Concrete Calculator – How Much Concrete Do I Need?
Slabs, footings, post holes and steps. Get the volume in cubic metres, plus bags, ready-mix or a mix-your-own breakdown.
Concrete Calculator
What are you pouring?
Shed and garden bases are usually 100mm thick; garages and driveways 150mm with mesh.
You need
0.99m³
0.9 m³ of concrete plus 10% waste
- 25kg pre-mix bags
- 91
- Ready-mix to order
- 1 m³
- Cement (25kg bags)
- 13
- All-in ballast
- 1.78 t
How we worked this out
Your shopping list
91 × 25kg bags of pre-mixed concrete
Keep them dry and off the floor — damp bags go hard in the bag.
£546
Concrete quantities at a glance
| Volume | 25kg pre-mix bags | Cement (25kg bags) | All-in ballast |
|---|---|---|---|
| 0.1 m³ | 10 | 2 | 0.18 t |
| 0.25 m³ | 23 | 4 | 0.45 t |
| 0.5 m³ | 46 | 7 | 0.90 t |
| 1 m³ | 91 | 13 | 1.80 t |
| 2 m³ | 182 | 26 | 3.60 t |
How this calculator works
Concrete quantities always come back to volume. For a slab or footing we multiply length × width × thickness. For a post hole we use π × radius² × depth, then multiply by the number of holes. For steps we treat each step as a solid block and multiply by the number of steps.
We then add 10% waste as standard. That covers an uneven sub-grade, spillage off the barrow and the fact that you cannot order another half-metre halfway through a pour without leaving a weak cold joint.
To convert volume into materials: one 25kg bag of pre-mix yields about 0.011m³, so we divide the volume by that and round up. For a mix-your-own 1:2:3, we allow 320kg of cement and 1.8 tonnes of all-in ballast per cubic metre.
Worked example: a 3m × 3m shed base
A 3m × 3m base at 100mm thick is 3 × 3 × 0.1 = 0.9m³. Add 10% and you need 0.99m³ of concrete.
As bags, that's 90 × 25kg pre-mix bags — around £540 and a very long weekend. Mixing your own needs about thirteen 25kg bags of cement and 1.8 tonnes of ballast (three 850kg bulk bags), roughly £310 plus mixer hire. Ready-mix at around £130/m³ delivered comes in near £130–£180 for the load, arrives in twenty minutes and will be stronger and more consistent than anything mixed by hand. Above about 1m³, ready-mix wins every time.
Tips from the trade counter
- Compact the base first. 100mm of well-whacked hardcore under a slab stops settlement cracks. Concrete is strong in compression, not in bridging voids.
- Use the least water you can. Sloppy concrete is easier to place and permanently weaker. Aim for a mix that holds its shape when you draw a trowel through.
- Add mesh for anything load-bearing. A121 or A142 mesh in a 150mm slab for a garage or driveway is cheap insurance.
- Don't pour in frost. Below 3°C concrete barely gains strength, and freezing before it sets ruins it. Cover with hessian or fleece overnight.
- Keep it damp for three days. Concrete cures by chemical reaction, not drying out. Covering it in hot weather makes a real difference to final strength.
Building Regulations and structural work
Garden slabs, shed bases and small garden wall footings are not usually notifiable work. Foundations for an extension, a garage, or anything supporting a structure are — they fall under Building Regulations and normally need building control approval and a designed depth based on the ground conditions.
Never pour footings near a drain run, a tree or a boundary without checking first. Clay soils and nearby trees often require deeper foundations than you'd expect. If it's structural, get it designed.
Frequently asked questions
How many 25kg bags of concrete make 1m³?
About 90 bags. A 25kg bag of pre-mixed concrete yields roughly 0.011m³, so anything over about 0.5m³ is usually cheaper and far easier as ready-mix.
What is the standard concrete mix ratio?
1:2:3 by volume — one part cement, two parts sharp sand, three parts coarse aggregate — which gives roughly C25 strength. That suits footings, shed bases and driveways.
How much cement do I need per cubic metre?
Around 320kg, which is thirteen 25kg bags, plus about 1.8 tonnes of all-in ballast for a 1:2:3 mix.
How thick should a concrete shed base be?
100mm over 100mm of compacted hardcore for a garden shed or summer house. Go to 150mm with mesh reinforcement for a garage or anything a vehicle will sit on.
How much concrete for a fence post?
A 300mm diameter hole 600mm deep is about 0.042m³ per post — roughly one and a half 20kg bags of postcrete, or four 25kg bags of concrete per post.
Should I order ready-mix or mix my own?
Under about 0.5m³, bagged is fine. Between 0.5 and 1m³, a mixer and loose materials save money. Over 1m³, ready-mix is cheaper, stronger and arrives in one pour.
Why add 10% waste?
Because sub-grade is never perfectly flat, some concrete is lost to spillage, and you cannot top up a part-set pour an hour later. Running out mid-pour creates a cold joint.
How long before I can walk on it?
Usually 24–48 hours for foot traffic and 7 days before heavy loads. Full strength takes 28 days. Keep it damp and covered in hot weather, and protect it from frost.