A reinforcing mat is bars running both ways at a spacing, and the thing that catches people is that the count is fenceposts rather than fence panels. A 12 foot span at 18 inch centers takes nine bars, not eight. This rebar calculator does both directions, takes the concrete cover off the ends, works out where the splices fall against your stock length, and tells you how many bars to buy.
Bars at a spacing are fenceposts
Divide the span by the spacing and you have counted the gaps, not the bars. There is always one more bar than there are gaps, because the mat has to be closed at both edges.
It is a small error on one run and a large one on a mat, because it happens in both directions at once. On a 20 by 12 foot slab at 18 inch centers it is two bars missing out of twenty-two, and they are the two along the edges, which are the ones you notice.
Concrete cover changes the count, not just the length
Cover is the depth of concrete between the steel and the face of the slab, and it is what keeps water and fire off the bar. Three inches is the usual figure where concrete is cast against the ground.
Most calculators treat it as a trim on the bar lengths. It is more than that: pulling the mat in from both edges narrows the span the bars are spread across, so it can drop a whole bar out of each direction. The calculator above applies it both ways, and the detail panel shows the two runs it ends up with so you can see what it did.
The lap is yours to set, and that is the point
Every other rebar calculator we have seen quietly assumes a lap splice of 40 bar diameters and never mentions it. That figure is commonly specified and it is a reasonable place to start, which is why it is pre-filled here. It is not a code requirement.
ACI 318 makes the required lap depend on the bar size, the concrete strength, the cover, the spacing between bars, whether the bar is epoxy coated and whether it sits near the top of a deep pour. Those conditions move the answer in both directions. There is no single multiplier that is right for every slab, so the field is exposed rather than buried, and the number that belongs in it is the one on your drawings.
Set it to zero and you get the bar lengths with no splice allowance at all, which is a useful sanity check against a quote.
Stock length decides how many splices you have
Twenty feet is the common stock length, with 10 feet widely carried and 40 and 60 available on order. Any run longer than one stick has to be lapped, and every lap adds its own overlap to the total.
This is where the arithmetic turns on itself. A 20 foot slab on 20 foot bars has no splices in the long direction at all. Stretch it to 21 feet and every single lengthwise bar suddenly needs a splice, and each of those splices eats another lap. The jump is real and the calculator shows it, so it is worth trying your slab against two stock lengths before ordering.
Tie wire
The crossings figure is every intersection in the mat, which is the upper bound on ties. Tie wire holds the bars in position while the concrete goes in and carries no load afterwards, so not every crossing is always tied, and a job that ties every other one halves that number.
What this is not
This is a quantity takeoff, not a design. It tells you how much steel a mat at a given size and spacing comes to. It does not tell you what size or spacing your slab needs, and nothing here should be read as saying a mat is adequate for a load.
The bar size, the spacing, the cover and the lap all come from a drawing or an engineer. Bring those four numbers here and this will price and order them for you.