Rebar Spacing for a Slab
Once you know the bar size and spacing your slab needs, the rebar calculatorturns your slab's dimensions into a bar count, total linear feet, and 20-ft stock bars to buy. This page is the step before that: typical spacing by slab type, so you have a starting point to check against your plans or local code before you run the numbers.
Typical bar size and spacing by slab type
These are general, code-typical starting points for residential slab-on-grade work, not a substitute for an engineer's plans or your local building department's requirements — always defer to stamped plans or the inspector's call where one applies.
| Slab type | Typical thickness | Typical bar size | Typical spacing |
|---|---|---|---|
| Patio | 4 in | #3 | 16–18 in |
| Walkway / shed floor | 4 in | #3 | 16–18 in |
| Standard driveway | 4–5 in | #3 or #4 | 12–18 in |
| Driveway carrying heavier vehicles | 5–6 in | #4 | 12 in |
| Garage floor | 4–5 in | #3 or #4 | 12–16 in |
| Footing (residential) | varies | #4 or #5 | 12 in |
ACI 318, Building Code Requirements for Structural Concrete — general residential slab-on-grade reinforcement practice, as summarized in CRSI (Concrete Reinforcing Steel Institute) placement guidance — lighter, lightly-loaded slabs (patios, walkways) tolerate wider spacing, while slabs carrying vehicle loads (driveways, garage floors) or acting as a structural footing use tighter spacing or a larger bar, or both. Thickness and load together decide which end of each range applies; where an engineer has already specified a spacing or bar size for your project, that specification governs regardless of what this table shows.
Bars per grid, by slab size
Bar count follows the same fence-post rule the calculator uses in both directions: ceil(dimension ÷ spacing) + 1. This table applies that rule to common slab footprints across the three spacing options above, so you can sanity-check a plan or a supplier's quote without opening the calculator first.
| Slab size | 12 in spacing | 16 in spacing | 18 in spacing |
|---|---|---|---|
| 10 × 10 ft (patio) | 22 bars | 18 bars | 16 bars |
| 12 × 12 ft (patio) | 26 bars | 20 bars | 18 bars |
| 10 × 20 ft (single driveway) | 32 bars | 25 bars | 23 bars |
| 16 × 20 ft (single-car garage) | 38 bars | 29 bars | 27 bars |
| 24 × 24 ft (two-car garage) | 50 bars | 38 bars | 34 bars |
| 20 × 30 ft (large driveway) | 52 bars | 40 bars | 36 bars |
Bar counts here are the two-way grid total (bars running each direction, added together) before the lap-splice allowance on linear footage. For example, the 16 × 20 ft garage floor at 12 in spacing needs 17 bars running the 16-ft way and 21 bars running the 20-ft way — 38 bars total — because each direction is counted independently against its own dimension.
Worked example: choosing spacing, then reading the grid
A homeowner is pouring a 16 × 20 ft single-car garage floor, 5 in thick, that will occasionally carry a loaded truck. That combination — vehicle load, mid-range thickness — sits toward the tighter end of the garage-floor row above: #4 bar at 12 in spacing rather than the looser 16 in option a lighter-duty floor might use. Reading the grid table at 16 × 20 ft and 12 in spacing gives 38 bars total (17 one way, 21 the other). From there, the rebar calculator converts that grid into linear feet, applies the standard 10% lap-splice allowance, and returns the number of 20-ft stock bars to order — the same slab dimensions and spacing, just carried one step further into a shopping list.
A lap splice note worth keeping in mind at this stage: any bar run longer than the 20-ft stock length needs two bars overlapped and tied together, not butted end to end. That overlap consumes extra material beyond the raw grid length, which is exactly what the calculator's 10% allowance is built to cover — it does not change the bar count above, only the total footage you need to buy.
What each spacing choice costs in steel
The bar counts above hide how quickly steel adds up. For the same 16 × 20 ft garage floor, here is the grid's total length and the 20-ft stock bars it takes once the 10% lap allowance is added:
| Spacing | Bars (each way) | Grid length | 20-ft bars with 10% lap allowance |
|---|---|---|---|
| 12 in | 17 + 21 | 676 ft | 38 |
| 16 in | 13 + 16 | 516 ft | 29 |
| 18 in | 12 + 15 | 480 ft | 27 |
Going from 18 in to 12 in spacing adds about 40% more steel to this slab, plus the extra ties and supports that go with it. Bar size is a separate multiplier on top: #4 bar weighs roughly 1.8 times as much per foot as #3. If you're choosing between a tighter spacing and a larger bar with no engineer's guidance, that is a question for the building department, not a place to save money by guessing.
Reading spacing on plans
Plans and permit drawings state reinforcement in a compact notation. "#4 @ 12" O.C. E.W." means #4 bar at 12 in on center, each way: the same spacing in both directions, which is the grid the tables on this page assume. "#4 @ 12" O.C. L.W., #3 @ 18" O.C. S.W." gives different bars for the long and short directions; run the calculator once per direction in that case. "T&B" means top and bottom, two separate mats, which doubles the steel. The bar number is the diameter in eighths of an inch, so #3 is 3/8 in and #4 is 1/2 in. A note such as "3 in clear" sets the concrete cover from the bars to the ground or form, which fixes where the grid starts at each edge.
Rebar and control joints
Rebar in a slab-on-grade doesn't stop concrete from cracking as it dries and shrinks. Its job is to hold cracks tight so the pieces stay level. The cracking itself is managed with control joints: grooves tooled or sawn into the surface so the slab cracks along a straight line. Concrete industry guidance commonly places them at intervals of roughly 24 to 36 times the slab thickness, so about 8 to 12 ft for a 4 in slab. That layout matters when planning rebar, because a garage floor or driveway divided into panels still gets one continuous grid underneath, and whether bars run through the joints is a detail your plans or inspector should settle. Neither closer spacing nor bigger bars replaces the joints.