Steps
A solid poured staircase of 5 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 | 81 |
| 60 lb | 54 |
| 80 lb | 41 |
| 25 kg | 59 |
| 30 kg | 49 |
At this size
41 × 80 lb bags is 0.89 yd³ and roughly 21 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. 0.89 yd³ is still under the usual one-yard delivery minimum, so plan the day around keeping the mixer fed rather than around a truck.
| Steps | 5 |
|---|---|
| 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 - 24.06 ft³ - is divided by 27 to give 0.89 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 41 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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