Rebar Calculator
Plan rebar for a concrete slab or footing: total linear feet, number of bars, and estimated weight. Enter the slab dimensions and your bar spacing (12 in is typical for residential, 6–8 in for driveways) and the calculator lays out a bidirectional grid with a waste allowance. Weights are based on #4 (½ in) rebar.
Rebar Grid Diagram
How the Formula Works
Count rows (bars running along width)
Rows = ⌊Length ÷ Spacing⌋ + 1 One bar at each spacing interval plus one at the far edge
Count columns (bars running along length)
Columns = ⌊Width ÷ Spacing⌋ + 1 Calculate total linear footage
Linear ft = (Rows × Width + Columns × Length) × (1 + Waste %) Each row spans the full width; each column spans the full length
Calculate weight
Weight (lbs) = Linear ft × (Weight per ft) Weight is dynamically calculated based on selected rebar size
Frequently Asked Questions
For a 10 × 10 ft slab with 12-inch spacing, you need (⌊10/1⌋+1) rows × 10 ft + (⌊10/1⌋+1) columns × 10 ft = 11 × 10 + 11 × 10 = 220 linear feet of rebar (plus 10% waste = 242 ft). The formula counts bars in both directions — use the calculator above for any slab size and spacing.
Standard rebar spacing depends on the application: residential slabs and patios 12 inches (300 mm), garage floors and driveways 6–8 inches (150–200 mm), structural slabs 6 inches (150 mm) or less as specified by an engineer. Closer spacing adds strength but increases material cost significantly.
#4 rebar (1/2 inch diameter) is the most common choice for residential slabs, driveways, and patios. #3 rebar (3/8 inch) is used for light-duty applications. #5 (5/8 inch) or larger is used for structural slabs, footings, and walls. This calculator uses #4 rebar for weight estimates — adjust if you are using a different size.
Divide the slab length by the desired spacing to get the number of sections, then add 1 for the starting bar. For a 10-foot slab with 12-inch spacing: 10 ÷ 1 = 10 sections + 1 = 11 bars. Repeat for the other direction. The result is the bar count for a bidirectional grid. This calculator handles both directions automatically.
Rebar is strongly recommended for most slabs. A plain concrete slab will crack under load and temperature changes — rebar holds the pieces together and maintains structural integrity after cracking. Exceptions: very small slabs (under 4 × 4 ft) and garden paths with no vehicle traffic may use wire mesh or fiber reinforcement instead.
Add 10% waste to account for bar overlaps (typically 12–18 inches at splices), cuts at edges, and any measurement errors. For slabs with many cut bars at irregular edges, increase to 15%. This calculator applies your chosen waste percentage automatically — the default is 10%.
#3 rebar: 0.376 lbs/ft (0.560 kg/m). #4 rebar: 0.668 lbs/ft (0.994 kg/m). #5 rebar: 1.043 lbs/ft (1.552 kg/m). #6 rebar: 1.502 lbs/ft (2.235 kg/m). This calculator uses #4 rebar weight for estimates. For other sizes, multiply your total linear footage by the weight per foot listed here.
Rebar should be placed at the middle to lower-middle of the slab depth. For a 4-inch slab, center the rebar at 2 inches from the bottom (providing 2-inch cover). Use rebar chairs or dobies to hold the bars at the correct height — never lay rebar on the ground before pouring, as it will end up at the bottom with no concrete cover.
References
- American Concrete Institute ACI 318 – Building Code Requirements for Structural Concrete (Reinforcement)
- Concrete Reinforcing Steel Institute CRSI Design Handbook