Concrete Stairs Calculator
Get the concrete volume for your steps in cubic yards, cubic meters and bags — using the waist‑slab method (structural slab plus step noses) with a builder's 10% waste allowance.
The vertical height of one step.
The horizontal depth of one step.
Slab thickness behind the steps.
Spillage and over-excavation; 10% is typical.
Cubic yards
0.22
yd³
Cubic meters
0.291
m³
Cubic feet
10.3
ft³
80-lb bags
18
≈ 0.6 ft³ each
60-lb bags
23
≈ 0.45 ft³ each
Includes waste
10%
added to volume
Under a cubic yard, bagged concrete mixed on site is typically the simplest option.
How concrete for stairs is calculated
A concrete stair is not a solid block of concrete. It is poured as a sloped structural slab — the waist (throat) — that runs up the underside of the flight, with a triangular step formed on top of it at every tread. So the concrete you actually pour is the waist slab plus the step triangles, not the whole filled wedge under the stair.
volume = width × (throat × √((N·riser)² + (N·run)²) + ½ × riser × run × N), then a waste allowance is added on top. Stairs Calc converts that millimetre volume into
cubic yards, cubic meters, cubic feet, and the number of 80‑lb and 60‑lb bags you would need
to mix it on site.
Worked example
Take a 5‑step flight with a 7″ riser, an 11″ run, a 36″ width and a 4″ throat. The waist slab comes to about 0.15 m³ and the five step triangles to about 0.11 m³, so the flight is roughly 0.26 m³. Adding a 10% waste margin gives about 0.29 m³ — around 0.38 cubic yards, or close to 18 eighty‑pound bags. Filling the whole wedge solid would have called for nearly triple that — which is exactly why the throat thickness matters.
Bags vs ready-mix: when to order a truck
Bagged concrete is convenient for small pours, but it adds up fast. An 80‑lb bag yields only about 0.6 ft³, so a single cubic yard (27 ft³) takes roughly 45 bags — a pallet of mixing, lifting and clean‑up. As a rule of thumb:
- Under about half a cubic yard — mixing bags on site is usually the simplest, cheapest option, especially if you already own a mixer or are mixing by hand in a tub.
- At a cubic yard or more — ordering ready‑mix from a truck is normally cheaper per yard and far faster, and you get one continuous, uniform pour instead of cold joints between batches.
Whichever route you take, order a little extra. The 10% waste allowance built into the calculator covers spillage, slightly over‑excavated forms and the concrete that clings to the mixer and tools — running short mid‑pour is far more expensive than a few leftover bags. To price the finished flight, the stairs cost calculator adds labor and railing, and the stairs square footage calculator sizes any tile or paint for the steps.
Frequently asked questions
How much concrete do I need for steps?
A concrete stair is a sloped structural slab — the waist (throat) — with a triangular step formed on top of each tread, so the volume is the slab plus the step triangles (not the whole solid wedge under the stair) and grows with the number of steps. A typical small flight runs a few tenths of a cubic yard; Stairs Calc gives the exact cubic yards, cubic meters and bag count with a 10% waste allowance.
How do you calculate concrete stairs volume?
Add the structural waist slab — its thickness (the throat) × the width × the inclined length of the flight — to the triangular nose on top of each step (½ × riser × run × width per step). Stairs Calc totals that geometry, adds 10% for waste, and converts to yards, meters and bags. This is far less than filling the whole wedge under the stair solid.
How many 80‑lb bags per cubic yard?
An 80‑lb bag of concrete yields about 0.6 ft³, so you need roughly 45 bags to make one cubic yard. Past about ½ cubic yard, ordering ready‑mix is usually cheaper and faster than bags.
Should I add rebar?
Reinforcing steel is recommended for concrete stairs to control cracking and tie the steps to any footing or slab. Size and placement should follow your local code and conditions; Stairs Calc estimates concrete volume only.
Related stair calculators
Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.
Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.
Last reviewed 2026-06-20 against IRC 2021/2024
Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.