Hash Collision Probabilities Please give help! how can I calculate the probability of collision? I need a mathematical equation for my studying. 

Assume, I am using SHA256 to hash 100-bits. Thus:

SHA256 {100} = 256-bits (hash values) I would like to know the probability of collision. 

This website: 
https://preshing.com/20110504/hash-collision-probabilities/

Gave an equation: $K^2 / 2N$ 

But I could not know what is $K$ and $N$? Is K is the output which is (256-bits) and $N$ is the input which is (100-bits)

Please Help and Thanks in advance...
$n$ is the output size of the given hash function. To find a collision you try randomly generated $k$ different inputs. When we say the output size is $n$, it means that the output space has $2^n$ elements.

What you see on that website is the general case of the collision probability. We normally talk about the 50% probability (birthday attack) on the hash collisions as 

$$ k = \sqrt{2^n}$$
You can also see the general result from the birthday paradox.

To have birthday attack with 50% percentage you will need $k = 2^{128} \approx 4.0 × 10^{38}$ randomly generated differently input for a hash function with output size $n= 256$