You don't need a calculator. That conversion is trivial.
r=4, w=2, x=1
no right = 0 = "-"
The "d" stands for "directory"
When you take one of the three permission digits, then it is one of these:
1 (x), 2 (w), 4(r)
3 (xw), 5 (rx), 6 (rw)
7 (rwx)
Then you just place three numbers there: one for owner, one for group, one for the "world".