Below is the formula I need to use but it tells me there are more levels of nesting than allowed.
What do I do to make it work?
=IF(L6=1,0,IF(L6=2,0,IF(L6=3,1,IF(L6=4,1,IF(L6=5,1,IF(L6=6,2,IF(L6=7,2,IF(L6=8,2,IF(L6=9,3,IF(L6=10,3,IF(L6=11,3,IF(L6=12,4,IF(L6=13,4,IF(L6=14,4,IF(L6=15,5,IF(L6=16,5,IF(L6=17,5,IF(L6=18,6))))))

Question

Below is the formula I need to use but it tells me there are more levels of nesting than allowed.
What do I do to make it work? 
=IF(L6=1,0,IF(L6=2,0,IF(L6=3,1,IF(L6=4,1,IF(L6=5,1,IF(L6=6,2,IF(L6=7,2,IF(L6=8,2,IF(L6=9,3,IF(L6=10,3,IF(L6=11,3,IF(L6=12,4,IF(L6=13,4,IF(L6=14,4,IF(L6=15,5,IF(L6=16,5,IF(L6=17,5,IF(L6=18,6))))))

espex · Answer

I won't pretend to know exactly what you're doing with this, however I can say that you are only showing paired parenthesis for these assignments:
IF(L6=13,4,IF(L6=14,4,IF(L6=15,5,IF(L6=16,5,IF(L6=17,5,IF(L6=18,6))))))
You have no other right parens, only left.

e.mccormick · Answer

Break things up into multiple cells.  Then each cell does a part.  In this way, you can have one that does the inner most operation that feeds the next that does a few more, etc. as needed until it is done and you have not exceeded the limits of Excel.