How to Calculate Rebar
For a concrete slab with a grid pattern, rebar runs in two perpendicular directions. The number of bars in each direction equals the slab dimension divided by spacing, plus one. This free tool handles all the math — just enter your slab dimensions and spacing to get bar counts, linear footage, and total weight.
Bars (direction 1) = ⌈Width ÷ Spacing⌉ + 1 Bars (direction 2) = ⌈Length ÷ Spacing⌉ + 1 Linear Feet = (Bars₁ × Length) + (Bars₂ × Width)
Rebar Size & Weight Chart
| Bar Size | Diameter | lbs / ft | Common Use |
|---|---|---|---|
| #3 | ⅜" | 0.376 | Residential footings, light slabs |
| #4 | ½" | 0.668 | Standard slabs, driveways, patios |
| #5 | ⅝" | 1.043 | Retaining walls, heavy slabs |
| #6 | ¾" | 1.502 | Structural columns, heavy duty |
Standard Rebar Spacing
Residential concrete driveways and patios typically use #4 rebar at 12" OC. Garage slabs use #4 at 18" OC. Always check local building codes — seismic zones and heavy loads require closer spacing or larger bars.
Frequently Asked Questions
What size rebar should I use for a concrete driveway?+
#4 rebar (½ inch diameter) at 12-inch spacing is standard for residential driveways and patios. For heavier use or expansive soils, use #4 at 12-inch OC both ways. Always check local building codes.
Do I need rebar in a 4-inch concrete slab?+
Rebar is recommended for most slabs — driveways, garage floors, and patios. While a plain 4-inch slab may technically hold up, rebar significantly reduces cracking from settling, temperature changes, and vehicle loads.
How far apart should rebar be spaced in a slab?+
Standard spacing is 12 inches on center (OC) for residential slabs. High-load applications like garage floors may use 12-inch spacing with heavier #5 rebar. Residential footings typically use 2 bars horizontally, centered in the footing.
Should I use rebar or wire mesh in a concrete slab?+
Rebar provides superior crack control and tensile strength, especially for slabs that may be subject to settlement or heavy loads. Wire mesh (welded wire fabric) is cheaper and faster to install but provides less structural reinforcement. Many contractors use a combination.