Rebar for a Concrete Slab: Size, Spacing and How Much
Typical rebar sizes and spacing for 4 to 6 inch slabs, where it sits in the slab, and how many bars a patio or pad takes.
Rebar doesn’t stop concrete from cracking. Concrete shrinks as it cures and moves with temperature, so small cracks are normal. What rebar does is hold cracks tight, so the two sides can’t separate or shift up and down. For patios, walkways and pads it’s cheap insurance.
Typical size and spacing
General guidance from Everything About Concrete:
| Slab thickness | Bar size | Spacing (on center) |
|---|---|---|
| 4” | #3 or #4 | about 18–24” |
| 5” | #4 | about 16–20” |
| 6” | #4 or #5 | about 12–18” |
The bar number is its diameter in eighths of an inch: #3 is ⅜”, #4 is ½”. A grid at 18” in both directions is a common choice for a 4” patio slab. For driveways and anything structural, check local code; many areas set minimums.
Where the rebar goes
- In the slab, not under it. Rebar lying on the gravel does nothing. Support it on plastic or concrete “chairs” so it sits roughly in the middle of the slab’s thickness, or slightly above.
- Away from the edges. Keep bars at least 2” from the edges of slabs exposed to weather so moisture can’t reach them and rust them.
- Tied at crossings. Tie the grid with wire at intersections, or at least every other one, so it doesn’t move during the pour.
- Overlapped where bars join. Where two bars meet end to end, overlap them; the planner lets you set the lap length.
Don’t rely on “pulling up” mesh or rebar during the pour; it rarely ends up where it should. Chairs do the job properly.
How many bars do you need?
4” slab, bars in both directions, 3” in from each edge, cut from 20 ft bars:
| Slab | 12” spacing | 18” spacing | 24” spacing |
|---|---|---|---|
| 10 × 10 ft | 11 bars (209 ft) | 8 bars (152 ft) | 6 bars (114 ft) |
| 12 × 12 ft | 26 bars (299 ft) | 18 bars (207 ft) | 14 bars (161 ft) |
| 12 × 16 ft | 30 bars (397 ft) | 21 bars (278 ft) | 16 bars (212 ft) |
| 20 × 20 ft | 42 bars (819 ft) | 28 bars (546 ft) | 22 bars (429 ft) |
The first number is how many 20 ft bars to buy; the number in brackets is the total length of bar that ends up in the slab. Where two pieces can’t be cut from one 20 ft bar (on a 12 ft slab, for example), the offcuts are bigger, so you buy more bars per foot of slab. The planner works out the cuts for your exact size.
Rebar or wire mesh?
Welded wire mesh is quicker to lay and fine for many patios, but it’s floppy and hard to keep at the right height. Rebar on chairs is stiffer and stays where you put it, which is why many builders prefer it even for small slabs. Some ready-mix suppliers also offer fiber-reinforced concrete, which helps control small surface cracks; it’s often used together with rebar rather than instead of it.
Don’t forget control joints
Reinforcement and control joints work together. Cut or tool joints about a quarter of the slab’s depth so cracks form in straight lines where you want them; see How Thick Should a Concrete Slab Be? for spacing.
Choose rebar or mesh, spacing and lap length in the Concrete Slab & Footing Planner to get your exact bar count and a cutting list.