How far apart the bars go — the standard mat, the code floor and ceiling, and what each spacing actually buys
← Back to the calculatorHeavy loads (RV, boat, equipment) tighten it to #4 at 12 in in a 6 in slab. The code fixes 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); between those two is the drawing’s call.
This page is about choosing the spacing for a driveway. 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 driveway 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 an 18 in driveway 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 |
Swipe the table sideways for more columns →
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 pour 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 driveway 3.6 in and thicker. A driveway is almost always 4 in to 6 in, which is exactly the range where 18 in is the ceiling.
| Slab thickness h | 5h (in) | Ceiling = lesser of 5h and 18 in |
|---|---|---|
| 2 in topping | 10 in | 10 in |
| 4 in light-duty driveway | 20 in | 18 in |
| 5 in car / truck driveway | 25 in | 18 in |
| 6 in RV / equipment driveway | 30 in | 18 in |
| 8 in heavy pad | 40 in | 18 in |
Swipe the table sideways for more columns →
A driveway slab on grade carries its loads by thickness and by the base under it. The bar does not make the slab stronger in bending — a 4 in slab with #4 at 12 in still cracks under the same wheel load as a 4 in slab with #4 at 18 in. What the steel does is hold the slab together across the cracks that shrinkage, curling and load open anyway: it keeps the two faces of a crack in line so the crack stays a hairline instead of stepping into a fault, and it keeps the panels tied so the slab behaves as one piece.
That is why the code treats a driveway mat as shrinkage and temperature reinforcement, not as structural steel. ACI 318-19 24.4.3.2 asks for a minimum ratio of steel area to gross concrete area of 0.0018 for Grade 60 deformed bar in a slab, and 24.4.3.3 caps the spacing at the lesser of 5h and 18 in. Both are floors, not designs: an engineered driveway can ask for more, never for less.
Read 0.0018 across the thicknesses a driveway actually comes in and it turns into square inches per foot of width, which is what a mat provides:
| Slab thickness | Required As (in²/ft) | #3 @ 12 in 0.110 | #4 @ 24 in 0.100 | #4 @ 18 in 0.133 | #5 @ 18 in 0.207 |
|---|---|---|---|---|---|
| 4 in light duty | 0.086 | pass | pass | pass | pass |
| 5 in car / truck | 0.108 | pass | short 7% | pass | pass |
| 6 in RV / equipment | 0.130 | short 15% | short 23% | pass, by 3% | pass |
| 8 in heavy pad | 0.173 | short 36% | short 42% | short 23% | pass |
Swipe the table sideways for more columns →
That table is the whole argument for the 18 in default. #4 at 18 in clears every thickness up to 6 in and fails at 8 in — at 8 in it is 23% light and the mat has to step up to #5. #3 at 12 in is the light-duty mat and stops working at 6 in. And the loosest common mat, #4 at 24 in, is short at 5 in and further short above it — which is why 24 in only appears with an engineer’s stamp behind it. None of this is a strength calculation; it is a minimum-steel check that a mat either passes or does not.
Three things follow for a pour, and they matter more than the spacing number:
The rest of this page is the spacing itself: the two code bounds that bracket it, what each spacing buys in steel, and a worked driveway at three spacings. If you want the bar count, the cut list and the order for the driveway you are about to pour, that is the how-much-rebar page.
In practice the spacing comes from what the driveway carries, not from a calculation you run on site. These are the combinations that show up on residential driveway drawings — ranges, not determinations.
| Driveway use | What it carries | Usual mat | Spacing, both ways | Thickness | Cover to mat |
|---|---|---|---|---|---|
| Light duty (bikes, foot traffic) | Crack control, no vehicles | #3 bar, or 6×6 W1.4 mesh | #3 at 12 in; mesh is 6 in by definition | 4 in | 2 in |
| Standard car driveway | Cars, light trucks | #4 bar | 18 in | 4–5 in | 2–3 in |
| Two-car / SUV / van | Heavier axle loads | #4 bar | 12–18 in | 5 in | 2–3 in |
| RV, boat, equipment pad | Heavy point loads | #4 bar (sometimes #5) | 12 in | 6 in | 3 in |
| On expansive or frost-prone ground | Extra movement, no designed flexural steel | Engineer sets bar and spacing | Per the drawing | Per design | Per drawing |
Swipe the table sideways for more columns →
Three things in that table matter more than the exact number. Two directions, always. A driveway 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 light-duty driveway to a standard one is #3 at 12 in to #4 at 18 in, and to a heavy one is #4 at 12 in — not a new bar size entirely.
The driveway row follows the same rule the spacing calculator works through: on a 20 ft × 40 ft driveway, #4 at 18 in is 14 + 27 bars, 1,100 ft and 734.8 lb; #4 at 12 in jumps to 20 + 40 bars, 1,600 ft and 1,068.8 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 on a driveway.
