Slab Rebar Quantity Calculator
Bar counts, total length and weight for two-way slabs.
Inputs
Result
Formula
| Bars in X = floor(Lx / spacing) + 1 | same 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.
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.
Related calculators
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.