Rebar Calculator
Calculate reinforcing bar length, count and weight for a slab grid.
Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.
What the Rebar Calculator does
Rebar quantity comes from the grid spacing across each direction of the slab, reduced by the concrete cover at the edges. Bar weight follows the standard d²/162 formula, which converts diameter in millimetres directly into kilograms per metre.
Formula
Effective span = Slab dimension − 2 × CoverBar count = ⌊Perpendicular span ÷ Spacing⌋ + 1Total length = Σ (Bar count × Bar length) × Layers × (1 + Lap %)Weight = Total length × Diameter² ÷ 162
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Slab length | number | Yes | Accepts more than 0. |
| Slab length unit | one of 6 options | Yes | — |
| Slab width | number | Yes | Accepts more than 0. |
| Slab width unit | one of 6 options | Yes | — |
| Bar spacing lengthwise | mm | Yes | Accepts 10 or more. |
| Bar spacing widthwise | mm | Yes | Accepts 10 or more. |
| Bar diameter | one of 6 options | Yes | — |
| Concrete cover each side | number | Yes | Accepts more than 0. |
| Concrete cover each side unit | one of 5 options | Yes | — |
| Number of layers | number | Yes | Accepts 1 or more. |
| Lap and waste allowance | % | Yes | — |
| Price per kg | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Slab length unit and Slab width unit.
- Enter Slab length, Slab width, Bar spacing lengthwise and Bar spacing widthwise and 3 more.
- Optionally add Price per kg.
- Select Calculate.
Worked example
A 6 m × 4 m slab, 200 mm spacing both ways, 12 mm bars, 50 mm cover, one layer, 10% laps.
- Length
- 6 m
- Width
- 4 m
- Spacing
- 200 mm
- Bar
- 12 mm
- Cover
- 50 mm
Effective 5.9 × 3.9 m → 20 bars × 5.9 m + 30 bars × 3.9 m = 235 m, +10% = 258.5 m → 229.8 kg.
Frequently asked questions
What does the d²/162 formula mean?
It gives the mass in kg per metre of a steel bar from its diameter in millimetres. A 12 mm bar is 144/162 = 0.888 kg/m.
Why deduct the cover?
Reinforcement must sit inside the concrete with a protective cover to prevent corrosion, so bars are shorter than the slab.