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Slab Rebar Quantity Calculator

Bar counts, total length and weight for two-way slabs.

Quick answer

With the default inputs: Span X 4 · Span Y 5 · Bar diameter 10 etc.Total rebar weight 195.5 kg

Edit the inputs below to recalculate.
Spacing mmBar diameter mm
s is the bottom-bar pitch, d the nominal diameter. Count = clear span ÷ pitch + 1: both ends get a bar, and forgetting that +1 is the usual error.

Inputs

%

Result

Total rebar weight
195.5kg
Bars in X34bars
Length per bar in X4.60m
Bars in Y27bars
Length per bar in Y5.60m
Layers1levels
Total length incl. waste316.8m
Unit weight0.617kg/m
Steel per m²9.77kg/m²

Counts use span/spacing + 1; real layouts also need the first bar offset (usually half a spacing), edge positions and opening trims. Bar length = span + 2× anchorage into support. Double layers also need stools or spacers, and top bars need hooks and anchorage. This result covers main bars only - not distribution steel, top bars over supports or trims.

Formula

Bars in X = floor(Lx / spacing) + 1same for Y
Bar length = other span + 2× anchorage
Total = count × length × layers×2 for double layer
Weight = total × (1+waste) × 0.00617d²
Slab 4×5 m, Φ10 at 150 mm, 0.3 m anchorage, single layer: 34 bars of 4.6 m in X (156.4 m) and 27 bars of 5.6 m in Y (151.2 m) give 307.6 m, 316.8 m with 3% waste, and at 0.617 kg/m the total is 195.5 kg, or 9.78 kg/m².

How to work it out

  1. Bars running in X are spaced across Y: count = ⌊(Y span ÷ spacing)⌋ + 1, length = X span + 2 × end anchorage into supports.
  2. Likewise in Y: the count comes from the X span and the length from Y plus both ends.
  3. Total length is the sum of both directions; double it for two layers.
  4. Weight = total length × 0.00617 × d² × (1 + waste).
  5. End anchorage per the standard drawings: bottom bars run to the beam or wall centreline and at least 5d; top bars anchor la.

Where it goes wrong

Material and cost

Slab steel runs 55-85 kg/m² — about 68 kg/m² for Φ10@150 two-way double layer. A 100 m² slab holds roughly 6.8 t, about 31,000 at 4,500 per tonne before waste and fabrication.

On site

Basis

FAQ

Why add one to the count?
Bars are physical objects - there is one at each end. A 4 m span at 150 mm has 26 spaces but 27 bars, and with the usual half-spacing start offset it comes to 28. Dividing alone loses a row, enlarging edge cover and unbalancing the load path.
Can top steel in a double-layer slab be estimated the same way?
Only if it really is continuous. Top bars over supports usually extend a quarter of the span, with temperature steel in between, so assuming full-length top steel overestimates by 30% or more. The double-layer option here fits rafts and basement slabs where bars do run full length.
Is about 10 kg/m² normal?
Residential cast-in-place slabs of 3-4 m span and 100-120 mm thickness typically run 8-12 kg/m²; basement roofs, civil defence slabs and long spans reach 20-35. A big deviation usually means a wrong spacing or diameter, or missing top bars - a useful sanity check.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Slab Rebar Quantity Calculator. 2026-10-25. https://jianzhujisuan.com/en/slab-rebar-quantity

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.