CalcJoist and Slab Calculator
Last updated: 2026-05-09
Use the Joist and Slab Calculator to get instant, accurate results. Enter your values below.
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Common Sizes — Click to Fill
| Floor length (m) (m) | Floor width (m) (m) | Joist spacing (m) (m) | Slab depth (m) (m) | |
|---|---|---|---|---|
| Caso 1 | 3.2 m | 2.4 m | 0.5 m | 0.2 m |
| Caso 2 | 5.6 m | 4.2 m | 0.5 m | 0.2 m |
| Caso 3 | 8.0 m | 6.0 m | 0.7 m | 0.25 m |
| Caso 4 | 12.0 m | 9.0 m | 1.0 m | 0.38 m |
| Caso 5 | 20.0 m | 15.0 m | 1.0 m | 0.5 m |
What is a Joist and Slab Floor?
A joist and slab floor (ribbed slab) uses closely spaced reinforced concrete joists with lightweight hollow blocks between them, topped with a structural compression layer. This system is lighter and uses less concrete than a solid slab of equivalent strength, allowing longer spans.
Calculation Method
Joists = Width ÷ Joist spacing + 1
Hollow blocks = Floor area ÷ (Spacing × Block height)
Standard joist spacing: 600-800mm centers. Compression layer: 40-50mm thick with reinforcing mesh.
Worked Example
Floor 8m × 6m, 700mm joist spacing, 250mm depth:
- Floor area = 48 m²
- Joists = 6 ÷ 0.70 + 1 = 10 joists
- Hollow blocks = 48 ÷ (0.70 × 0.25) ≈ 274 units
- Compression layer = 48 × 0.04 = 1.92 m³
- Cement = 1.92 × 7 ≈ 14 bags
Common Mistakes
- Not counting additional joists at edges and ends
- Incorrect placement of anti-crack mesh in the compression layer
- Insufficient propping during curing
- Blocking reinforcement continuity across beams
Standards
Calculated per EN 1992-1-1 / CTE DB-SE-AE.