Footing
A 30 ft run of 24 in × 12 in strip footing. The volume is simply the trench cross-section times the run; here it is worked out with the bag counts.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 200 |
| 60 lb | 134 |
| 80 lb | 100 |
| 25 kg | 146 |
| 30 kg | 121 |
At this size
100 × 80 lb bags is 2.22 yd³ - about 50 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 100 bags on site cannot keep ahead of the setting edge, so the pour ends up full of cold joints. Ordered by the yard it is part of a single load. Have the crew, the access and the discharge points sorted before anything arrives.
| Run | 30 ft |
|---|---|
| Width | 24 in |
| Depth | 12 in |
A strip footing is a long rectangular prism under a wall; a spread footing is a pad under a column. Either way the volume is length × width × depth. Enter the total run of the footing as the length, and the trench cross-section as width × depth.
That volume - 60 ft³ - is divided by 27 to give 2.22 cubic yards, and by each bag's yield to give the bag count. An 80 lb bag of premix yields about 0.60 cubic feet, so this job takes 100 of them; bag counts always round up, because you cannot buy part of a bag.
Ordering ready-mix instead? Suppliers sell by the cubic yard (or cubic metre) and usually have a minimum load and a short-load surcharge - worth a call before you choose between bags and a truck.
Formwork
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