Rebar Spacing Calculator

Rebar reinforcement prevents concrete slabs from cracking under load, temperature changes, and soil movement. This calculator tells you how many linear feet of rebar you need for a grid pattern based on your slab dimensions and chosen spacing. Rebar grids are laid in both directions across the slab, creating a mesh that distributes stress evenly. The calculator counts the number of bars running lengthwise and widthwise at your specified spacing, then totals up the linear footage. This helps you figure out how many standard rebar sticks to buy, since they typically come in 10-foot or 20-foot lengths. For most residential slabs like driveways, garage floors, and patios, number 3 rebar (3/8 inch diameter) at 18-inch spacing is the standard. Heavier applications like shop floors or slabs that will support vehicles may call for number 4 rebar (1/2 inch) at 12-inch spacing. Always check your local building code or engineered plans for the required size and spacing. Rebar should sit roughly at the middle of the slab thickness. For a 4-inch slab, that means supporting the rebar grid about 2 inches off the ground using rebar chairs or dobies (small concrete blocks). Rebar sitting on the ground does almost nothing structurally. When two pieces of rebar overlap at a splice, the standard minimum overlap is 24 inches or 30 bar diameters, whichever is greater. Factor this into your total footage if your slab dimensions exceed your rebar stick length. Buy an extra 10 percent to account for overlaps, cuts, and waste.