How Much Rebar Do I Need? Sizing, Spacing & a Worked Slab Example

Q: How much rebar do I need for a 10x10 slab?

A 10x10 ft slab with #4 rebar on an 18-inch grid needs 16 ten-foot bars, or 160 linear feet total — about 107 lb. That is eight 20-ft sticks cut in half. Add chairs to hold the grid and plan lap splices of about 15 inches wherever bars join.

Q: What size rebar do I need for a concrete slab?

#3 (3/8 in) or #4 (1/2 in) bar covers most residential slabs on grade — patios, walkways, shed floors. #4 is the common default. Anything structural (footings, retaining walls, suspended slabs) should be sized by an engineer, not a rule of thumb.

Q: How far apart should rebar be spaced in a slab?

12 to 24 inches on center is the typical range for slabs on grade; 18 inches on center is the common residential default. Closer spacing (12 in) suits heavier loads and thinner slabs; 24 in is the loose end for light-duty work.

Q: How much does rebar weigh per foot?

#3 bar weighs 0.376 lb/ft, #4 weighs 0.668 lb/ft, and #5 weighs 1.043 lb/ft. Multiply by your total linear feet to get the total weight — useful when hauling it yourself or checking delivery limits.

Q: Can I use wire mesh instead of rebar?

For light-duty slabs (walkways, small patios) 6x6 welded wire mesh is a common and cheaper alternative, but it must be chaired up into the middle of the slab — mesh left lying on the ground does nothing. Rebar gives a stronger, more forgiving grid for driveways and anything carrying real load.

Q: How much concrete cover does rebar need?

Keep at least 3 inches of concrete between rebar and soil (concrete cast against earth), which is why bars are cut a few inches short of the slab edges. Rebar too close to the surface will rust, expand, and crack the concrete from the inside.

Planning estimate, not engineering advice. Concrete is strong in compression and weak in tension — rebar (reinforcing steel bar) supplies the tensile strength that keeps slabs from cracking apart under load and soil movement. This guide covers which bar size to buy, how to space the grid, and a fully worked 10×10 ft example with real numbers. Pair it with our concrete calculator for the yardage and our slab cost estimator for the budget.

Rebar sizes: what the numbers mean

Rebar is sold by number, where the number is the bar's diameter in eighths of an inch. #4 bar is 4/8" = 1/2" in diameter. The three sizes you'll see at any home center:

#3 — 3/8" diameter, 0.376 lb/ft. Light-duty: small walkways, thin slabs, tying other bars.
#4 — 1/2" diameter, 0.668 lb/ft. The residential default for slabs on grade: patios, driveways, shed floors.
#5 — 5/8" diameter, 1.043 lb/ft. Heavier slabs, footings, and anything an engineer specs.

For a typical backyard slab, #4 is the safe default. The weight per foot matters when you're hauling it: a 20-ft #4 stick weighs about 13.4 lb — manageable alone, but 160 linear feet totals over 100 lb, so plan the trip.

Spacing rules

Rebar goes in as a grid — bars running both directions, tied at intersections with wire ties. The standard guidance for slabs on grade:

12 to 24 inches on center is the normal range ("on center" means measured from the center of one bar to the center of the next).
18 inches on center is the common residential default for 4-inch slabs.
Go to 12 inches for heavier loads (driveways, equipment pads) or thinner slabs.
24 inches is the loose end — fine for a garden walkway, not for anything structural.

The grid sits in the middle to lower-middle of the slab (roughly the bottom third for slabs on grade), held up on chairs or dobies — small supports that keep the steel off the ground while you pour. Rebar lying on the dirt does nothing; it must be embedded in concrete on all sides.

The 3-inch cover rule

Keep at least 3 inches of concrete between rebar and soil for concrete cast against earth. That means every bar gets cut a few inches short of the slab edge — a 10-ft-wide slab gets bars about 9 ft 6 in long. Steel too close to the surface draws moisture, rusts, expands, and cracks the concrete from the inside (called spalling). The cover rule is the cheapest insurance in the whole project.

