How this calculator works
It shrinks the slab by the edge clearance on every side, then lays bars both ways at your spacing, always adding one bar so the spacing is never wider than you asked for. Each run is cut from stock bars: two short runs come out of one bar where they fit, and runs longer than a bar are lapped 40 bar diameters.
Bar weights are nominal standard figures (#4 is 0.668 lb per foot; 12 mm is 0.888 kg per metre). Ties are one per crossing and chairs about one per 9 sq ft, so the grid sits in the middle of the slab instead of on the ground.
bars each way = (span − 2 × clearance) ÷ spacing, rounded up, + 1
stock bars = runs ÷ runs per bar (or with 40 × diameter laps when a run is longer than a bar)
ties = bars one way × bars the other wayWorked example
A 20′ × 10′ slab with 3″ clearance leaves a 19′ 6″ × 9′ 6″ grid. At 18″ spacing that is 8 bars running the length and 14 running the width: 289 ft of #4 bar, about 193 lb. The 19′ 6″ runs take one 20′ bar each (8 bars) and the 9′ 6″ runs come two to a bar (7 bars), so the list buys 15 bars, 112 ties and 21 chairs.
Tips before you buy
- Check the bar size and spacing on your plans or with your building department. Driveways and footings often have specific requirements.
- Rebar only works inside the concrete. Use chairs or bricks so the grid sits near mid-depth of the slab, and walk on it carefully during the pour.
- Wear gloves and cap the exposed ends with safety caps while you work around the bars.
- Bars are heavy and long. A 20′ bar may need a truck or delivery; ask whether the store will cut them to length.
Questions
- What size rebar should I use in a slab?
- #3 or #4 bar is typical for residential slabs, patios and driveways, often at 12 to 18 inches on center. Heavier loads call for larger bars or closer spacing. Local codes and your plans have the final say.
- How much should rebar overlap?
- A common rule is a lap of 40 bar diameters, which is 15 inches for #3 and 20 inches for #4. Structural drawings may call for more. Tie the overlapping bars together in at least two places.
- Do I need rebar or is wire mesh enough?
- Welded wire mesh controls small shrinkage cracks in thin slabs such as walkways. Rebar adds more strength and is common in driveways and slabs that carry heavier loads. Both must sit inside the slab, not on the ground, to do any good.