Column
A single 16 in diameter round column (sonotube) 8 ft tall. The volume is the circle area π·(diameter/2)² times the height; multiply the bag counts by the number of piers you are pouring.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 38 |
| 60 lb | 25 |
| 80 lb | 19 |
| 25 kg | 28 |
| 30 kg | 23 |
At this size
19 × 80 lb bags is 0.41 yd³, which is about 10 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 0.41 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
| Diameter | 16 in |
|---|---|
| Height | 8 ft |
| Columns | 1 |
A round column or sonotube form holds a cylinder of concrete: area π·(diameter/2)² times its height, multiplied by how many you are pouring. Enter the tube's inside diameter - that is the finished column diameter.
That volume - 11.17 ft³ - is divided by 27 to give 0.41 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 19 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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