Effective Health is a calculation that tanks have been using for years to help them make their gearing decisions. Simply put, it’s a mathematical calculation of HP and armor that calculates how much raw damage the tank can take before dying. Where HP is your pet’s maximum HP and Ar% is his armor mitigation percentage, the formula for effective health is:
For example, if your pet has 75% armor and 50,000 HP, he can take 200,000 raw damage before dying.
Effective health is generally a good representation of how easy it feels to heal a tank; the higher a tank’s effective health, the more unavoided hits in a row they can endure between heals. Since we’re not able to increase our pet’s avoidance with gear, effective health is the main way we’re able to improve out pets.
How to use EH:
What knowing how EH works lets us do is compare the benefits off additional armor vs. additional stamina. Lets look at the case that came up in the comments recently.
Lets assume a pet with 50,000 health and 60% armor with one empty trinket slot. That’s an EH of 125,000. Adding the Glyph of Indomitability with full armor talents and raid buffs brings the armor up to 61.03% for an EH of 128,304 or an increase of 3,304. If you instead added the Bitter Balebrew Charm the HP goes up to 51,733 for an EH of 129,333 or an increase of 4,333.
In this case, the Balebrew Charm comes ahead on EH, but depending on your stats, build, and buffs which trinket would comes out ahead can change. Knowing how to calculate your pet’s EH will also help you make decisions on which pet talents to take if you need to choose between HP and armor.
When not gear for effective health:
Traditional tanks have one major reason to not rely solely on effective health that we don’t have to worry about, which is they have the options of choosing between EH and avoidance when picking their gear. We really only have two reasons.
1. A large percentage of the damage in is magic damage.
The more of the damage you take is magic based, the less your effective your Armor is and the lower you’re your effective health.
2. The damage in is steady, but too high for the healer.
The main purpose of a high effective health is to smooth out incoming damage. If the incoming damage is smooth, say from DoTs or multiple light hitting mobs, it may be worth dropping EH for armor to reduce the total amount of damage. A prime example would be in soloing when your healing output is capped at what you can do with Mend Pet, Silverback and possibly 2-piece T5.
Overall, effective health is a great tool to help make your pet both as durable as possible and as easy to heal as possible. As always though, there are always exceptions to the rule, so its important that it be just one of the factors you take into consideration.