Raft Foundation Rebar Calculator
Top and bottom layers plus stools in one calculation.
Inputs
Result
Formula
| Bars in X per layer = floor(Lx / s) + 1 | same 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.
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.
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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.