Why did my Pokémon's CP go down when I evolved it?
The Basic CP Formula
Detailed analysis of the Combat Power (CP) formula in Pokémon GO reveals why this phenomenon occurs. Basically, the CP formula heavily weighs the Attack stat compared to Defense and Stamina.
Notice that the Attack stat is a direct multiplier, whereas Defense and Stamina are square-rooted. This means that a change in Attack has a much larger impact on the final CP than an equivalent change in Defense or Stamina.
The Evolution Stat Shuffle
While most evolutions result in an increase across all stats, there are certain species where the evolution actually has a lower base Attack than its pre-evolution. Usually, this is compensated by a significant increase in Defense or Stamina, but because of the CP formula's weighting, the CP can still drop.
Example: Trapinch to Vibrava
A classic example is evolving Trapinch into Vibrava. Even though Vibrava looks like a bigger, stronger dragonfly, its base Attack is actually lower than the feisty little ant pit Trapinch!
| Stat | Trapinch | Vibrava | Change |
|---|---|---|---|
| Attack | 162 | 134 | -28 |
| Defense | 78 | 99 | +21 |
| Stamina | 128 | 137 | +9 |
In this case, Trapinch loses 28 points in Base Attack. Although it gains Defense and Stamina, the linear drop in Attack penalizes the CP calculation more than the square-root gains in bulk can compensate for, resulting in a lower CP for Vibrava at the same level.
Other Examples
This also happens with several "cocoon" evolutions where the middle stage is dormant and defensive:
Should I Worry?
No! CP is just a number representing "Combat Power" based on this specific formula. Lower CP doesn't necessarily mean the Pokémon is worse, especially for PVP leagues like Great League where a lower Attack allows you to power up the Pokémon to a higher level, resulting in greater overall bulk (Stat Product).
In fact, low Attack is often preferred for Great League and Ultra League PVP builds!