For a standard 4-inch residential concrete slab, you’ll typically need #3 rebar spaced in a grid pattern at 18 to 24 inches on center. The exact amount depends on your slab dimensions, the spacing you choose, and whether you have thickened edges or footings. A 10-by-10-foot slab with 18-inch spacing, for example, requires roughly 150 linear feet of rebar.
Choosing the Right Rebar Size
Most residential slabs use one of two sizes. The “#” number refers to the bar’s diameter in eighths of an inch:
- #3 rebar: 3/8-inch diameter, weighs 0.376 pounds per foot. This is the standard choice for 4-inch slabs like patios, walkways, and garage floors.
- #4 rebar: 1/2-inch diameter, weighs 0.668 pounds per foot. Used for 6-inch slabs, driveways with heavy vehicle traffic, or any slab carrying structural loads.
For a typical backyard patio or shed slab, #3 rebar at 18 to 24 inches on center is sufficient. If you’re pouring a driveway that will support trucks or heavy equipment, step up to #4. Foundation walls and stem walls typically call for #4 rebar at 12 to 18 inches on center, though your local building code may require tighter spacing.
How to Calculate Total Rebar
Rebar is laid in a grid, with bars running in both directions. The math is straightforward once you know your slab dimensions and chosen spacing. Here’s how it works for a slab that’s L feet long and W feet wide, with bars spaced S feet apart.
First, figure out bars running the length of the slab: divide the slab width by the spacing, then add 1. That gives you the number of bars, and each bar is as long as the slab. Then do the same in the other direction: divide the slab length by the spacing, add 1, and each of those bars is as wide as the slab. Add both totals together for your full linear footage.
Worked Example: 20 x 20-Foot Slab
Say you’re pouring a 20-by-20-foot slab with #3 rebar at 18-inch (1.5-foot) spacing.
Bars running one direction: (20 ÷ 1.5) + 1 = 14.3, rounded up to 15 bars, each 20 feet long. That’s 300 linear feet. Bars running the other direction: same calculation, another 300 linear feet. Total: 600 linear feet of rebar.
At 0.376 pounds per foot, that’s about 226 pounds of #3 rebar. Standard rebar comes in 20-foot sticks, so you’d need 30 sticks for this slab. If you chose 24-inch spacing instead, the same slab would need about 440 linear feet, or 22 sticks.
Worked Example: 10 x 12-Foot Slab
For a smaller project like a shed pad, 10 by 12 feet with 18-inch spacing: bars along the length give you (10 ÷ 1.5) + 1 = 8 bars at 12 feet each, totaling 96 feet. Bars along the width: (12 ÷ 1.5) + 1 = 9 bars at 10 feet each, totaling 90 feet. Grand total: 186 linear feet, or roughly 70 pounds of #3 rebar.
Add Extra for Overlaps
Standard rebar sticks are 20 feet long. If your slab is longer than that in either direction, you’ll need to overlap (splice) bars where they meet. The required overlap length varies based on bar size, concrete strength, and rebar grade, but a common rule of thumb for residential work is 24 to 30 inches of overlap for #3 and #4 bars. Your engineer or local code will specify the exact requirement.
Plan to buy 10 to 15% more rebar than your grid calculation shows. This covers overlaps, waste from cutting, and any bars you need for edge reinforcement. For the 20-by-20 example above, that means ordering roughly 33 to 35 sticks instead of 30.
Thickened Edges and Footings
If your slab has thickened edges (common for monolithic foundation slabs where the perimeter is deeper than the center), those edges need their own reinforcement separate from the main grid. Typically, thickened edges get two horizontal bars: one near the bottom of the trench and one higher up, closer to the height of the main slab mesh. Both should sit on chairs about 1.5 inches off the gravel base.
To estimate this extra rebar, measure the perimeter of your slab and multiply by 2 (for two bars). A 20-by-20-foot slab has an 80-foot perimeter, so you’d need roughly 160 additional linear feet of rebar for the edges, plus overlap allowances.
Positioning Rebar in the Slab
Rebar doesn’t sit on the ground. It needs to be suspended within the concrete at the right height, which means you’ll also need rebar chairs (small plastic or metal supports). For concrete poured directly on the ground, building codes require a minimum of 3 inches of concrete cover below the rebar. In a 4-inch slab, that puts the rebar very close to the top surface, so many contractors position it roughly in the middle or slightly above center.
Space your chairs about 3 to 4 feet apart in each direction. For the 20-by-20-foot slab, that works out to roughly 36 to 49 chairs. They’re inexpensive, so buy a few extra. The point is keeping the rebar from sagging to the bottom during the pour, where it does almost nothing structurally.
Quick Reference by Slab Size
These estimates assume #3 rebar at 18-inch spacing, the most common residential configuration, and include a 10% waste factor:
- 10 x 10 feet: ~165 linear feet, about 62 lbs
- 10 x 20 feet: ~290 linear feet, about 109 lbs
- 20 x 20 feet: ~660 linear feet, about 248 lbs
- 20 x 30 feet: ~950 linear feet, about 357 lbs
- 30 x 30 feet: ~1,320 linear feet, about 496 lbs
If you switch to 24-inch spacing, reduce these numbers by roughly 25%. If you step up to #4 rebar, the linear footage stays the same but the weight increases by about 78% per foot.
When Spacing or Size Should Change
The 18-to-24-inch grid with #3 rebar works for slabs that sit on well-compacted soil and carry light loads. You should tighten the spacing or use larger bars when the slab will support vehicles heavier than a passenger car, when the soil is expansive clay or poorly compacted fill, when the slab spans over soft spots or utility trenches, or when local codes require it for your climate zone.
The structural code minimum for temperature and shrinkage reinforcement is a steel-to-concrete area ratio of 0.0018 for standard grade rebar. In practical terms, for a 4-inch slab that’s 12 inches wide, you need at least 0.0864 square inches of steel per linear foot. A single #3 bar has a cross-sectional area of 0.11 square inches, which means one bar every 15 inches or so meets the bare minimum. Most builders space at 18 inches for a comfortable margin above code minimums while keeping material costs reasonable.