[2020] Calculator for how much ap you'd need to match the damage of a magic pen item

We'll start with AP

AP is quite simple. It adds a flat amount of damage.

Slightly more complex: The amount that damage matters requires you to consider the base damage of the ability, the AP ratio, and the amount of AP you already have. So for example, if you already have 100 AP, your 200 base damage ability with a 1.0 ratio essentially has 300 base damage and you need to consider what further AP does from there.

Magic penetration is simple multiplication.

Magic penetration has two factors: How much total damage your ability deals (200 + 100x1.0 = 300) and how much magic resist your opponent has. It's actually fairly easy to break down flat pen and percent pen in a similar fashion. Ultimately, all either do is multiply the amount of damage you already deal.

The short answer to your question is two statements:

1. The higher damage your ability deals, the more valuable magic penetration is.
2. The lower the AP ratio on the ability is, the more magic penetration matters.

Whether you choose flat penetration or percent penetration is mostly just a factor of how much magic resist your opponent has. Finding the optimal amount of AP, flat pen, and percent pen is just a constantly-moving math problem. In general, the suggestion I'll provide is that flat AP is most valuable in the early game. And similarly, any late game build should absolutely have some form of magic penetration in it. Ultimately, magic penetration can only ever multiply the damage you already have. If you have no damage worth dealing, there's no need to multiply it.