How many hash calculation is needed to reach one BTC?
Double SHA 256 follows a uniform distribution that means all hashes are equally likely to occur. So, your chances of finding the block header hash that is less than the target is same for every round of hashing.

With the current mining difficulty of 10183488432890, the target bits are 0x171ba3d1. This means you will need to find the block header hash that is less than or equal to 0x0000000000000000001ba3d10000000000000000000000000000000000000000. For one round of double hash that is a probability of 2.28631x10-23. Let's call it P. So the probability of not finding a valid hash is (1-P). For N hash attempts, the probability of not finding the valid hash is (1-P)N. To be sure that you find a block we need to ensure that (1-P)^N -&gt; 0. Since P -&gt; 0,  (1-P)^N = 1-N*P (using Taylor expansion). Solving both equations you get N = 1/P = 4.3738x1022

At your hash rate of 1 TH/s it will take you 4.3738x1010 seconds which is 1386.91 years.