How it works
Slab volume = length × width × thickness. Hole volume = π × radius² × depth. We multiply by the number of pours, add your waste allowance, then divide by the typical yield of each premix bag.
| Bag | Typical yield |
|---|---|
| 80 lb | 0.60 cu ft |
| 60 lb | 0.45 cu ft |
| 40 lb | 0.30 cu ft |
| 25 kg | about 0.0115 m³ |
| 20 kg | about 0.0092 m³ |
Yields vary by brand, so check the bag. Add 5–10% for spillage and uneven ground.
FAQ
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 in thick is about 1.23 cubic yards (33.3 cu ft). That is 56 bags of 80 lb premix before waste, or about 62 with 10% extra.
How much concrete for a fence post hole?
A 10 in hole 36 in deep holds about 1.6 cu ft, roughly three 80 lb bags, minus the volume of the post itself.
Should I order ready-mix instead of bags?
Above roughly 1 m³ (1.3 yd³), ready-mix delivered by truck is usually cheaper and much less work than mixing bags.