While there is some good information in the answers to the above questions, I feel none of them give the complete picture.
In Diablo 3, there are a number of different ways to increase your exp gain. Assuming your level is fixed (at 70, most likely), we have the following effects:
- "Strength in Numbers" buff (group bonus)
- "Increases bonus experience by ..%" from equipment and/or ruby in head slot
- "Monster kills grant +... experience" from equipment and/or Gem of Ease
- Difficulty bonus (a % bonus)
These all modify the base experience received from slaying a monster (again, assuming for simplicity that player level isn't taken into account). For simplicity, I will refer to them as (in the same order).
Now, how these work on their own is pretty straightforward, but how do they stack when two or more are present? Knowing the exact way they stack could make a big difference when evaluating different item properties.
For example, having "Monster kills grant +... experience" on a few pieces of gear could make a big difference if the calculation is something like:
Net gain = (Base + Bonus) * Group% * Bonus% * Difficulty%
However, it would be almost useless at higher difficulties if the formula was more like:
Net gain = Base * Group% * Bonus% * Difficulty% + Bonus
The names aren't very descriptive, but if I had to guess, my guess would be something like:
Net gain = (Base + Bonus) * (Bonus% + Difficulty%) * Group%
However, this is mostly guesswork. Is there any official source (or someone doing a bunch of science) that clears this up? I also remember someone saying there was a difference in how experience gain was calculated in Greater Rifts compared to the rest of the game. Any truth to this?
(Note: When multiplying by X%, I naturally mean multiplication by 1.00 + 0.01*X. Multiplying by (X% + Y%) refers to a multiplication by 1.00 + 0.01*(X + Y))
Edit: While broad in scope, the question primarily applies to the current (as of August 2015) state of the game. So patch 2.2.1 primarily (which also applies to season 4/patch 2.3.0, since there are no changes to the exp formula planned, only base exp balancing and bonus exp being averaged over the party).