Footing
A 30 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 | 89 |
| 60 lb | 60 |
| 80 lb | 45 |
| 25 kg | 65 |
| 30 kg | 54 |
At this size
45 × 80 lb bags is 0.99 yd³ and roughly 23 mixer batches. This is the crossover point: bagging it is a full day with a mixer and at least two people, and the concrete has to keep arriving faster than the placed edge stiffens. 0.99 yd³ is still under the usual one-yard delivery minimum, so plan the day around keeping the mixer fed rather than around a truck.
| Run | 30 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 - 26.67 ft³ - is divided by 27 to give 0.99 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 45 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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