How to lay out a rebar grid
A slab grid is two sets of bars at right angles. Bars running the length of the slab are spaced across its width, so you need the width in inches divided by the spacing, plus one for the bar at the far edge — the classic fence-post problem, where four spaces need five posts. The other set works the same way using the length. Multiply each count by the bar length it runs and add the two together for total linear feet, then divide by the length of stick you are buying and round up. Standard mill lengths are 20 feet, with 10 foot sticks stocked at most home centres for smaller jobs.
Two practical points the raw math misses. First, bars have to overlap where one stick ends and the next begins, and a common specification is a lap of at least 40 times the bar diameter — about 20 inches for #4 bar. That is what the lap allowance above covers, so leave it at roughly 10% unless your slab is much longer than a single stick, in which case go higher. Second, rebar only does its job if it sits in the middle third of the slab thickness and stays there during the pour: set it on chairs or dobies rather than the older habit of pulling it up with a hook as the concrete goes in. Keep bars about 2 to 3 inches back from every edge so they stay covered by concrete and do not rust.
Bar size, spacing and lap length should follow your engineer's drawings or local code — this is a material estimate, not a structural design.