TryBuildCalcWaist Slab CalculatorWaist slab concrete and steel estimator
Calculate concrete, steel, and shuttering quantity for a staircase waist slab, steps, and optional landing.
🕒 Last updated: August 9, 2026
Basic Dimensions
ℹ️Vertical distance the staircase must rise between the two floors it connects.
ℹ️Commonly 150-175 mm for residential staircases; the calculator adjusts this slightly so the height divides evenly into whole steps.
ℹ️Commonly 250-300 mm; comfort rule of thumb: 2 × riser + tread between 500-700 mm.
ℹ️Clear width of the flight, commonly 0.9-1.2 m for residential staircases.
ℹ️Commonly 125-200 mm for residential spans — confirm against your structural drawing.
ℹ️Set to more than 1 if several identical flights share the same dimensions.
Material Details
Reinforcement
ℹ️Main bars run along the slope; commonly 100-200 mm centre-to-centre.
ℹ️Distribution bars run across the width, perpendicular to main bars; commonly 150-250 mm centre-to-centre.
Landing
Cost
For 1 staircase with 20 steps rising 5.32 m horizontally, you need approximately 1.381 m³ of concrete and 42.1 kg of reinforcement steel.
Concrete
Waist slab volume: 0.916 m³
Step blocks volume: 0.399 m³
Total concrete: 1.381 m³ (48.77 cft)
Cement: 11.1 bags
Sand: 0.580 m³ (20.48 cft)
Aggregate: 1.160 m³ (40.96 cft)
Steel Reinforcement
Main Bars (Waist Slab): 8 × 10 mm
Distribution Bars (Waist Slab): 32 × 8 mm
Total steel weight: 42.1 kg
Shuttering
Waist soffit: 6.11 m²
Waist sides: 1.83 m²
Step riser faces: 2.85 m²
Total contact area: 10.79 m² (116.1 sqft)
| Bar Type | Diameter | Count | Cutting Length | Total Length | Weight |
|---|---|---|---|---|---|
| Main Bars (Waist Slab) | 10 mm | 8 @ 150 mm c/c | 6.068 m | 48.54 m | 29.96 kg |
| Distribution Bars (Waist Slab) | 8 mm | 32 @ 200 mm c/c | 0.960 m | 30.72 m | 12.14 kg |
Assumptions Used
Step blocks counted = steps − 1 (top riser is flush with the upper floor) | Steel weight: d² ÷ 162 (kg/m) | Bar count follows slab spacing convention (span − 2×cover) ÷ spacing + 1 | Concrete dry volume factor: 1.54 | No lap splices modeled — see Limitations.
Checking riser/tread comfort or headroom before finalizing the layout? Staircase Design Guide →
Staircase Concrete & Steel Visualization
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Staircase Concrete & Steel Calculator.
Staircase waist slab, without a landing
This page focuses on the waist slab itself — the inclined reinforced-concrete slab that runs the full length of a straight staircase flight, with the landing option switched off by default.
The waist slab is one of the two biggest drivers of a staircase's concrete volume, since it runs the full inclined (slope) length rather than just the horizontal run — a small increase in thickness gets multiplied across a comparatively large sloped area.
- Waist volume = slope length × width × waist thickness, where slope length is the flight's diagonal (Pythagorean) length.
- Turn the landing option back on if your flight actually has a mid-landing.
- Main bars run along the slope; distribution bars run across the width.
How Is the Staircase Quantity Calculated?
The calculation happens in four parts — geometry, concrete quantity, steel reinforcement, and shuttering area — then an optional cost estimate on top.
Step 1 — Geometry
Steps = Round(Total Height ÷ Riser)
Adjusted Riser = Total Height ÷ Steps
Horizontal Run = (Steps − 1) × Tread
Slope Length = √(Horizontal Run² + Total Height²)
The riser is adjusted slightly so the total height divides evenly into a whole number of steps — the same convention used by this site's pure-geometry Staircase Calculator. The horizontal run uses (Steps − 1) treads, not Steps, because the top riser is flush with the upper floor slab and doesn't add its own going — the same convention used for step blocks in Step 2.
Step 2 — Concrete Volume
Waist Volume = Slope Length × Width × Waist Thickness
Step Blocks Volume = 0.5 × Riser × Tread × Width × (Steps − 1)
Landing Volume = Landing Length × Landing Width × Waist Thickness (if included)
Dry Volume = Wet Volume × 1.54
Step blocks are counted as (Steps − 1) because the top riser is flush with the upper floor slab, which already accounts for that last riser's height. The landing reuses the waist slab thickness rather than a separate input, since the two are typically cast together as one continuous structure.
