Model the stepped cross-section
Treat each step as an incremental rectangular prism so lower steps carry the volume beneath upper treads.
Use finished tread depth, riser height, stair width, and actual step count.
Calculate net concrete first
Establish geometric volume before applying waste or purchase rounding.
Keep the assumption visible so it can be replaced when project or product data is available.
Apply an explicit allowance
Spillage, form irregularity, and placement conditions can justify extra concrete.
Do not use a waste percentage to hide uncertain dimensions.
Convert to the purchase unit
Ready-mix is normally planned in cubic yards; bagged mix should use the exact package yield.
Round only at the final sales unit.
Separate quantity from design
The estimate does not establish stair geometry compliance, reinforcement, foundations, or structural capacity.
Verify the stair design and applicable requirements independently.