How the math works
A concrete stair run is a triangular block: the cross-section is a right triangle with legs of tread depth and riser height, times the number of steps. Volume is width × ½ × tread × riser × count. Tread and riser are quoted in inches (typical code is 11″ tread × 7″ riser), so we convert both to feet before multiplying and then divide by 27 for cubic yards.
The waste factor (default 10%) is especially important on stairs — forming errors and over-fill are common.
When to order ready-mix vs. bags
A short front-porch stair (3–4 steps, 3 feet wide) is under a tenth of a yard — clearly a bag job. A wider or taller stair run — say 6 steps at 4 feet wide — approaches the short-load threshold. Above that, ready-mix wins on consistency and finish. Stairs need to be poured and finished in one go so the treads bond; a hand-mixed pour that stalls half-way risks cold joints.
If you’re combining stairs with a landing or walkway, adding those to the same delivery almost always tips the calculation toward ready-mix.
Getting a real quote
National ballparks in the defaults. For a firm number, call a local producer — most will quote over the phone once you have your yardage.