The use of this calculator depends on the maximum length you can get your bars.
If you can get 60' long bars the total length of your slab can be 60'.
If you can get 40' long bars the total length of your slab can be 40'.
If you can get 20' long bars the total length of your slab can be 20'.
If the length of your slab is longer than the length of the bar you will have to splice (over-lap) the bars together with tie wire to form a continuous length. The amount of lap needed depends on the re-bar size, the grade used, grid spacing, concrete strength, and whatever your Engineer comes up with, what he or she thinks what is the minimum amount of lap needed to do your project.
If your project "needs" exceeds the max bar length required you will have to add the amount of overlap needed to the total length of re-bar needed and also include a bit for waste.
Note:Purchasing rebar in long custom cut lengths is not always the cheapest route to go.
Even with additional lineal feet needed for using a regular available bar size of 20' for splicing, the cost for this could be cheaper than purchasing longer custom cut lengths.