Rebar Calculator
Estimate rebar counts, total straight length and nominal steel weight for a rectangular reinforcement grid.
How to Use
- Enter the rectangular slab or reinforcement-grid length and width from the project drawing.
- Enter the specified spacing for bars running in each direction. The two spacings may be different.
- Enter the edge cover shown on the drawing. The calculator subtracts cover from both sides before laying out the grid.
- Enter the bar diameter and number of reinforcement layers exactly as specified by the design.
- Optionally add a percentage allowance for cutting waste or other purchasing margin. This allowance does not model lap splices, hooks, bends or anchorage.
- The result shows the bar count in each direction per layer, total bars placed, total straight length and nominal steel weight.
Formula
- Effective length
- L_eff = L − 2 × cover
- Effective width
- W_eff = W − 2 × cover
- Bars along length
- N_L = ceil(W_eff ÷ spacing_L) + 1
- Bars along width
- N_W = ceil(L_eff ÷ spacing_W) + 1
- Total straight length
- [(N_L × L_eff) + (N_W × W_eff)] × layers
- Nominal bar mass per meter
- m = ρ × π × d² ÷ 4, using steel density 7850 kg/m³
Worked Example
A 6 m × 4 m rectangular grid with 200 mm spacing each way, 50 mm edge cover and 12 mm bars in one layer.
- Effective length = 6.00 − 0.10 = 5.90 m; effective width = 4.00 − 0.10 = 3.90 m.
- Bars running along length are spaced across 3.90 m: ceil(3.90 ÷ 0.20) + 1 = 21 bars.
- Bars running along width are spaced across 5.90 m: ceil(5.90 ÷ 0.20) + 1 = 31 bars.
- Straight bar length = (21 × 5.90) + (31 × 3.90) = 244.8 m.
- Nominal 12 mm bar mass is approximately 0.888 kg/m, so the estimated steel weight is about 217 kg.
Result: 21 bars along length, 31 along width, about 244.8 m total straight length and about 217 kg nominal steel weight.
About This Tool
What this tool does
This calculator converts a rectangular reinforcement layout into estimated bar counts, straight lengths and nominal steel weight. It is intended for quantity takeoff when the reinforcement design values are already known.
When to use it
Use it for preliminary quantity takeoffs for rectangular slabs, mats or similar orthogonal reinforcement grids where bar spacing, cover, diameter and layers are specified on the drawings.
What the result means
The bar counts show how many straight bars are needed in each direction per layer while keeping the calculated spacing no greater than the entered target spacing. Total length is the sum of those straight bars after edge cover, multiplied by the number of layers and any optional allowance. Weight is a nominal geometric estimate from bar diameter and steel density.
Assumptions & limitations
The tool assumes a rectangular orthogonal grid of straight bars with uniform spacing. It does not include lap splices, development length, hooks, bends, chairs, starter bars, openings, trimming bars, congestion, bar schedules, code requirements or structural adequacy. Do not use it to choose reinforcement. Use project drawings and the responsible engineer for design requirements, and use actual mill or supplier mass tables when procurement precision is critical.