Rebar Calculator (Slab Grid)
Enter the slab size, bar spacing and edge cover.
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Rebar Calculator (Slab Grid) by TheCalcTools
How the Rebar Calculator (Slab Grid) works
A grid has bars in both directions. The number running the length is the width (minus cover at both edges) ÷ spacing + 1, each as long as the slab minus cover; the same the other way.
Weights per foot are the ASTM A615 nominal values. Add lap splices where bars join (often 40 bar diameters) and follow your engineer’s design.
The formula
Worked example
Using the default values shown in the calculator:
| Slab length (ft) | 20 |
|---|---|
| Slab width (ft) | 12 |
| Bar spacing (in) | 12 |
| Edge cover (in) | 3 |
| Bar size | #4 (1/2 in): 0.668 lb/ft |
| Stock bar length (ft) | 20 |
- A 20 × 12 ft slab at 12 in spacing with 3 in cover: 12 + 20 bars, 464 ft in total.
- #4 bar weighs about 310 lb.
Result:
| Total rebar length (ft) | 464 |
|---|---|
| Bars running the length | 12 |
| Bars running the width | 20 |
| Stock bars to buy (no laps) | 24 |
| Weight (lb) | 310 |
Frequently asked questions
Does a patio slab need rebar?
Many residential slabs use wire mesh or fibre instead; driveways and structural slabs typically use rebar. Check local requirements.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.