Step 3 — Steel Reinforcement
Main Bar Count = ⌈(Width − 2×Cover) ÷ Spacing⌉ + 1
Distribution Bar Count = ⌈(Slope Length − 2×Cover) ÷ Spacing⌉ + 1
Cutting Length = Span − 2×Cover
Unit Weight (kg/m) = Diameter² ÷ 162
Main bars run along the slope and are spaced across the width; distribution bars run across the width and are spaced along the slope — the same bar-count convention this site's slab steel calculator uses for a flat spanning slab. Lap splices are not modeled; see Limitations.
Step 4 — Shuttering Area
Waist Soffit = Slope Length × Width
Waist Sides = 2 × Waist Thickness × Slope Length
Riser Faces = Riser × Width × (Steps − 1)
Landing Soffit + Edges (if included)
Shuttering supports the underside and both open sides of the inclined waist slab, plus a form against each step's vertical riser face until the concrete gains enough strength to be self-supporting.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section. Each table shows the calculation, the values substituted in, and the result it produces.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Floor-to-floor height | 3 m | Fixes the total rise, so it sets the step count and the slope length |
| Riser / tread | 150 mm riser × 280 mm tread | Riser sizes the steps and step-block volume; tread sizes the horizontal run |
| Width | 1 m | Multiplies every area/volume across the flight (waist slab, steps, shuttering) |
| Waist slab thickness | 150 mm | Sets the waist volume and the shuttering side area |
| Mix ratio / wastage | 1:1.5:3, 5% wastage | Converts wet volume to cement bags and adds a buffer for site losses |
| Cover / bar diameter / spacing | 20 mm cover, 10 mm main @ 150 mm, 8 mm distribution @ 200 mm | Sets bar cutting length (span minus cover) and how many bars fit across each dimension |
| Number of staircases | 1 | Multiplies every final quantity for identical repeated flights |
Step 1 — Geometry
A 3 m rise needs to be split into a whole number of 150 mm-ish steps first, since a flight can't have a fractional step. That step count then fixes how far the flight runs horizontally (one tread per step, except the top one, which lands flush with the upper floor) and, from there, the actual slope length the waist slab and steel must span.
| Calculation | Substitution | Result |
|---|---|---|
| Total height | 3 m | 3.000 m |
| Steps | Round(3.00 ÷ 150 mm) | 20 steps |
| Horizontal run | (20 − 1) × 280 mm | 5.320 m |
| Slope length | √(5.32² + 3.00²) | 6.108 m (20.04 ft) |
Step 2 — Concrete
The waist slab is treated as a flat inclined slab: 6.11 m long (the slope length from Step 1) × 1m wide × 150mm thick. The 19 triangular step blocks sit on top of it and are each half a riser × tread rectangle, so the volume formula carries a 0.5 factor. Wastage is applied once, after the waist and step volumes are added together.
| Calculation | Substitution | Result |
|---|---|---|
| Waist volume | 6.11 × 1m × 150mm | 0.916 m³ |
| Step blocks volume | 0.5 × 150mm × 280mm × 1m × 19 | 0.399 m³ |
| With 5% wastage | 1.315 × 1.05 | 1.381 m³ (11.1 bags) |
Step 3 — Steel
Main bars run along the slope length and are spaced across the 1m width, so their count comes from the width and their cutting length comes from the slope length (minus cover on each end). Distribution bars run the other way — spaced along the slope, counted across the width — using the same cover and unit-weight (diameter² ÷ 162) formula.
| Bar Type | Substitution | Weight |
|---|---|---|
| Main Bars (Waist Slab) | 8 × 6.068 m × 10²÷162 | 29.96 kg |
| Distribution Bars (Waist Slab) | 32 × 0.960 m × 8²÷162 | 12.14 kg |
| Total steel | Sum of all rows | 42.10 kg |
Step 4 — Shuttering
Formwork is needed under the waist slab (soffit), against both open long edges of the slope (sides), and against each step's vertical face until the concrete cures — that's why the riser face area uses the same 19 step-block count as the concrete step volume above.
| Calculation | Substitution | Result |
|---|---|---|
| Waist soffit | 6.11 × 1m | 6.11 m² |
| Waist sides | 2 × 150mm × 6.11 | 1.83 m² |
| Riser faces | 150mm × 1m × 19 | 2.85 m² |
| Total shuttering | Sum of all rows | 10.79 m² (116.14 sqft) |
Therefore, 1 staircase with 20 steps needs 1.381 m³ of concrete, 42.1 kg of steel, and 10.79 m² of shuttering.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.