Here is a little bit of Info from smogon on it.
" This section will explain from where the equations for the normalized base stats were derived, and hence may be skipped upon first reading.
The equation for the actual stat of a Level 100 Pokemon having 31 IVs in all stats is the following:
Stat = (Base Stat × 2 + 31 + Num + floor(EV ÷ 4)) × Nature
where Num = 110 if the stat is HP, and 5 otherwise.
This means that for the stats other than HP, assuming a neutral nature:
Stat with 0 EVs = 2 × Base Stat + 36 = 2 × (Base Stat + 18)
Stat with 252 EVs = 2 × Base Stat + 99 = 2 × (Base Stat + 49.5)
To calculate the statistical damage, we multiply the attack or special attack stat by the move power. However, since the stat is always twice a number, the statistical damage will always end up as being twice a number as well. Hence, the normalized stats can be defined as being half the actual stats for simplicity purposes (rounding 49.5 up to 50):
Normalized Stat with 0 EVs = Base Stat + 18
Normalized Stat with 252 EVs = Base Stat + 50
For the HP stats, we write them down in this way first:
HP Stat with 0 EVs = 2 × Base HP + 141 = 8 × (Base HP ÷ 4 + 17.625)
HP Stat with 252 EVs = 2 × Base HP + 204 = 8 × (Base HP ÷ 4 + 25.5)
Since the statistical defense is equal to the HP and Defense stats multiplied by each other, we again can use one eighth of the HP stat multiplied by half the Defense stat instead, and the statistical defense will always be one sixteenth of what it should be. Hence, we can define the normalized HP stat as being one eighth of the real HP stat, rounding the numbers from 17.625 to 18 and from 25.5 to 25. This is simple since we still end up adding 18 for the 0 EV normalized HP. Thus:
Normalized HP with 0 EVs = Base HP ÷ 4 + 18
Normalized HP with 252 EVs = Base HP ÷ 4 + 25
Here is a short summary of all the formulae given in this article:
Normalized Base Stat = Base Stat + 18 if the stat is not HP
Normalized Base HP = Base HP ÷ 4 + 18
Normalized Base Stat with 252 EVs = Normalized Base Stat + 32 if the stat is not HP
Normalized Base HP with 252 EVs = Normalized Base HP + 7
Statistical Damage = Normalized Base (Special) Attack × Move Power
Statistical (Special) Defense = Normalized Base HP × Normalized Base (Special) Defense
Maximum Percentage Damage = Statistical Damage × 10.5 ÷ Statistical (Special) Defense
Minimum Percentage Damage = Statistical Damage × 9 ÷ Statistical (Special) Defense
As one can see, the above formulae are all relatively straightforward (especially compared to the actual Stats formula and Damage formula).
The statistical damage can be used as is to compare the damage dealt by different Pokemon using various moves, from just their base stats.
The statistical defense or special defense can be used as is to compare the physical or special sturdiness, or lack of, of different Pokemon from just their base stats.
I hope you find this article of some use. Thanks for reading. " - Smogon