Rebar Calculator
Work out the bars and total rebar length for a slab grid.
Estimate the rebar needed for a reinforced concrete slab laid out as a grid in two directions.
How the Math Works
The Rebar Calculator uses a precise mathematical formula to determine the number of reinforcement bars needed in each direction of a slab grid. The calculation floor((span - 2*edge)/spacing) + 1 works by first accounting for the edge distance - the space left between the slab boundary and the first bar, typically 2-3 inches for proper concrete cover. It then divides the usable span (total length minus both edge distances) by the spacing between bars. The floor function ensures we only count complete intervals, and adding 1 gives us the total number of bars needed, including the last bar at the edge. This approach guarantees structural integrity while minimizing material waste.
Practical Applications
To use this calculator practically, start by measuring your slab's dimensions and determining the required edge distance (commonly 2-3 inches for interior slabs, 3-4 inches for exterior). Enter the clear span (total length minus edge distances), the spacing between bars (typically 12-18 inches on center for slabs), and the calculator will output the exact number of bars needed in each direction. For example, a 20-foot slab with 3-inch edges and 18-inch spacing would require floor((240-6)/18) + 1 = 14 bars in each direction. Always round up to the nearest available bar length standard and add 5-10% for cuts and waste.
Day-to-Day Use
This calculator saves homeowners and contractors significant time and money on everyday construction projects. Instead of over-ordering rebar and wasting hundreds of dollars on unused materials, you'll order exactly what's needed. Whether you're building a backyard patio, garage slab, driveway, or shed foundation, this tool prevents costly mistakes like having too few bars (which compromises structural integrity) or too many (which wastes money and creates clutter). The calculator also helps you plan your project timeline - knowing exactly how many bars to purchase means no waiting for deliveries mid-project, keeping your renovation on schedule and within budget.
Worked example
20×10 ft slab, 12 in spacing, 3 in edge → ~11 + 21 bars, ~430 ft of rebar.
FAQ
What rebar spacing for a slab?
Residential slabs commonly use #3–#4 bar at 12–18 in on center.