How to use it
- Enter the slab size.
- Set the grid spacing and the edge clearance.
- Pick the stock length and bar size.
- Set the lap length used where bars are spliced.
The formula
Bars across one direction = floor((dimension − 2 × edge) ÷ spacing) + 1
Total length = bars × run length + lap splices
Stock bars = total length ÷ stock length (rounded up)
Worked example
| Slab length | 20 ft |
| Slab width | 12 ft |
| Bar spacing (on center) | 12 in |
| Edge clearance | 3 in |
| Stock bar length | 20 ft |
| Bar size | #4 (1/2 in) — 0.668 lb/ft |
| Lap splice length | 18 in |
Result
| 20 ft bars to buy | 24 bars |
| Total rebar length | 464 ft |
| Bars running the length | 12 |
| Bars running the width | 20 |
| Approx. weight | 310 lb |
Tips before you order
- Support bars on chairs or dobies so they sit in the middle third of the slab.
- Many specs call for a lap of 40 bar diameters — 20 in for #4 bar — but follow your engineer or local code.
Questions
What rebar spacing for a 4 inch slab?
Residential slabs often use #3 or #4 bar at 12–18 in on center, but slabs on grade don't always require rebar — check your plans.