How to use this calculator
Enter your slab's length and width in feet, choose a rebar size and grid spacing, and the calculator instantly shows how many bars you need in each direction, the total linear feet, weight, and how many 20-foot rebar sticks to buy.
The calculator assumes a grid pattern with the same spacing in both directions. If your slab dimension exceeds 20 feet, it automatically accounts for splice overlaps (24 inches per splice) in the linear footage and piece count.
Rebar sizes at a glance
| Size | Diameter | Weight/ft | Common uses |
|---|---|---|---|
| #3 | 3/8" | 0.376 lb | Patios, walkways, light slabs |
| #4 | 1/2" | 0.668 lb | Driveways, garage floors, most residential |
| #5 | 5/8" | 1.043 lb | Heavy-duty slabs, commercial, foundations |
| #6 | 3/4" | 1.502 lb | Structural walls, columns, footings |
Why concrete needs reinforcement
Concrete is extremely strong in compression but weak in tension. When a slab flexes under load or shifts with the ground, the bottom of the slab experiences tension forces that can cause cracking. Rebar handles those tension forces, keeping cracks tight and preventing the slab from breaking apart.
Without reinforcement, even a well-poured slab on stable soil will eventually develop cracks that widen over time. Rebar doesn't prevent all cracking, it controls it, keeping cracks small enough that the slab stays structurally sound.
Rebar vs. fiber mesh: which is better?
Rebar (steel reinforcing bar): Provides true structural reinforcement. It holds cracked concrete together and prevents pieces from shifting. Required by code for driveways, garage floors, and any slab that carries significant load. The downside is the labor to cut, place, and tie the grid.
Fiber mesh: Synthetic or steel fibers mixed into the concrete. They reduce shrinkage cracking during curing but do not provide structural reinforcement. Fiber mesh is a supplement, not a replacement for rebar in load-bearing applications.
Rule of thumb: Use rebar for anything that supports weight, driveways, garage floors, foundations, structural slabs. Fiber mesh alone may be adequate for non-structural flatwork like sidewalks or small patios on stable, well-compacted soil.
Frequently asked questions
How much rebar do I need for a 20x20 slab?
For a 20x20 foot slab with #4 rebar at 12-inch spacing, you need 21 bars in each direction (42 total bars), requiring 42 pieces of 20-foot rebar. The total weight is approximately 561 pounds. At 16-inch spacing, you would need about 32 total bars.
What size rebar should I use for a concrete slab?
#4 rebar (1/2 inch) is the standard for residential driveways and garage floors. #3 rebar (3/8 inch) works for patios and walkways. #5 rebar (5/8 inch) is used for heavier applications like commercial floors or slabs under heavy equipment.
What spacing should rebar be in a concrete slab?
Most residential slabs use 12-inch or 16-inch on-center spacing in a grid pattern. Driveways and garages typically use 12-inch spacing. Patios and walkways can use 18 or 24-inch spacing. Check local building codes for your specific project.
How much overlap do you need when splicing rebar?
The general rule is 40 bar diameters of overlap. For #4 rebar, that's 20 inches. Many contractors use 24 inches as a safe standard for residential work. Always stagger splice locations so they don't all line up in the same spot.