Give the slab size, the spacing and the cover, and this returns how many bars run each way, exactly where they sit, the total linear metres once laps are added, and how many ties you will get through.
Notes
How the count is worked out
The first and last bars sit one cover in from each edge, and the rest are spaced evenly between them. So the count is the clear span divided by the spacing, rounded down, plus one — the extra bar catches the one at the far edge.
Laps
Bars longer than the stock length have to overlap, and the lap is usually expressed as a number of bar diameters. Set it from your own specification — the default here is a common figure, not a rule.
Cover is not optional
Cover is what stops water reaching the steel. Too little and the slab spalls in a few winters; too much and the steel is not doing structural work where it should be. The right figure depends on exposure and comes from the design, not from a calculator.
What this does not do
It does not size reinforcement. Bar diameter, spacing and layer position are structural decisions that depend on loads and ground conditions — this tool lays out what you were told to place.