Example: ten triangular wedges plus a stated waist
Ten wedges at 1.2 m width, 280 mm tread and 170 mm riser contain 10 × 1.2 × 0.280 × 0.170 ÷ 2 = 0.2856 m³. For a separately specified waist with 2.8 m horizontal run, 1.7 m rise and 150 mm normal thickness, sloping length is √(2.8² + 1.7²) = 3.27567 m; waist volume is 0.58962 m³. Their combined model is 0.87522 m³ before landings and allowance.
Identify the model used by the calculator
The tool multiplies the number of steps by width × tread × riser ÷ 2. This treats each repeated step above a sloping reference line as a right-triangular prism. It excludes the slab beneath that reference line, landings and other structural thickening.
A solid stair filled all the way to a horizontal base is a different geometry. Its entire volume is not the triangular-wedge sum. Read a section drawing and identify exactly which region you are calculating before transferring the result into a concrete order.
Calculate the designed waist slab separately
Where the waist slab has a constant thickness measured perpendicular to its sloping face, its volume is sloping length × width × that thickness. Use the actual designed dimensions and the correct extent of the slab. A vertical thickness is not interchangeable with a normal thickness on a sloped element.
The sloping length follows from the horizontal run and vertical rise of the region being measured. Landings may extend beyond or overlap the flight reference lines; resolve those intersections so the same concrete is not counted twice. Do not use the example below to choose a structural thickness.
Check risers, treads and the landing convention
Draw or count the actual wedges in the flight. A flight may have a different number of exposed treads and risers depending on the way its landings meet it. Enter the count that corresponds to this geometric wedge model, not an automatically copied staircase label.
Nosing projections, shaped edges, voids, intermediate supports and changes of width need their own measurement. Reinforcement, cover, formwork and temporary support requirements come from the design and construction method, not from the estimated concrete volume.
Combine quantities before order rounding
Keep wedge, waist and landing lines visible in your takeoff, then add their unrounded volumes. Apply a chosen placement allowance to the relevant combined quantity. Rounding every small wedge as a separate ready-mix order would greatly overstate the total.
Use the slab tool for a rectangular landing only when its dimensions match the drawing. If you use separate calculator results, take their net volumes rather than adding already-rounded purchase quantities and adding waste again.
Do it in order
- Mark step wedges, sloping slab, landings and any special geometry on the section.
- Calculate each non-overlapping region with matching units.
- Sum net quantities, add one explicit allowance and round the combined order.
Planning boundary
This guide explains quantity-model limits. It does not design stair geometry, structural support, reinforcement or formwork.
Sources checked
Is the stair calculator's result the entire flight?
It is the triangular-step model only. Add the actual waist slab, landings and other specified concrete separately.
Can I choose stair dimensions from the example?
No. Use the project's design and applicable requirements. The example only demonstrates volume arithmetic.