How much rebar for a 20×20 slab?

The 18 in grid here is only an example to show the method — your plans or building department set the real spacing and bar size. Change the spacing above and the count updates.

Why the bars fit in single sticks

With the bars stopping 3 in short of each form, every run is 19 ft 6 in — just under a 20 ft stick. That means no splices and very little waste: each bar is a full stick with 6 in cut off. Across 19.5 ft at 18 in spacing there are 14 bars each way (13 spaces of 18 in is 19.5 ft exactly), 28 bars in all.

Tighten the grid to 12 in and it becomes 21 bars each way — 19½ ft doesn’t divide evenly by 12 in, so the count rounds up to keep every gap at 12 in or less — 42 sticks and 441 ties, and the steel weight goes from about 374 lb to 561 lb of #4. Going to #3 bar on the same grid drops the weight by almost half; going to #5 adds more than half again. Those choices are structural, so they come from the plans.

Handling the grid

Twenty-foot sticks are awkward to carry and transport: many yards will cut them in half for a pickup, but then each run needs a splice. If you can, have them delivered full length. Set the grid on chairs at the height the plans call for so it ends up inside the 4 in or thicker slab — see how much concrete for a 20×20 slab for the concrete — and compare how much wire mesh for a slab if your plans allow mesh instead.