Rebar Spacing Calculator
Enter your slab size and on-center spacing to get the exact number of bars each way, total linear feet, 20-ft sticks to order, tie count and steel weight. Uses the fence-post formula the trade relies on.
Your slab
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To find the bars in one direction: take the slab dimension, subtract twice the edge cover, divide by the spacing, and add one. The “+1” is the part people forget — a grid always has one more bar than the gaps between them, exactly like fence posts. Do it both ways and add the two totals for a full mat. For a typical residential slab, #4 bar at 12–18 inches on center with 3 inches of cover is standard.
The fence-post formula
Walk through a 20 × 20 ft driveway with 3-inch cover and 12-inch spacing:
- Effective grid = 20 ft − (2 × 3 in) = 19.5 ft each way.
- Bars one way = floor(19.5 ft ÷ 1 ft) + 1 = 19 + 1 = 20 bars.
- Both ways = 20 + 20 = 40 bars total for the mat.
- Linear feet = 40 bars × 19.5 ft = 780 ft, before laps and waste.
Miss the “+1” and you’ll be one bar short in each direction — a small error that leaves an unreinforced strip at the slab edge.
Typical spacing and bar size
| Project | Bar size | Spacing (OC) | Cover |
|---|---|---|---|
| Patio, walkway | #3 | 18–24 in | 3 in |
| Slab on grade, shed pad | #4 | 18 in | 3 in |
| Driveway | #4 | 12–16 in | 3 in |
| Garage floor | #4 | 12 in | 3 in |
| Foundation, footing | #5 | 12 in | 3 in |
Closer spacing means more steel and better crack control, but bars set too close stop concrete flowing around them. These are common field defaults — your local code and the slab’s load always take precedence.
Placing the grid correctly
- Keep 3 inches back from every edgeSteel that reaches the edge has no concrete cover and will rust. The grid stops short so the pour buries it fully.
- Set it mid-slab, on chairsRebar lying on the gravel at the bottom of the pour does almost nothing. Chairs or dobies lift it to the lower-middle third where it carries tension.
- Tie every crossingWire ties stop the grid shifting when concrete is placed. Count roughly one tie per intersection — the calculator estimates this for you.
- Lap long runsAny run over a 20-ft stick needs a lapped overlap. Add it to your order with the lap splice calculator.
Frequently asked questions
How far apart should rebar be in a slab?
Residential slabs commonly use 18 inches on center; driveways and garage floors tighten to 12 inches for heavier loads. Your local code and slab thickness set the requirement.
What is the “+1” in the rebar formula?
A grid always has one more bar than the number of gaps between bars — like fence posts. Ten feet of fence at 5-ft spacing needs 3 posts, not 2. Skip the +1 and you’re a bar short each way.
How much edge cover should rebar have?
3 inches is the common field value for slabs on grade, so the steel stays fully buried and protected from rust. The grid is kept back that far from every edge.
What size rebar for a driveway?
#4 bar (½ inch) at 12–16 inches on center is the residential driveway standard. #3 suits patios; #5 is for foundations and footings.
How many rebar ties do I need?
Roughly one tie per grid intersection. A 20 × 20 slab at 12-inch spacing has about 400 crossings, so plan for around 400 ties plus a few spares.