How far apart slab bars go — the defaults, the code floor and ceiling, and what each spacing actually buys
← Back to the calculatorThe code sets the floor (a clear gap at least the bar diameter, never under 1 in) and the ceiling for a crack-control mat (the lesser of 5h and 18 in); everything between those two is the drawing’s call.
This page is about choosing the spacing. If you already have the spacing and need the bar count, the cut list and the order, that is the rebar spacing calculator. Here: what the typical numbers are and why, the two code bounds that actually bite, and what loosening or tightening a mat really does to the steel.
Two numbers in ACI 318-19 bracket the answer, and they are the only spacing numbers that are not the engineer’s choice. Between them, a residential slab can sit anywhere.
The floor — ACI 318-19 25.2.1. The clear gap between parallel bars in a layer must be at least the bar diameter db, and never less than 1 in. That is a construction rule as much as a design one: concrete has to pour and consolidate between the bars. At a 12 in mat it never binds — a #4 bar at 0.500 in is nowhere near it — but it is what decides layouts when big bars crowd together.
| Bar | db (in) | Floor = max(db, 1 in) |
|---|---|---|
| #3 | 0.375 | 1.00 in |
| #4 | 0.500 | 1.00 in |
| #5 | 0.625 | 1.00 in |
| #6 | 0.750 | 1.00 in |
| #8 | 1.000 | 1.00 in |
| #9 | 1.128 | 1.13 in |
| #10 | 1.270 | 1.27 in |
| #11 | 1.410 | 1.41 in |
| #14 | 1.693 | 1.69 in |
| #18 | 2.257 | 2.26 in |
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The ceiling — ACI 318-19 24.4. Where a slab carries no designed flexural steel and the mat is there for shrinkage and temperature, the code caps the spacing at the lesser of 5h and 18 in. Five times the thickness is the big number on any slab you would build by hand: a 4 in slab allows 20 in, a 6 in slab allows 30 in — so the 18 in cap is what binds for every slab 3.6 in and thicker.
| Slab thickness h | 5h (in) | Ceiling = lesser of 5h and 18 in |
|---|---|---|
| 2 in topping | 10 in | 10 in |
| 4 in patio / walkway | 20 in | 18 in |
| 5 in garage floor | 25 in | 18 in |
| 6 in driveway / RV pad | 30 in | 18 in |
| 8 in heavy pad | 40 in | 18 in |
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In practice the spacing comes from the slab’s job, not from a calculation you run on site. These are the combinations that show up on residential drawings — ranges, not determinations.
| Slab | What it carries | Usual mat | Spacing, both ways | Thickness | Cover to mat |
|---|---|---|---|---|---|
| Patio, walkway, shed floor | Crack control only, foot traffic | #3 bar, or 6×6 W1.4 mesh | 18 in bar; mesh is 6 in by definition | 4 in | 2 in |
| Garage floor, basement slab | Foot traffic, a parked car | #4 bar | 12–18 in | 5 in | 2 in |
| Driveway | Vehicles, snow load | #4 bar (or #3 for light duty) | #3 at 12 in, or #4 at 18 in; a heavier driveway goes to #4 at 12 in | 4–6 in | 2–3 in |
| RV or equipment pad | Heavy point loads | #4 bar | 12 in | 6 in | 3 in |
| Suspended or structural slab | Design loads, spans | Engineer sets bar and spacing | Per the drawing | Per design | Per drawing |
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Three things in that table matter more than the exact number. Two directions, always. A slab cracks in both directions, so the mat is a grid — bars running the length and bars running the width, at the same spacing unless the drawing says otherwise. The mat sits high. Shrinkage cracks start at the top surface, so the mat goes in the upper third with about 2–3 in of cover — held there by chairs, because steel lying on the subgrade does nothing. Heavier use tightens the spacing before it adds bar. The step from a patio to a driveway is 18 in of #3 to 12–18 in of #4, not a new bar size entirely.
The driveway row follows the same rule the spacing calculator works through: on a 24 ft × 24 ft driveway, #4 at 12 in is 24 bars each way, 1,152 ft and 769.5 lb; #4 at 18 in drops to 16 bars each way, 768 ft and 513.0 lb. The spacing does most of the work.
The steel actually in the slab is set by the pair, not by either number alone: steel per direction in square inches per foot is the bar’s area divided by the spacing in feet. That is why “what spacing?” and “what bar?” are one question.
Read the near-equivalences that fall out: #3 at 12 in and #4 at 24 in both put about 0.11 in²/ft in the slab; #4 at 12 in and #5 at 18 in both put about 0.20. One bar size up buys back roughly one spacing step. That trade matters on site because bar size is what you can change on an order, while spacing is what the drawing fixes — but when the drawing says 12 in and you are quoting the swap, the steel does not lie.
Spacing is still the bigger single lever. Steel goes as 1 ÷ spacing, so loosening 12 in to 18 in cuts the steel by a third and 12 in to 24 in halves it, whatever bar you started on. Halving the spacing doubles it — the most expensive change you can make to a mat, which is why it is the engineer’s call and not a field decision. The weight calculator turns any of these combinations into pounds.