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 a driveway because bar size is what you can change on an order, while spacing is what the drawing fixes — but when the drawing says 18 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 18 in to 24 in drops the steel by a quarter 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 driveway most people actually pour — 20 ft × 40 ft, 5 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 + 40 | 1,600 ft | 1,068.8 lb | 0.200 in²/ft | 1,336 lb |
| 18 in | 14 + 27 | 1,100 ft | 734.8 lb | 0.133 in²/ft | 919 lb |
| 24 in | 10 + 20 | 800 ft | 534.4 lb | 0.100 in²/ft | 668 lb |
Swipe the table sideways for more columns →
Two things the table shows that a spacing alone does not. The 18 in driveway counts 41 bars total against 60 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 11 in short of the far edge, at 24 in they end 23 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 919 lb per 1,000 sq ft on this driveway, 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. The effect fades as the pad grows.
For the full order — stock lengths, laps, chairs and tie wire — run the same driveway through the how-much-rebar page, and check the takeoff weight against the rebar weight chart before you place the order.
Spacing is one dial on a driveway, and reaching for it alone is how driveways crack anyway.
For a standard car driveway the usual mat is #4 bar at 18 in on centre in both directions, in a 4 in to 5 in slab with the mat in the upper third and 2 in to 3 in of cover. Heavy loads move it tighter: an RV, boat or equipment pad goes to #4 at 12 in in a 6 in slab. Light-duty, no-vehicle drives step down to #3 at 12 in or welded wire mesh. The drawing or the local code governs; check your drawing against the 18 in default rather than the other way round.
18 in on centre both ways is the most common answer for a #4 mat in a 4 in to 5 in driveway; 12 in is the heavy-duty answer. 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 driveway 3.6 in and thicker is 18 in.
#4 bar (1/2 in, designation #13) is the standard for car and light-truck driveways; #3 for light duty with no vehicles; #5 where very heavy point loads sit on the slab. Bar size and spacing are one decision, because steel per direction is the bar area divided by the spacing: #3 at 12 in and #4 at 24 in both put about 0.11 in²/ft in the slab, so one bar size up buys back roughly one spacing step.
4 in for light duty, 5 in for cars and pickups, 6 in for RVs, boats and equipment. Thickness carries the load and the mat controls the cracks, so a 6 in slab at 18 in out-performs a 4 in slab at 6 in. The bar does not buy depth: going tighter than 12 in on a thin slab buys crack width, not strength.
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 driveway 3.6 in and up that is 18 in. A 24 in spacing appears only where an engineer designs the mat structurally and the spacing follows from the loads and bar sizes on the drawing. On a residential driveway, treat 24 in as a designed number that needs a drawing behind it.
Yes. A slab on grade 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. One-way mats belong to specific designed members, not a driveway. The grid is what the spacing calculator and the takeoff both assume.
For #4 bar on a 20 ft × 40 ft driveway, 12 in on centre is 20 + 40 bars, 1,600 ft and 1,068.8 lb; 18 in is 14 + 27 bars, 1,100 ft and 734.8 lb. That is 1,336 lb of steel per 1,000 square feet at 12 in against 919 lb at 18 in. Loosening 12 in to 18 in cuts the steel by about a third, because steel goes as 1 divided by spacing.
On a light-duty, no-vehicle driveway that needs crack control only, often yes: a 6×6-W1.4×W1.4 sheet carries about 0.028 square inches of steel per foot in each direction, roughly a seventh of #4 at 12 in. Mesh orders as sheets or rolls with edge laps and no offcut. A driveway that sees vehicles, or any drawing that specifies deformed bar, usually stays on bar.
A driveway slab on grade does not need bar to carry its loads — the thickness and the base do that. It needs it for crack control: ACI 318-19 24.4.3.2 asks for a minimum ratio of 0.0018 of the gross concrete area as shrinkage and temperature steel, which is 0.086 in² per foot in a 4 in slab and 0.130 in a 6 in slab. #4 at 18 in provides 0.133 and clears every thickness up to 6 in; #4 at 24 in provides 0.100 and is short from 5 in upward. On a light-duty, no-vehicle drive, welded wire mesh is the usual substitute; on anything that takes vehicles or has a drawing behind it, stay on bar.
In the upper third, about 2 in to 3 in below the finished surface, held on chairs. A slab cracks at the top surface, so steel near the bottom does nothing for crack width: a #4 resting on the subgrade leaves about 4.5 in of unreinforced concrete above it. The code floor is 1½ in for #5 and smaller exposed to weather (ACI 318-19 Table 20.5.1.3.1), so 2 in to 3 in is practice chosen to keep the mat high without the bars showing through at the surface.
It does not set the spacing for your driveway. 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 driveway-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.