Steps
A solid poured staircase of 4 steps, 3 ft wide, with comfortable 7 in rises and 11 in treads. The volume is the stacked step cross-section times the width.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 54 |
| 60 lb | 36 |
| 80 lb | 27 |
| 25 kg | 39 |
| 30 kg | 33 |
At this size
27 × 80 lb bags is 0.59 yd³, which is about 14 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 0.59 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
| Steps | 4 |
|---|---|
| Rise | 7 in |
| Run | 11 in |
| Width | 3 ft |
A solid poured staircase is a stack of rectangular blocks: the cross-section area is rise × run × n(n+1)/2 for n steps, and the volume is that area times the stair width. Seven-inch rises with eleven-inch runs are a comfortable, code-typical stair.
That volume - 16.04 ft³ - is divided by 27 to give 0.59 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 27 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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