact trench width: measure the dug trench, not the drawing.
- Order slightly over, not under. A second ready-mix delivery for 0.25 m³ costs far more than the extra on the first.
- Keep 25 kg cement bags dry and off the ground; damp cement goes lumpy and weak.
Questions
How many 25 kg bags of cement are in 1 m³ of concrete?
For a 1:5 mix, about 15 bags of cement and 2 tonnes of ballast per m³ of finished concrete. A stronger 1:4 mix needs about 18 bags and just under 2 tonnes of ballast.
How many 20 kg bags of pre-mixed concrete make 1 m³?
About 110 bags. That is why bagged concrete only makes sense for small jobs like fence posts.
How thick should a concrete slab be?
A shed or garden-room base is usually 100 mm on 100 mm of compacted hardcore. Garage floors and driveways are often 125–150 mm. Follow your drawings or Building Control where they apply.
Why add 10% waste?
Uneven ground, spillage and concrete left in the mixer or pump line. On a well-prepared, level base 5% may be enough; on rough ground use 10–15%.
Results are estimates for planning and ordering. Check quantities with your supplier and product data sheets, and anything structural or regulated with a qualified professional. See our terms.