What Size Rebar for a Slab?

Rebar sizing basics: what the numbers mean

Rebar is sized by the number of eighths of an inch in its diameter, so #3 rebar is 3/8 inch thick and #4 is 1/2 inch, with #5 (5/8 inch) reserved for heavier work like footings and foundation walls. That number is rolled right onto the bar at the mill, so it is the first thing to check against your plans before you order.

For ordinary flatwork, only three sizes come up in practice. #3 covers light-duty slabs where the steel mainly holds shrinkage cracks tight, #4 is the residential default for anything carrying real weight, and #5 shows up in footings and thick structural slabs rather than a typical patio or driveway pour.

Bar grade matters as much as diameter. Grade 60 rebar, meeting ASTM A615, is the standard for residential concrete in the US and yields at 60,000 psi, more than enough for slab-on-grade work where the steel is not carrying a structural load the way a beam or column would.

Rebar size and spacing by slab type and thickness

The right call depends on what sits on the slab, not just how thick it is. #3 rebar on an 18 to 24 inch grid handles patios, walkways, and shed floors, while #4 rebar on a 12 to 18 inch grid is the standard for driveways, garage floors, and any slab that sees vehicle weight.

These spacings follow the ACI 318 tables for structural concrete, even though a slab-on-grade resting on compacted soil is not technically a structural slab. Most contractors size to code anyway, since the extra steel costs little and buys real protection against cracking. Plug your own dimensions into the rebar calculator to get the exact bar count and total length for your grid.

Does a 4 inch slab need rebar, or is wire mesh enough?

A 4 inch slab does not always need rebar. On stable, well-compacted soil with only foot traffic and patio furniture, welded wire mesh controls shrinkage cracking just as well and costs less to install.

Rebar earns its keep once the slab carries real load or sits on unpredictable ground. A driveway, an uneven subgrade, or a climate with hard freeze-thaw cycles all call for rebar over mesh, because the steel bridges soft spots and resists bending under wheel loads in a way mesh cannot.

If you are pouring a driveway rather than a patio, the sizing shifts toward heavier bar and tighter spacing: see what size rebar for a driveway for spacing built around vehicle loads specifically.

Using both rebar and mesh in the same 4 inch slab is usually redundant and makes placement harder, not better. Pick one system, set it in the bottom third of the slab on chairs, and skip doubling up.

How much rebar for a 10x10 concrete slab

A 10x10 foot slab reinforced with #3 rebar on an 18 inch grid, kept 3 inches back from each edge, needs about 133 linear feet of rebar before waste. Add a 10% allowance for overlaps and cutting and that comes to roughly 145 to 150 feet, call it seven 20-foot sticks.

That works out to 7 bars running each direction, 14 bars total, each cut to about 9.5 feet. It is a small enough job that most suppliers will pre-cut it to length rather than making you trim full sticks on site.

For the layout math behind that count, and how it changes at tighter spacing, rebar spacing for a slab walks through the grid pattern in more detail.

How much rebar for a 30x40 slab

A 30x40 foot slab, a common footprint for a detached garage or small shop floor, covers 1,200 square feet. On a #3 grid at 18 inches on center, that comes to roughly 1,740 linear feet of rebar with a 10% overlap allowance, about 87 sticks of 20-foot bar.

Stepping up to #4 bar on a 24 inch grid, which many contractors prefer at this size since the slab is thicker and the spans are longer, cuts that down to around 1,300 feet, roughly 65 sticks, while still meeting the heavier-duty spacing shown in the table above.

Larger pours like this are also where ready-mix delivery and control joint layout start to matter as much as the rebar grid itself. For other project sizes between these two examples, how much rebar do I need breaks down the math for common slab dimensions.

Placement, cover, and grade: getting the install right

Rebar belongs in the bottom third of the slab, not sitting on the gravel base and not floating loose at mid-depth. Set it on plastic or wire bar chairs so it holds position when the concrete is poured and vibrated, never on bricks or scrap wood, which trap moisture and telegraph rust stains up through the finish.

Keep bars 2 to 3 inches back from every edge so concrete fully encases the steel; exposed rebar at an edge or a saw cut is a common source of rust staining and spalling years later. Overlap splices by at least 12 inches, roughly 40 times the bar diameter, for a proper lap.

Confirm you are actually getting Grade 60 bar meeting ASTM A615, the standard specification for concrete reinforcement in the US. Cheaper unmarked or mystery-grade rebar is a false economy on a pour meant to last decades.

What rebar costs by size in 2026

Rebar pricing tracks steel tariffs and mill costs, so it moves from year to year. As of 2026, #3 rebar runs about 0.40 to 0.65 dollars per linear foot, or 8 to 13 dollars for a 20-foot stick, while #4 runs about 0.55 to 0.85 dollars per foot, roughly 10 to 14 dollars a stick.

Buying full 20 or 40-foot lengths and cutting on site keeps cost down, since suppliers often charge 1.50 to 3 dollars per cut for pre-trimmed pieces. For a typical 10x10 patio slab, that puts the rebar itself in the 60 to 100 dollar range before chairs, tie wire, and delivery.

Frequently asked questions

References

• Rebar (Wikipedia)
• ASTM A615/A615M Standard Specification for Concrete Reinforcement
• ACI 318: Building Code Requirements for Structural Concrete
• CRSI: Concrete Reinforcing Steel Institute

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