Rebar Calculator
Slab dimensions and grid spacing in — total linear feet, bar count, sticks to buy by stock length, and weight out. Lap splice and cutting waste are applied on top, and every US size maps to its metric equivalent.
The method
Bars in one direction = ceil(slab width ÷ spacing) + 1, each spanning the full length; the other direction mirrors it. Total linear feet = bars × their span in both directions, then × (1 + lap) × (1 + waste).
Worked example: 30 × 20 ft slab, #4 at 16" o.c. → 16 bars each way → 640 lin ft → with 15% lap and 5% waste ≈ 773 ft ≈ 517 lb.
Rebar size table
| US size | Diameter (in) | Weight (lb/ft) | Metric | Weight (kg/m) |
|---|---|---|---|---|
| #3 | 0.375 | 0.376 | Ø10 | 0.62 |
| #4 | 0.500 | 0.668 | Ø13 | 0.99 |
| #5 | 0.625 | 1.043 | Ø16 | 1.56 |
| #6 | 0.750 | 1.502 | Ø19 | 2.24 |
| #8 | 1.000 | 2.670 | Ø25 | 3.98 |
| #11 | 1.410 | 5.313 | Ø36 | 7.92 |
Frequently asked questions
- What spacing does a patio slab need?
- Common residential practice is #4 at 18–24" on center in both directions on 4" of gravel; driveways often use #4 at 16–18". Spacing and size must come from your structural drawing — this tool only counts what a given grid uses.
- Why add lap allowance?
- Bars come in 20–60 ft sticks and must overlap (typically ~40 bar diameters) at splices. 15% covers typical residential laps; long slabs need more.
- How is rebar weight calculated?
- From published lb-per-foot values per bar size (a #4 weighs 0.668 lb/ft). Metric bars follow kg/m ≈ d²÷162 for diameter in mm.