Calculate the exact number of rebar pieces, total linear feet, and overlap requirements for any slab, footing, or wall.
Rebar Sizing Guide — Which Size for What
| Size | Diameter | Weight per ft | Use For |
|---|---|---|---|
| #3 | 3/8 inch | 0.376 lbs | Light slabs, shed floors, sidewalks |
| #4 | 1/2 inch | 0.668 lbs | Most residential slabs and footings |
| #5 | 5/8 inch | 1.043 lbs | Driveways, garage floors, heavy patios |
| #6 | 3/4 inch | 1.502 lbs | Structural columns, RV pads, foundations |
Rebar Spacing Standards — ACI 318
ACI 318 (American Concrete Institute Building Code) sets rebar spacing standards based on slab thickness and load. The general rule: rebar should be spaced at intervals no greater than 3× the slab thickness, up to a maximum of 18 inches.
- 12-inch grid: Heavy load applications — RV pads, hot tub pads, structural slabs over 6 inches thick
- 16-inch grid: Driveways, garage floors, slabs supporting permanent structures
- 18-inch grid: Standard residential slabs, patios, walkways (most common)
- 24-inch grid: Light-duty applications, shed floors, decorative slabs
For walls, vertical rebar is typically placed at 12 to 24-inch centers depending on wall height and lateral load. Add horizontal rebar at the same spacing for two-way reinforcement.
Lap Splice and Overlap Requirements
When rebar pieces meet, they must overlap (lap splice) by a code-required amount to transfer load between bars. The standard rule: lap splice = 40× bar diameter. Round up for safety.
- #3 rebar: 15-inch overlap minimum
- #4 rebar: 20-inch overlap minimum
- #5 rebar: 25-inch overlap minimum
- #6 rebar: 30-inch overlap minimum
Tie laps with rebar tie wire at 3 points — both ends and center. Crossings of perpendicular bars should be tied at every 4th intersection minimum, every intersection in seismic or high-load zones.
Buying Rebar — Sticks vs Coil vs Cut
Rebar is sold three ways. 20-foot sticks are the standard at home improvement stores — cheapest per foot but require a rebar cutter or hacksaw to size. Pre-cut to length from a steel supplier costs more per foot but eliminates field cutting on large jobs. Coil rebar uncoils into long lengths but requires a straightener machine — best for commercial concrete contractors, not DIY.
For a 20×20 foot slab with 18-inch grid #4 rebar, expect to need 14-15 twenty-foot sticks. At $8-12 per stick, total rebar cost runs $112-180 plus tie wire and chairs.
Wire Mesh vs Rebar — When Each Works
Welded wire mesh (typically 6×6 W1.4×W1.4) is acceptable for non-load-bearing residential slabs like sidewalks and small patios. It is cheaper and easier to install than rebar — sheets are 5×10 feet and roll out flat. The downside: mesh sags under foot traffic during pouring, often ending up at the bottom of the slab where it provides no benefit. For any slab supporting a vehicle, structure, or concentrated load, use rebar.