The base stats are converted into real stats by the following formula at Level 100:
(Base Stat 2) + 5
Unless HP, which is: (Base Stat 2) + 110
So if you have a Pokemon with 170 Base Attack, you would plug it into the formula as such:
*(1702) + 5
340 + 5
345 = n** (for later)
So it would have a stat of 345. However, this is with 0 IVs. This stat could raise by as much as 31 with a perfect IV. So you could have a stat anywhere between 345-376 now.
However, EVs come into play as well. Every 4 EVs on a Pokemon raise the stats by 1, meaning 252 EVs raise the stat by 63. Also, natures raise a random stat by 10% and lower a random stat by 10%. There are also 5 natures that raise or lower no stats. They are Serious, Hardy, Docile, Bashful and Quirky. Source.
So, for example, say you have a Kyruem-Black with 27 IVs in Attack, a Quirky nature, and 184 EVs in Attack. Remember the variable "n" earlier? This saves us work, as we know Kyruem-B's minimum stat.
The equation we would use is:
n + 27 + (184 / 4)
n + 27 + 46
n + 73 (Since n = 345,)
345 + 73
Therefore, your Kyruem-Black would have an attack stat of 418.