Hollow-block (hourdi) slab rebar calculator
This calculator estimates a one-way hollow-block (hourdi) slab: blocks = area ÷ (rib spacing × block length), rounded up; concrete = topping + ribs, that is the gross volume minus the block volume; then steel = concrete volume × a typical ratio of 80 kg per m³. The result is a preliminary estimate only; the structural drawings govern.
Calculator
- Reinforcing steel
- 1,538 kg (1.54 t)
- Concrete (topping + ribs)
- 19.23 m³
- Hollow blocks
- 1,924 blocks
- Topping / ribs concrete
- 10 / 9.23 m³
Method, step by step
- Block height = total slab thickness minus the topping thickness.
- Rib spacing = block width + rib width; the module area = rib spacing × block length.
- Blocks = slab area ÷ module area, rounded up.
- Topping concrete = area × topping thickness; rib concrete = area × (rib width ÷ rib spacing) × block height.
- Steel weight = total concrete × the steel ratio in kg per m³; divide by 1000 for tonnes.
Worked example
Inputs
- Slab area (m²): 200
- Total slab thickness (cm): 25
- Topping thickness (cm): 5
- Rib width (cm): 12
- Block width (cm): 40
- Block length (cm): 20
- Block height = total thickness - topping = 25 - 5 = 20 cm.
- Rib spacing = block width + rib width = 40 + 12 = 52 cm; the module (one block with its share of rib) = 0.52 × 0.2 = 0.104 m².
- Blocks = 200 ÷ 0.104 = 1,924, rounded up.
- Topping concrete = 200 × 0.05 = 10 m³; rib concrete = 200 × (12 ÷ 52) × 0.2 = 9.23 m³; total = 19.23 m³.
- Check: gross volume 200 × 0.25 = 50 m³, minus the block volume 30.77 m³, gives 19.23 m³.
- Steel weight = 19.23 × 80 = 1,538 kg (1.54 t).
Result
- Reinforcing steel
- 1,538 kg (1.54 t)
- Concrete (topping + ribs)
- 19.23 m³
- Hollow blocks
- 1,924 blocks
- Topping / ribs concrete
- 10 / 9.23 m³
Assumptions and default ratios
| Ratio | Default value |
|---|---|
| Hollow-block slab steel ratio (typical) | 80 kg per m³ of concrete |
- The steel ratio of 80 kg per m³ of concrete is an editable typical value for preliminary estimates, not a design value. The actual quantity comes from the structural drawings and the bar bending schedule.
- The slab is one-way with parallel ribs. Solid strips around columns, hidden beams and solid edges are not included; add them as solid concrete if present.
- The block count has no waste allowance, and the block and rib sizes in the example are illustrative; enter your project's sizes from the drawing.
Frequently asked questions
How do I count hollow blocks per square metre?
Each block occupies a module as wide as the rib spacing (block width + rib width) and as long as the block. With a 40 cm wide, 20 cm long block and a 12 cm rib, the module is 0.52 × 0.2 = 0.104 m², about 9.62 blocks per m².
Why does a hollow-block slab need less concrete than a solid slab of the same thickness?
The blocks replace part of the concrete between the ribs. In the worked example a 200 m² slab, 25 cm thick, needs 19.23 m³ of concrete instead of 50 m³ if it were solid.
What steel ratio does the calculator use?
By default 80 kg per m³ of actual concrete (not of the full thickness), a typical ratio for preliminary estimates that you can change. The real ratio depends on spans and loads and comes from the structural drawings.
Does this replace the bar bending schedule?
No. The result is for early pricing and comparison; steel orders and payment certificates rely on the actual lengths in the schedule.
Add the result to your project BOQ in Ta3mir
In Ta3mir the same calculators work inside your project: send a result to a BOQ line in one click and save your own default ratios for every calculation.