Estimator Inputs
This estimator is intended for early planning and procurement. Always confirm detailing and code requirements with your structural design.
Example Data Table
| Scenario | Inputs | Outputs (estimated) |
|---|---|---|
| Typical roof slab |
4.50 m × 3.00 m, cover 20 mm, thickness 125 mm Main: 12 mm @ 150 mm, Distribution: 10 mm @ 200 mm Stock 12 m, lap factor 40, waste 5%, straight ends |
Main bars: ~21 bars, cut ~2.96 m each Distribution bars: ~15 bars, cut ~4.46 m each Total steel: ~140–165 kg (varies by detailing) |
| Long span with splices |
10.00 m × 4.00 m, cover 25 mm, thickness 150 mm Main: 16 mm @ 150 mm, Distribution: 12 mm @ 200 mm Stock 12 m, lap factor 45, waste 7%, 90° hooks |
Splices may apply depending on cut lengths Lap steel increases total length and weight Use the report download for full breakdown |
Formulas Used
- Slab area: Area = Length × Width
- Bars count: N = floor((Perpendicular dimension − 2×Cover) ÷ Spacing) + 1
- Base cut length: Lbase = (Run dimension − 2×Cover) + 2×(Hook allowance)
- Hook allowance: 0 (straight), 9d per end (90°), 12d per end (135°)
- Lap length: Llap = (Lap factor) × d
- Cut length with laps: Lcut = Lbase + (Number of laps) × Llap
- Total bar length: Ltotal = N × Lcut
- Unit steel weight: w = d² ÷ 162 (kg/m), where d is in mm
- Total steel weight: W = (Main + Distribution) × (1 + Waste%)
How to Use This Calculator
- Enter slab length, width, thickness, and cover values.
- Select the direction where main reinforcement will run.
- Provide bar diameters and spacing for main and distribution steel.
- Set stock length and choose whether splices should be included.
- Adjust lap factor, anchorage option, and waste allowance if needed.
- Click Estimate Reinforcement to view results above the form.
- Download CSV or PDF to share estimates with your team.
Technical Notes and Planning Guide
1) Why one-way slabs need a clear bar plan
In one-way behavior, most bending occurs across the short span, so the main reinforcement is placed parallel to the short edge. This estimator lets you define the main direction and then calculates bar counts, cut lengths, and total steel weight to support ordering and site planning.
2) Typical spacing and diameter ranges
For light-to-moderate building slabs, main reinforcement commonly falls between 10–16 mm diameter with spacing between 100–200 mm. Distribution steel is often 8–12 mm at 150–250 mm. Always follow the structural drawing and local code, but these ranges help validate inputs.
3) Cover, durability, and constructability
Clear cover influences durability and bar cut length. Moving from 20 mm to 25 mm reduces effective clear span by 10 mm, which slightly reduces each bar length but may improve corrosion protection. Use cover values consistent with exposure conditions, concrete quality, and detailing standards.
4) Bar count logic and edge conditions
The calculator uses N = floor((dimension − 2×cover) ÷ spacing) + 1. This keeps the first and last bars inside the cover line. If you must maintain maximum spacing at the edge, you can reduce spacing slightly to keep bar distribution uniform.
5) Cut length, hooks, and lap steel
Base cut length is the clear run plus anchorage. Straight ends add zero; 90° hooks add about 9d per end; 135° hooks add about 12d per end, where d is bar diameter. If the bar exceeds stock length, lap steel is added using a lap factor (commonly 40d–50d).
6) Steel weight checks using kg/m²
A practical cross-check is steel intensity. Light residential slabs may fall around 8–15 kg/m², while heavier loads or thicker sections can increase to 15–25 kg/m² or more. Compare the result’s kg/m² with project expectations and revise spacing, diameters, and detailing if needed.
7) Waste allowance and procurement realism
Cutting loss, bending, laps, and site handling create waste. Many projects carry 3–7% waste for straight bar work, but congested detailing or multiple splice locations can push waste beyond 8–10%. This tool applies a simple percentage to total steel to support safer ordering.
8) How to interpret the report outputs
Review both directions separately: bar count, cut length per bar, total length, and weight. If splicing is enabled, check the number of laps and lap length assumptions. Export CSV for estimating sheets and PDF for approvals, then confirm final quantities against shop drawings.
FAQs
1) Does this replace structural design?
No. It estimates reinforcement quantities for planning. Final bar sizes, spacing, laps, and anchorage must come from the approved structural design and local code requirements.
2) Which direction should main bars run?
Main bars typically run parallel to the short edge, resisting bending across the short span. If your drawing indicates otherwise, select the direction that matches the detailing.
3) What lap factor should I use?
Common planning values are 40d to 50d, depending on steel grade, concrete strength, and detailing rules. Use the project specification or design notes if available.
4) Why do hooks increase cut length?
Hooks add extra bar length to improve anchorage. This estimator uses typical allowances of 9d per end for 90° hooks and 12d per end for 135° hooks.
5) How is steel weight calculated?
It uses the standard approximation kg/m = d²/162 with d in millimeters. Total weight equals total bar length times unit weight, adjusted by waste percentage.
6) Why might my site quantity differ?
Site quantities vary due to extra laps, openings, drop panels, crank bars, trimming bars, congestion, and bar bending schedules. Use this as an initial estimate and reconcile with drawings.
7) Can I estimate multiple slabs?
Yes. Run the calculator for each slab and export CSV results. Combine totals in a spreadsheet for procurement. For repeated slabs, keep inputs consistent for clean comparisons.