Concrete Stairs Calculator — ft, yd³, bags, cost
How to use this calculator
Select a stair type (straight, L-shaped, or U-shaped), enter total rise, stair width, riser height, tread depth, and waist slab thickness. The calculator determines the number of risers, total run, stair angle, and concrete volume. Results update instantly with an interactive side-elevation diagram.
Cost — pick ready-mix per yd³, 80-lb bags, or 60-lb bags for material cost.
Reinforcement — main bars along the waist + distribution bars per ACI 318-19.
Labor — rate per step, per yd³, or a flat price. Grand total sums all active sections.
Saved Calculations
| Time | Type | Rise | Steps | Vol yd³ | Concrete $ | Rebar $ | Labor | Total |
|---|
How to Calculate Concrete for Stairs
This concrete stairs calculator estimates volume in cubic yards, number of pre-mixed bags (60 lb and 80 lb), total weight, material cost, reinforcement, and labor for cast-in-place concrete stairs. Supports straight runs, L-shaped (with one landing), and U-shaped (with two landings) configurations. Enter total floor-to-floor rise, stair width, riser height, tread depth, and waist slab thickness in feet, inches, and fractions with 1/8″ precision.
Stair Concrete Volume Formulas
The total concrete volume is the sum of three components: Steps — each step is a triangular prism: 0.5 × riser height × tread depth × width. Waist slab — the structural slab under the stairs: waist thickness × slope length × width, where slope length = √(total rise² + total run²). Landings — for L/U shapes: landing width × landing depth × waist thickness. Number of risers = total rise ÷ riser height (rounded to nearest whole number).
Building Code Requirements (IBC / ACI 318-19)
Per the International Building Code: maximum riser height is 7.75 inches (7¾″), minimum tread depth is 10 inches, minimum stair width is 36 inches for residential. The stair angle should be between 30° and 37° for comfortable use. Reinforcement follows ACI 318-19 with main bars along the waist slab and distribution bars perpendicular, using standard 20-foot bar lengths with 40db lap splices.
FAQ
How thick should the waist slab be? For residential stairs up to 10 ft rise, 6 inches is typical. For longer spans or commercial use, 8 inches or more may be needed — consult a structural engineer.
How many bags of concrete for stairs? A typical straight-run residential staircase (9 ft rise, 42″ wide, 14 risers) needs approximately 1.5–2.0 yd³, which equals roughly 68–91 bags of 80-lb mix or 88–118 bags of 60-lb mix.
