Concrete Stairs Calculator — ft, yd³, bags, cost
How to use this calculator
Pick the stair shape (straight, L-shaped, or U-shaped), then enter total floor-to-floor rise, stair width, riser height, tread depth, and waist slab thickness in feet, inches, and ⅛″ fractions. The calculator equalizes the risers, reports total run, slope length and stair angle, checks IBC limits and the 2R+T comfort rule, and converts to yd³, 60/80 lb bags, ACI 318-19 reinforcement, and priced labor. Tap a dimension on the diagram to jump to its input.
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
A cast-in-place flight is poured as a sloped waist slab with triangular treads cast on top, so concrete take-off is more than the simple length × width × thickness used for a flat slab. This tool splits the pour into steps, waist, and landings, applies your waste allowance, and converts to cubic yards, 60/80 lb bags, weight, ACI 318-19 reinforcement, and priced labor. Use the same dimensions you set out on the stringer template; for the surrounding flatwork pair it with the concrete volume calculator and a concrete bag calculator to true up your order.
Stair Concrete Volume Formula
Volume is the sum of three parts. Steps: each tread is a triangular prism, so step concrete = treads × ½ × riser × going × width. Waist slab: waist thickness × slope length × width, where slope length = √(total rise² + total run²). Landings: on L/U flights, landing width × landing depth × waist thickness for each landing. Number of risers = total rise ÷ target riser, rounded to a whole number; the calculator then back-solves the actual riser so every step is equal. A 5–10% waste allowance covers form spillage and over-excavation at the base.
Code Limits and Reinforcement (IBC / ACI 318-19)
IBC caps the riser at 7¾″ and sets a 10″ minimum tread and 36″ minimum width for residential stairs; a comfortable flight runs 30–37°. Reinforcement follows ACI 318-19: main bars run up the waist on the tension face with distribution steel perpendicular, 1.5–2″ cover, 40db lap splices, and standard 20 ft bar stock. For supporting members size them with the concrete beam calculator and column calculator, and detail any cheek walls with the concrete wall calculator.
FAQ
How thick should the waist slab be? For residential flights up to a 10 ft rise, 6″ is typical (rule of thumb: span/20 to span/25 along the slope). Longer spans or commercial loading may need 8″ or more — confirm with a structural engineer.
How many bags of concrete for a staircase? A straight 9 ft-rise flight, 42″ wide with 14 risers, needs roughly 1.5–2.0 yd³ — about 68–91 bags of 80 lb mix or 88–118 bags of 60 lb mix. Bagged mix is practical only for small jobs; above ~1 yd³, ready-mix is cheaper and faster.
On pour day
Three numbers contractors most often miss on a stair pour. Short-load fee — most ready-mix plants charge $80–150 if you order under ~3 yd³, so a typical 9 ft-rise residential flight (1.5–2 yd³) almost always carries the fee; price it in or pair with adjacent flatwork on the same truck. Stair formwork is the real cost driver — riser boards, side stringers, kickers and bracing typically run $25–60 per linear foot of stair (2025 USA), and the cast-in-place stair line item averages $300–700 per step all-in (forms + concrete + rebar + finish). Mix grade — exterior stoops and porches in freeze-thaw climates need 4000 psi air-entrained concrete per ACI 318-19 §19.3; specifying it after the truck is on site means a short-load reschedule. Save runs to History and export the take-off as text, CSV, A4 JPG, or PDF.
