What is Rebar
Reinforcing bar is the ribbed steel rod embedded in concrete to carry the tension loads plain concrete can't handle on its own. Concrete resists compression well and tension poorly; rebar reverses that weakness, so a slab, column, or beam can flex slightly under load instead of cracking. The ribs on the bar surface exist for one reason — they lock the steel to the surrounding concrete so the two materials move as one unit under stress.
Why One Calculator Isn't Enough
A single "rebar calculator" that only handles slab grids answers one question out of a dozen a real project asks. This site splits the problem the way an estimator actually works through it:
- Weight & Takeoff — how much steel, by bar size and length
- Estimating — how much rebar a specific pour needs (slab, footing, driveway, mat)
- Detailing — spacing, development length, lap length, cutting length, chairs, tie wire
- Members — column, beam, wall, and pool-specific reinforcement
- Commercial — turning a bar schedule into tonnage and cost
How Rebar Weight is Calculated
For metric bar sizes, weight per metre follows a fixed relationship to the bar's diameter:
kg/m = d² ÷ 162.28
where d is the bar diameter in millimetres.
A 12mm bar weighs approximately 0.888 kg per metre; a 20mm bar weighs approximately 2.466 kg per metre. Multiply that figure by total bar length, then by quantity, to get total steel weight for a schedule.
For imperial bar sizes (#3–#18), use the published ASTM weight-per-foot table rather than the theoretical formula — mill tolerances and the ribbed profile mean the certified weight differs slightly from a pure cylinder calculation, and that certified figure is what a supplier will actually bill against.
Worked Example
A 6m × 4m slab, 12mm bars, 200mm spacing each way, 40mm edge cover:
- Bars along the length: floor((6 − 2×0.04) / 0.2) + 1 = 30 bars
- Bars along the width: floor((4 − 2×0.04) / 0.2) + 1 = 20 bars
- Total bar length: (30 × 4m) + (20 × 6m) = 240m
- Total weight: 240 × 0.888 kg/m ≈ 213 kg
Standards This Site Covers
Calculations follow IS 456:2000 for development and lap length (τbd-based anchorage) alongside ACI 318 conventions for US bar sizing and spacing practice, with BS 4449 bar designations noted where relevant. Pick the code your jurisdiction requires — the underlying steel doesn't change, but the required lap and development lengths do.
Rebar Weight Chart (kg per metre)
Metric
| Ø | kg/m | kg/ft | 12 m |
|---|
Imperial (US #)
| # | Ø | lb/ft | kg/m |
|---|
Frequently Asked Questions
How do you calculate rebar weight?
Multiply the bar's weight-per-metre (metric: d² ÷ 162.28) or weight-per-foot (imperial: use the ASTM table) by its total length, then by the number of bars.
How much rebar does a concrete slab need?
Bars in each direction = floor((slab dimension − 2 × edge cover) ÷ spacing) + 1. Run the calculation in both directions and add the weight of each.
What's the difference between development length and lap length?
Development length is how far a single bar must extend into concrete to transfer its full design force. Lap length is how far two bars must overlap when splicing — it's usually a multiple of development length, not a separate calculation from scratch.
Should rebar be welded at joints?
Generally no. Standard reinforcing steel has a carbon content that makes it brittle under uncontrolled welding heat, which can leave a joint weaker than the bar itself. Tie wire is the standard method for holding a grid together at intersections.
Do I need rebar for a patio or walkway?
Light pedestrian slabs with no vehicle loading often skip reinforcement or use it only at control joints. Anything expecting vehicle weight, freeze-thaw cycling, or expansive soil should be reinforced — check local code before skipping it.
What bar size is standard for a residential slab?
#3 to #4 (10mm–12mm) is typical for residential slabs and driveways; heavier structural elements move up to #5–#8 (16mm–25mm) depending on load.
How is rebar cost estimated?
Total weight (or tonnage) × current price per kg or per ton. Steel pricing moves with the market, so treat any calculator's cost output as an estimate to check against a current supplier quote, not a locked-in number.
Can this calculator replace an engineer's design?
No. These tools estimate material quantities for ordering and budgeting. Structural design — bar size, spacing, and lap requirements for a specific load case — must come from engineered drawings and a qualified engineer.