How much rebar do I need?

A mat is two directions, not one

Rebar in a slab is laid as a grid, so you are buying two sets of bars at right angles. Bars running the length of the slab are spaced across its width, and bars running the width are spaced along its length. Each direction gets its own count, and the totals rarely match.

The count in each direction is the span divided by the spacing, plus one — the same off-by-one that catches people with fence posts. A 10 foot width at 18 inch spacing takes 6 spaces, which means 7 bars. Forget the plus one and you are a bar short in both directions.

The tie count is the number of intersections, which is simply one count multiplied by the other. Every intersection wants a wire tie to stop the mat shifting when the concrete goes in. They are cheap; run out mid-pour and you will regret it.

Two things this does not cover. Laps: where two bars are spliced end to end, codes typically want an overlap of around 40 bar diameters — roughly 20 inches for #4 bar — so a slab longer than your bar length needs that extra length added at every splice. And cover: the mat has to sit inside the concrete, not on the ground, usually on chairs at mid-depth. Rebar resting on dirt does nothing and will rust.

None of this replaces an engineer. For anything structural — footings, retaining walls, anything carrying load — the bar size, spacing and cover come from a design, not from a calculator.