Concrete Stairs Calculator — ft, yd³, bags, cost

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.

Geometry screening and reinforcement takeoff

The model-code selector separates residential IRC geometry from general IBC egress geometry. Local amendments, occupancy, occupant load, accessible-means requirements and stair exceptions still govern. Reinforcement is a quantity takeoff from user-selected bars, stock length, lap and anchorage allowances; it is not an ACI structural design. Development length, splice class, cover, support reinforcement and anchorage must come from the project drawings.

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.