Concrete Calculator
Concrete volume for slabs, footings and columns — in m³ and bags.
The Concrete Calculator works out the volume of concrete for the three common pours — rectangular slabs and footings, and round columns — then converts it to the number of 40 kg or 25 kg bags you will need if you are mixing by hand.
Under-ordering concrete means a half-finished pad and an emergency second pour; over-ordering from a ready-mix truck means paying for concrete you cannot use. For small DIY jobs (path slabs, fence posts, small footings), bags give you exact control, and knowing the count keeps the trip count honest.
The estimates assume standard bag yields — roughly 20 liters of concrete per 40 kg bag — and a 5–10% buffer is wise for uneven ground. Runs entirely in your browser, metric or the bag math you see at the hardware store.
Volume
0.80 m³
≈ 28.3 ft³
≈ 40 kg bags
40
if mixing by hand
≈ 25 kg bags
64
smaller sack option
Ready-mix estimates: a 40 kg bag yields ~20 L of concrete; order 5–10% extra for uneven ground and spillage. For large pours, ready-mix trucks are more economical than bags.
How to use the Concrete Calculator
- Choose the shape: slab/pad, footing, or round column.
- Enter the dimensions in meters (length, width, depth — or diameter).
- Read the volume in cubic meters.
- Check the 40 kg and 25 kg bag counts.
- Add 5–10% for real-world losses.
Frequently asked questions
How do I calculate concrete for a slab?
Volume = length × width × depth. A 4 m × 2 m slab at 0.1 m (10 cm) deep is 0.8 m³. Divide by the bag yield (~0.02 m³ per 40 kg bag) for the bag count: about 40 bags here.
How much does a 40 kg bag make?
Roughly 20 liters (0.02 m³) of mixed concrete at standard ratios. A 25 kg bag makes about 12.5 liters. Check the bag label — yields vary slightly by mix design.
When should I order a ready-mix truck instead?
For pours over roughly 1 m³, trucks are usually cheaper and faster than dozens of bags. Below that, bags win on control and freshness. Either way, order 5–10% extra for uneven ground and spillage.
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