算BuildCalc

Raft Foundation Rebar Calculator

Top and bottom layers plus stools in one calculation.

Quick answer

With the default inputs: Raft length X 20 · Raft width Y 30 · Main bar diameter 16 etc.Total weight incl. stools 20.60 t

Edit the inputs below to recalculate.
Spacing mmMain bar diameter mm
Top and bottom layers each run two ways; the pitch is per layer per direction. Stool spacing is separate — do not fold it into the main pitch.

Inputs

Result

Total weight incl. stools
20.60t
Bars in X per layer151bars
Length per bar in X21.00m
Bars in Y per layer101bars
Length per bar in Y31.00m
Main bar total length12604m
Main bar weight19.91t
Stool count651pcs
Stool weight0.69t
Steel per m²34.34kg/m²

Assumes continuous top and bottom steel both ways. It excludes extra bars under columns and walls, sump reinforcement and edge details, which often add 15-25%. Stools are estimated at one per square metre with a length of twice the slab thickness plus feet. Rafts typically run 90-160 kg/m²; a much lower figure usually means missing extra bars.

Formula

Bars in X per layer = floor(Lx / s) + 1same for Y
Bar length = other dimension + 2× anchorage
Total = (X + Y) × count × layers×2 for top and bottom
Stools = (Lx/s+1) × (Ly/s+1)each 2× thickness + 0.2 m
20×30 m raft, 500 mm thick, Φ16 at 200 mm, 0.5 m extension, Φ12 stools at 1 m: 151 bars of 21 m in X and 101 bars of 31 m in Y, doubled for two layers gives 12604 m and 19.90 t; 651 stools of 1.2 m add 0.69 t, total 20.60 t or 34.3 kg/m².

How to work it out

  1. Measure each direction separately: count = ⌊(the other dimension ÷ spacing)⌋ + 1, length = that dimension + 2 × projection.
  2. Two layers, top and bottom, so double the total length.
  3. Chair count = ⌊slab area ÷ chair spacing²⌋; each chair is 2 × slab depth + 0.2 m, allowing 0.1 m flat at top and bottom.
  4. Total weight = all bar lengths × 0.00617 × d² × (1 + waste).
  5. Projection follows the design: typically 1.0-1.5 m beyond the wall face and never less than half the slab depth.

Where it goes wrong

Material and cost

Mat foundations run 90-140 kg/m² of steel at around 1 m thickness, among the heaviest elements in housing. A 20×30 m mat at 0.5 m holds about 600 m³ of concrete and 60 t of steel — roughly 500,000 in materials.

On site

Basis

FAQ

Why only 34 kg/m² when rafts are usually 100+?
Because this covers continuous main steel and stools only. Extra bars under columns and walls are often denser than the main mat, and sumps and lift pits add more. The tool verifies the continuous portion quickly; it does not replace take-off from drawings.
Do stools have to be counted, or will spacers do?
Spacers only set the bottom cover; the top mat must be carried by stools or steel trestles. Above 800 mm thickness, rebar stools are too flexible and structural trestles are needed. In many regions stools are treated as temporary works and not measured as permanent steel, but they are still bought - cost them separately.
Where does the first bar go?
Bottom steel usually starts half a spacing from the blinding edge to maintain cover; the top mat edge follows the raft extension less cover. The perimeter also needs edge closure (U-bars or short extras), routinely forgotten and worth 2-3% of the total.

Cite this page

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

Building Calculator. Raft Foundation Rebar Calculator. 2026-10-25. https://jianzhujisuan.com/en/mat-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.