Worked example: 10×10 ft slab, #4 at 18" on center

Here's the full estimate, step by step:

Bars per direction. 10 ft = 120 inches. 120 ÷ 18 = 6.67 spaces, so round up to 7 spaces = 8 bars per direction (one at each edge).
Total bars. 8 bars × 2 directions = 16 bars, each 10 ft nominal.
Total linear feet. 16 × 10 = 160 linear feet (order quantity; the 3-inch cover trim comes out of this).
Total weight. 160 ft × 0.668 lb/ft = ~107 lb of #4 bar.
Sticks to buy. Rebar comes in 20-ft sticks: 160 ÷ 20 = 8 sticks, each cut in half to make your 16 ten-foot bars.
Lap splices. If you ever need to join two bars end to end, overlap them — the rule of thumb is about 30 bar diameters, which is 15 inches for #4 bar. Our example needs no splices (each bar is one piece), but longer slabs will.
Rough cost. A 10-ft #4 stick runs roughly $5–$9 at home centers (2026 pricing — confirm locally). Sixteen sticks puts the steel at roughly $80–$150, plus a bag of wire ties and a pack of chairs. This is a planning estimate, not a quote.

General formula for any rectangular slab: bars per direction = (width in inches ÷ spacing) rounded up, + 1. Total feet = bars × 2 directions × slab length. Add 5–10% for cutting waste and splices.

Rebar vs. wire mesh

For light-duty slabs — walkways, small patios — 6×6 welded wire mesh is a common cheaper alternative. One honest warning: mesh only works if it's chaired up into the slab. Mesh unrolled flat on the ground and poured over ends up at the very bottom, doing nothing; this is the most common reinforcement failure in DIY slabs. Rebar on chairs is more forgiving and plainly stronger — for driveways or anything carrying real load, use the bar.

When to call an engineer

This guide covers slabs on grade — flatwork sitting on prepared soil. Anything else needs professional sizing: footings, foundation walls, retaining walls, suspended or structural slabs, and slabs carrying heavy point loads. Rebar sizing for structural work follows ACI 318 and local building code; a rule of thumb is not a substitute. When in doubt, the engineer's fee is the cheapest part of the project.

Frequently asked questions

How much rebar do I need for a 10x10 slab?

A 10×10 ft slab with #4 rebar on an 18-inch grid needs 16 ten-foot bars — 160 linear feet total, about 107 lb. That's eight 20-ft sticks cut in half. Add chairs to hold the grid and plan lap splices of about 15 inches wherever bars join.

What size rebar do I need for a concrete slab?

#3 (3/8 in) or #4 (1/2 in) covers most residential slabs on grade — patios, walkways, shed floors — with #4 the common default. Anything structural (footings, retaining walls, suspended slabs) should be sized by an engineer.

How far apart should rebar be spaced in a slab?

12 to 24 inches on center is typical for slabs on grade; 18 inches is the common residential default. Use 12-inch spacing for heavier loads and thinner slabs.

How much does rebar weigh per foot?

#3 bar is 0.376 lb/ft, #4 is 0.668 lb/ft, and #5 is 1.043 lb/ft. Multiply by total linear feet for the haul weight — our 10×10 example comes to about 107 lb.

Can I use wire mesh instead of rebar?

For light-duty slabs, 6×6 welded wire mesh works — but only if chaired up into the middle of the slab. Mesh left on the ground does nothing. Rebar is stronger and more forgiving for driveways and loaded slabs.

How much concrete cover does rebar need?

At least 3 inches of concrete between rebar and soil for concrete cast against earth — cut bars a few inches short of the slab edges. Steel near the surface rusts, expands, and cracks the concrete.

Price out the whole slab

Now estimate the concrete to go with the steel: how many yards and bags you'll need, then what the full slab will cost — materials plus an all-in range.