Rebar in a residential slab is laid as a grid, and the quantity question has a clean answer once you know the spacing. What this page will not do is tell you what that spacing should be — that comes from a plan or from your building department, and it is the one number you should not guess.
Why it is plus one, not just divided
The common mistake is dividing the slab width by the spacing and ordering that many bars. A twenty foot slab at eighteen inch spacing gives 13.3, so people order thirteen.
But a grid needs a bar at both edges, not just at each interval. Thirteen intervals means fourteen bars. This is the same off-by-one that catches people counting fence posts, and on a large slab it is several hundred dollars of steel.
The calculator uses the floor of the division plus one, in both directions, and shows the arithmetic so you can check it against your plan.
Both directions, and why it doubles
A slab grid runs bars both ways. Bars running the length of the slab are each as long as the slab is long, and there are as many of them as fit across the width. Then the same again in the other direction.
That means the total linear footage is roughly twice what you would calculate for a single direction, and it is why rebar is usually the largest single line item in a small slab pour after the concrete itself.
Laps are not waste, they are required
Where two bars meet end to end, they have to overlap — not butt together. The lap length is typically specified as a multiple of the bar diameter, often around forty times, which for a number four bar is about twenty inches.
The ten percent default in this calculator covers laps on a typical residential slab plus the offcuts. On a long slab where every run needs a splice, increase it. The calculator lets you change that figure directly.
Offcuts from rebar are mostly unusable — a three foot piece is too short to lap into anything — so unlike lumber, the waste really is waste.
Ties and chairs
Every intersection in the grid gets a wire tie. The calculator counts them, because it is a number nobody thinks about until they are on their knees in the sun. A twenty by thirty slab at eighteen inches has over three hundred intersections.
Tie wire is sold by the pound and a tie takes roughly half an ounce, so figure the weight from the count rather than buying one roll and hoping.
The grid also has to sit in the middle of the slab thickness, not on the ground. Bar chairs or dobies hold it there. Steel lying on the subgrade does nothing at all — it is below the tension zone and might as well not be in the pour.
Rebar versus mesh
Welded wire mesh is the cheaper alternative and it is adequate for some flatwork. It comes in rolls or sheets and is faster to place.
The tradeoff is that mesh is thin, it is hard to keep at the right height, and rolled mesh wants to curl back down as you walk on it. Sheets behave better than rolls for that reason.
Which one your slab needs is, again, a call for whoever is specifying the slab.