How much rebar a slab grid needs
For a rebar grid, count bars in each direction and add their lengths. Bars across the width: floor(width × 12 ÷ spacing) + 1, each running the full length; bars along the length: the same formula on the length, each running the full width. A 20 × 10 ft slab at 12-inch spacing needs about 32 bars and 430 linear feet of rebar. The pour itself is sized by the concrete calculator.
How to use this calculator
Enter the slab length and width in feet and the bar spacing on-center in inches (12–18 inches is common for residential slabs). The result shows total linear feet and the bar count in each direction. Divide linear feet by your bar stock length (often 20 ft) to get pieces to buy. Slab dimensions come from the square footage calculator if the shape is irregular.
Choosing spacing and bar size
Tighter spacing and larger bars add strength. Typical residential slabs use #3 or #4 rebar at 12–18 inch spacing. Heavier loads (driveways, footings) use closer spacing and bigger bars. Follow local code and any engineer's specification for structural work. Round piers use vertical bars and ties instead, sized alongside the sonotube calculator.
Add for overlaps and waste
Bars are spliced where they overlap (typically 40× the bar diameter), and there's cutting waste, so add about 10% to the linear footage. Keep bars about 3 inches from slab edges and supported mid-depth on chairs.
Welded wire mesh is sometimes used instead of a bar grid on simple slabs on grade, but the two are not interchangeable wherever an engineer has specified reinforcement.
Frequently asked questions
- How do I calculate rebar for a slab?
- Count bars in each direction as floor(side × 12 ÷ spacing) + 1, multiply each count by the bar length, and add. A 20×10 ft slab at 12-inch spacing is about 430 linear feet.
- What spacing should rebar be?
- 12–18 inches on-center is common for residential slabs. Heavier loads use closer spacing. Follow local code or an engineer's spec.
- How much extra rebar should I order?
- Add about 10% for lap splices and cutting waste.
- How far from the edge should rebar sit?
- About 3 inches from slab edges, and supported on chairs so it sits in the middle of the slab thickness.
- How do I convert linear feet of rebar to weight?
- Multiply by the bar's weight per foot: #3 rebar is about 0.376 lb per foot and #4 about 0.668 lb. So 430 linear feet of #4 bar weighs roughly 287 pounds.