🏗️ Rebar Calculator
Enter your slab dimensions and grid spacing and get the number of bars each way, the total length, the weight for your bar size, and the number of grid intersections.
Total bars
30
10 lengthwise + 20 widthwise
Total length
385 ft
Weight (#4)
257.2 lb
Grid intersections (tie wires)
200
Bars each way = floor((side − 2×edge) ÷ spacing) + 1. Weights per ASTM bar size. Add 40× bar-diameter for lap splices on long runs, and check your local code for spacing and cover. 🔒 In your browser.
How the rebar calculator works
For a reinforcing grid, the bars in each direction are spaced across the opposite side: bars = floor((side − 2 × edge clearance) ÷ spacing) + 1. The tool counts the lengthwise and widthwise bars, multiplies each by its effective length to get the total linear metres or feet, and multiplies by the mass per length of the chosen ASTM bar size (#3–#8) for the weight. The number of grid intersections tells you roughly how many tie wires you need.
This estimates a single flat mat for a slab or footing. Add lap-splice length (commonly about 40 times the bar diameter) where bars are joined on long runs, and always follow your local building code and engineer’s drawings for bar size, spacing and concrete cover — this tool is a material estimate, not a structural design.
Frequently asked questions
How do I calculate how much rebar I need?
Count the bars in each direction as floor((side − 2 × edge clearance) ÷ spacing) + 1, multiply each count by its length, and add them for the total linear feet or metres. For a 20 × 10 ft slab at 12″ spacing that’s roughly 21 + 11 bars.
What spacing should rebar be?
Residential slabs commonly use #3 or #4 bar on a 12–18 inch (30–45 cm) grid, but the correct spacing depends on the load and your local code. Use the drawings or engineer’s spec; this tool lets you enter any spacing.
How much does rebar weigh?
It depends on the bar size: #3 ≈ 0.376 lb/ft (0.56 kg/m), #4 ≈ 0.668 lb/ft (0.994 kg/m), #5 ≈ 1.043 lb/ft (1.552 kg/m). The tool multiplies your total length by the mass per length of the size you pick.
What is edge clearance (concrete cover)?
The gap between the bar and the edge of the slab, which keeps steel protected inside the concrete — typically about 3 inches (7.5 cm) from a soil-cast edge. The bars start and stop that distance in from each side.
Should I account for lap splices?
Yes, on runs longer than a single bar. Where two bars overlap, add a lap length of roughly 40 times the bar diameter (check your code). This estimator counts bars and length for the mat; add splice length separately.