Take the patio most people actually build — 20 ft × 24 ft, 4 in thick, #4 bar both ways, first bar centre 1 in in from the edge — and run it at three spacings. The bars-per-way count comes from the layout rule the spacing calculator uses: whole bays across the clear distance, plus one bar.
| Spacing | Bars each way | Total length | Weight (#4) | Steel per direction | lb of #4 per 1,000 sq ft |
|---|---|---|---|---|---|
| 12 in | 20 + 24 | 960 ft | 641.3 lb | 0.200 in²/ft | 1,336 lb |
| 18 in | 14 + 16 | 656 ft | 438.2 lb | 0.133 in²/ft | 913 lb |
| 24 in | 10 + 12 | 480 ft | 320.6 lb | 0.100 in²/ft | 668 lb |
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Two things the table shows that a spacing alone does not. The 18 in patio counts 30 bars total against 44 at 12 in — a third fewer pieces to lift, tie and cut — because the count goes as 1 ÷ spacing, exactly like the weight. And the last bay drifts: at 12 in the rows end 10 in short of the far edge, at 24 in they end 22 in short. Neither is wrong; both belong on the layout so the crew knows whether the row is being centred or run off one reference edge.
One quiet detail in the last column: 18 in comes to 913 lb per 1,000 sq ft on this patio, above the 891 lb the 1 ÷ spacing rate gives. The count rounds up to whole bars, so a loose mat on a small pad carries slightly more than the rate says — the sparser the layout, the heavier the last bay. On a 24 ft × 20 ft slab the site’s spacing page shows the same 913 lb at 18 in; the effect fades as the pad grows.
For the full order — stock lengths, laps, chairs and tie wire — run the same patio through the how-much-rebar page, and check the takeoff weight against the rebar weight chart before you place the order.
A 16-second run through the whole page: the 12 in default, the 1 in floor, the 18 in ceiling, and the three-spacing table on the 20 ft × 24 ft patio. Same numbers as above.
Spacing is one dial on a slab, and reaching for it alone is how slabs crack anyway.
The residential default is #4 bar at 12 in on centre in both directions, with the mat in the upper third of the slab and 2–3 in of cover over it. A lighter crack-control mat steps down to #3 at 18 in, or to welded wire mesh on a non-structural slab. The drawing or the local code governs; check your drawing against the 12 in default rather than the other way round.
12 in on centre both ways is the most common answer for a #4 mat in a 4–6 in slab. Lighter mats go to 18 in; 24 in appears on the loosest designed mats. The two bounds are ACI 318-19 25.2.1 on the floor — clear gap at least the bar diameter, never under 1 in — and ACI 318-19 24.4 on the ceiling — a shrinkage and temperature mat spaced no wider than the lesser of 5h and 18 in, which for any slab 3.6 in and thicker is 18 in.
ACI 318-19 25.2.1 sets it at the bar diameter db, never less than 1 in. For #4 that is 0.5 in of bar against the 1 in floor, so up to #8 the floor is what binds; above #8 the bar takes over — #10 needs 1.27 in, #11 needs 1.41 in, #14 needs 1.69 in, #18 needs 2.26 in. At a 12 in mat none of it matters; it matters on walls and columns where large bars crowd.
Not for a crack-control mat under ACI 318-19 24.4 — the ceiling there is the lesser of 5h and 18 in, and for a slab 3.6 in and up that is 18 in. A 24 in spacing appears where an engineer designs the mat structurally and the spacing follows from the loads and bar sizes on the drawing. On a residential job, treat 24 in as a designed number that needs a drawing behind it.
Neither alone — the steel is the pair. Steel per direction equals bar area ÷ spacing, so #3 at 12 in and #4 at 24 in both give about 0.11 in²/ft, and #4 at 12 in and #5 at 18 in both give about 0.20. Spacing is still the more aggressive lever: 12 in to 18 in cuts the steel a third, 12 in to 24 in halves it.
For #4 bar, 0.200 in² per foot in each direction at 12 in against 0.133 at 18 in — the same bar spread over a longer interval. That is 1,336 lb per 1,000 sq ft at 12 in against 891 lb at 18 in; on the 20 ft × 24 ft patio above, 641.3 lb against 438.2 lb. Chairs, wire and ties track the same step, because they count bars and intersections, not area.
Often yes on a non-structural patio, walkway or shed floor, where the job is crack control. A 6×6-W1.4×W1.4 sheet puts about 0.028 in²/ft per direction — a seventh of #4 at 12 in — and it orders as sheets or rolls with edge laps, no stock lengths and no offcut. Structural slabs and driveways usually stay on deformed bar because the drawings say so.
In the upper third, with about 2–3 in of cover from the top of the slab to the bar — the mat has to sit near the surface where shrinkage cracks start. Chairs hold it there, and a chair’s height is the cover plus half a bar diameter. Steel laid on the subgrade does nothing, whatever the spacing is.
It does not set the spacing for your slab. The structural drawing or the local code does, and on anything structural neither a web page nor a rule of thumb is a substitute for the engineer. It covers the residential slab-on-grade case — a crack-control mat — not designed suspended slabs, post-tensioned slabs, or mats on expansive or problem soils. All weights are nominal, from the bar designation and standard steel density; coated bar weighs more and mill tolerance moves real bars a fraction of a percent. And it does not decide anything about your pour — it tells you what the numbers on the drawing mean.