Footing
A 10 ft run of 16 in × 8 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 | 30 |
| 60 lb | 20 |
| 80 lb | 15 |
| 25 kg | 22 |
| 30 kg | 18 |
At this size
15 × 80 lb bags is 0.33 yd³, which is about 8 batches in a 6 ft³ site mixer at two bags a batch. That is a comfortable half-day for two people - one mixing, one placing - and at 0.33 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
| Run | 10 ft |
|---|---|
| Width | 16 in |
| Depth | 8 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 - 8.89 ft³ - is divided by 27 to give 0.33 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 15 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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