Why the volume is a triangle, not a box
Seen from the side, a solid concrete stair is a right triangle: it climbs the total rise while running out the total run. The stepped top surface and the sloped underside cancel out, so the concrete fills exactly half of the enclosing box. Multiply that triangular area by the stair width to get the volume.
Turning cubic feet into bags and yards
Compute the volume in cubic feet, add roughly 10 percent for waste, then divide by the bag yield. Cubic yards are the volume in cubic feet divided by 27.
| Concrete volume | 80 lb bags (0.60 ft³) | 60 lb bags (0.45 ft³) | Cubic yards |
|---|---|---|---|
| 5 ft³ | 9 | 12 | 0.19 |
| 10 ft³ | 17 | 23 | 0.37 |
| 15 ft³ | 25 | 34 | 0.56 |
| 20 ft³ | 34 | 45 | 0.74 |
| 27 ft³ | 45 | 60 | 1.00 |
Bag counts already round up and include no waste allowance; add about 10 percent on top for a real pour.
Getting the step dimensions right first
- Riser and tread. US residential code caps risers at 7¾ in and needs treads of at least 10 in. Keep every riser within 3/8 in of the others or the stairs become a trip hazard.
- The 2R + T check. Twice the riser plus the tread should land near 25 in (about 63 cm). That is the comfort band; well outside it and the stairs feel steep or stretched.
- Width sets the volume. Volume scales directly with width, so a 4 ft stair needs a third more concrete than a 3 ft one at the same rise and run.
Common questions
How is concrete for stairs calculated?
A run of stairs fills a triangular wedge, so the concrete volume is half the total rise times the total run times the width. Halving it is the key step people miss, since the staircase is a triangle in cross-section, not a full box.
How many bags of concrete do I need for steps?
Divide the volume in cubic feet by the yield printed on the bag. An 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45. So 12 cubic feet of stairs needs roughly 20 of the 80 lb bags.
Should I add extra for waste?
Yes. Add about 10 percent for spillage, over-excavation and the fillet at the base. Order to the next full bag or the next quarter cubic yard rather than cutting it close.
How many cubic feet are in a cubic yard of concrete?
A cubic yard is 27 cubic feet. Ready-mix is sold by the cubic yard, so a small stair pour of under half a yard is usually cheaper to mix from bags.
