I have 100 hundred different items. How to I calculate all possible combinations?

Question

ELI5 the formula to figure out all possible combinations without repeating any items but no upper or lower limit of the number to include. Order does not matter.

Answer 1

You can pick up $k$ items out of $100$ where $k=0,1,2,\ldots ,100$

Hence, all the possibilities are:

$$\sum_{k=0}^{100} \binom{100}{k}=2^{100}$$