How this instrument works
Poured concrete stairs are built as a run of rectangular treads stacked on rectangular risers, and from the side that stack traces a rough staircase-shaped profile rather than a solid rectangular block. A contractor pricing the pour needs the volume of concrete inside that stepped shape, not the volume of the rectangular box that would contain it — and the stepped shape works out to almost exactly half of that bounding box, which is where the 0.5 factor in this instrument comes from.
This instrument multiplies the number of steps by the tread depth, riser height and stair width, halves the result for the triangular profile, then divides by 27 to convert cubic feet to cubic yards, the unit ready-mix concrete is sold in. It is a genuine rule of thumb long used for fast material takeoffs, not a substitute for a structural engineer's pour drawing, which is why the volume readout is deliberately labelled 'simplified'.
What the simplified estimate leaves out is the solid base beneath the stepped profile — the footing, sub-base or landing slab that most real stair pours also include, along with formwork thickness. A contractor ordering ready-mix typically adds this instrument's figure to a separate base-slab calculation, then rounds the combined total up to the nearest quarter or half yard, since trucks deliver in fixed increments and a short pour part-way through a stair flight is far costlier to fix than a small surplus.
- Enter Number of steps — the count of individual treads in the flight, five by default.
- Set Tread depth (ft) and Riser height (ft) to match the actual step dimensions; a 12 in tread and 7 in (0.583 ft) riser are common residential defaults.
- Enter Stair width (ft), the horizontal width of the flight, and watch Volume, simplified triangular estimate (cubic feet) update instantly.
- Read Concrete needed (cubic yards) beneath it — the figure to hand a ready-mix supplier, rounded up to their minimum order.
- Remember this estimate excludes the base slab or footing under the stairs; add that volume separately (this site's cubic yard instrument handles a simple rectangular base) before ordering.
Worked example — 5 steps, 1 ft tread, 0.583 ft riser
Enter 5 into Number of steps, 1 into Tread depth (ft), 0.583 into Riser height (ft) — about 7 inches — and 4 into Stair width (ft). Volume, simplified triangular estimate (cubic feet) reads 5.83: five steps times a 1×0.583 ft tread-riser rectangle times 4 ft width times 0.5 for the triangular stack.
Concrete needed (cubic yards) reads 0.2159 — 5.83 ft³ divided by 27. That is a little over a fifth of a cubic yard for the stepped profile alone; a contractor would still add the volume of whatever base or landing slab supports this particular flight before placing an order with a ready-mix supplier.
Questions
Why does this formula multiply by 0.5?
Because a flight of stairs, viewed from the side, traces a stepped shape that fills roughly half of the rectangular box formed by steps × tread × riser × width. Stacking rectangular treads and risers this way leaves roughly half the bounding box as empty space above each step, so halving the full box volume gives a reasonable estimate of the solid concrete inside the stepped profile itself.
Does this include the base or footing under the stairs?
No — the readout is explicitly labelled 'simplified triangular estimate' because it only covers the stepped tread-and-riser profile, not the solid base, footing or landing slab that most real stair pours also require. Add that volume separately, for example with this site's plain cubic-yard instrument for a rectangular base, before ordering concrete for an actual job.
What tread depth and riser height should I use for a standard step?
Most residential building codes target roughly a 10–11 in tread and a 7–7.75 in riser, which is why this instrument defaults to a 1 ft (12 in) tread and a 0.583 ft (7 in) riser. Measure your own stringer layout rather than relying on the defaults, since even a small change in riser height compounds across every step in the flight.
How much extra concrete should I order beyond the calculated volume?
Many contractors add 5–10% on top of a calculated volume to cover form leakage, an uneven sub-base, or minor over-excavation, then round up to their ready-mix supplier's minimum load or nearest quarter-yard increment. Because this instrument's figure already excludes the base slab, apply that buffer after adding the base volume, not to the stepped estimate alone.
Is this the same formula an engineer would use for a structural pour?
No — this is a fast rule of thumb for material ordering, not a structural design calculation. An engineer's pour drawing accounts for the exact stringer geometry, reinforcement clearances, and any landing or base slab as one combined shape, which can differ meaningfully from the stacked-triangle approximation used here, especially for stairs with unusual dimensions or open risers.
Why divide by 27 at the end?
Ready-mix concrete is sold by the cubic yard, and 1 cubic yard equals exactly 27 cubic feet, since a yard is 3 feet along each of three dimensions (3×3×3). The instrument computes the stepped volume in cubic feet first, because tread, riser and width are naturally measured in feet and inches on a job site, then divides by 27 only at the final step to hand you an order-ready cubic-yard figure.