How much rebar do I need?

Count the bars each way and add one, because a grid at 16 in on centre across a 20 ft slab is 15 spaces but 16 bars. A 20 × 20 ft slab at 16 in gives 16 bars each way, roughly 640 linear feet, which is 32 twenty-foot sticks before laps.

No. Where bars are spliced, the overlap is specified as a multiple of the bar diameter — commonly around 40 diameters, but the actual figure comes from the drawings or the engineer, and it changes with bar size, concrete strength and whether the splice is in tension. Add the laps to the linear footage before you order.

Take both from the drawings. Slab reinforcement is engineered around the loads, the subgrade, the concrete mix and what the slab is for; a driveway, a footing and a structural slab are not the same problem. This tool counts the mat that a specified spacing produces — it does not decide the spacing.

One at each intersection if you are tying every crossing, which is what the intersection count here gives you. Many crews tie every other intersection in a staggered pattern on flat slab work, which halves the ties and the labour; on anything structural, follow the spec.