Footing
A 20 ft run of 16 in × 8 in strip footing. The volume is the trench cross-section times the run. That is 17.78 cubic feet, or 0.66 cubic yards: 30 bags of 80 lb premix or 40 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 60 |
| 60 lb | 40 |
| 80 lb | 30 |
| 25 kg | 44 |
| 30 kg | 36 |
Same job, other sizes
| Size | ft³ | yd³ | yd³ to order (+10%) | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|---|---|
| 10 ft run | 8.89 | 0.33 | 0.36 | 17 | 22 |
| 15 ft run | 13.33 | 0.49 | 0.54 | 25 | 33 |
| 20 ft run (this page) | 17.78 | 0.66 | 0.72 | 33 | 44 |
| 24 ft run | 21.33 | 0.79 | 0.87 | 40 | 53 |
| 30 ft run | 26.67 | 0.99 | 1.09 | 49 | 66 |
| 40 ft run | 35.56 | 1.32 | 1.45 | 66 | 87 |
| 50 ft run | 44.44 | 1.65 | 1.81 | 82 | 109 |
| 60 ft run | 53.33 | 1.98 | 2.17 | 98 | 131 |
| 80 ft run | 71.11 | 2.63 | 2.9 | 131 | 174 |
| 100 ft run | 88.89 | 3.29 | 3.62 | 163 | 218 |
Each foot of this 16 × 8 in footing holds 0.89 ft³, so one cubic yard pours about 30.4 ft of it.
One 80 lb bag fills about 8 in of run.
Change the dimensions in the footing calculator. Why every count rounds up: how many bags of concrete you need.
At this size
30 × 80 lb bags is 0.66 yd³, which is about 15 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 0.66 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
| Run | 20 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 - 17.78 ft³ - is divided by 27 to give 0.66 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 30 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.
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