How much rebar for a 30×40 slab?

A shop or pole-barn slab this size is where rebar counts get interesting, because every run is longer than a 20 ft stick. The 18 in grid and 24 in lap are placeholders to show the math; your plans set both.

Splices change the count

Runs along the 40 ft side are 39 ft 6 in; along the 30 ft side, 29 ft 6 in. Neither fits in one 20 ft stick. At the 24 in lap used here, two 20 ft bars only reach 38 ft, so each 39.5 ft run takes three bars and two laps (43.5 ft of bar), while each 29.5 ft run takes two bars and one lap (31.5 ft). The laps add about 8% to the steel. Leave the lap at 0 and the calculator warns you, because the count would be short.

There are 21 bars running the length and 28 running the width (the spaces round up so no gap is wider than 18 in), 49 runs in all and about 1,796 ft of bar including laps: 91 sticks of 20 ft, counting each direction’s cut-offs separately. If your supplier stocks 40 ft bars, the long runs fit in one stick each and the count drops; ask, and try the 40 ft option above.

Planning the steel

About 1,216 lb of #4 bar — over half a ton — needs a delivery and a plan for where to stack it. Stagger the splices so they don’t all fall on one line across the slab, tie every crossing, and set the grid on chairs at the height in your plans. The concrete for the same slab is on how much concrete for a 30×40 slab, and the smaller example on how much rebar for a 20×20 slab.