Some folks might be interested in that. How big a boost does Advantage represent ?
Unlike many other games and many things in 5E, where you'd get a straight up bonus of +X to something, Advantage means your roll two d20 and keep the higher of the two. This means that it is somewhat not expressed in the same language/unit/currency as a standard "+X to thingy rolls", although some conversions can be made.
One way look at how much Advantage represents is to look at the effective bonus. When rolling two d20 and keeping the max, how much bigger is the result compared to rolling one d20 ? The answer is
the effective bonus is on average 3.325 (having an average at 13.825 instead of 10.5 for the simple d20).
Another way, quite popular, to look at how much Advantage represents is to look at the equivalent bonus. If p is your chance of success on an attack roll, benefitting from Advantage increases that hit chance by an amount i(p) = p*(1-p). Because of the uniformity of the d20, this can easily translated into an equivalent bonus : the standard bonus that would have increased your hit chance as much. Having Advantage means you have an equivalent bonus b(p) = 20*p*(1-p).
A couple of theoretical examples.
- Say your hit chance is p = 50%, or equivalently your roll target is t = 11. Then i(p) = 25%, your equivalent bonus is a +5.
- Say your hit chance is p = 65%, or equivalently your roll target is t = 8. Then i(p) = 22.75%, your equivalent bonus is a +4.5.
- Say your hit chance is p = 80%, or equivalently your roll target is t = 5. Then i(p) = 16%, your equivalent bonus is a +3.2.
- Say your hit chance is p = 100%, or equivalently your roll target is t = 1. Then i(p) = 0%, your equivalent bonus is a +0.
Clearly, Advantage is less beneficial when your base hit chance is high (or low). Also to note, everything is symmetric if your p is below 50%. Advantage won't help you much if you have p=5%, and will essentially give you +4.5 if p = 35%. Advantage is at its most beneficial when p = 50%.
So if you typically find your hit chance between 50% and 80% without Advantage, you will essentially benefit from a +5 to +3.2.
But how about real monsters ? Let's assume you have a level 1--3 character, with 16 in the Ability Score used for their Attack Roll, and you are Proficient, so your total AS+Proficiency bonuses are +5. I have selected some monsters that I feel a low-level party could encounter.
- Generic Monster CR1 (DMG p274) with AC=13. That means your roll target is t = 8, your hit chance is p = 65%.
- Zombie : AC = 8, t = 3, and the hit chance is p = 90%.
- Acolyte, Badger, Giant Vulture : AC = 10, t = 5, and the hit chance is p = 80%.
- Harpy, Brown Bear : AC = 11, t = 6, and the hit chance is p = 75%.
- Bandit : AC = 12, t = 7, and the hit chance is p = 70%.
- Skeleton : AC = 13, t = 8, and the hit chance is p = 65%.
- Giant Spider : AC = 14, t = 9, and the hit chance is p = 60%.
- Goblin, Gnoll : AC = 15, t = 10, and the hit chance is p = 55%.
- Bugbear : AC = 16, t = 11, and the hit chance is p = 50%.
- Goblin Boss : AC = 17, t = 12, and the hit chance is p = 45%.
- Hobgoblin : AC = 18, t = 13, and the hit chance is p = 40%.
Note that I took these AC from the Monster Manual and didn't relauch my BG3. Larian overall reduced some AC, so you might have higher hit chance in practice.
I feel the typical range of hit chance is about 40%-80%. That means that Advantage typically gives you an equivalent bonus between 3.2 and +5. Now, let's assume you find yourself in a fight against single-type enemies, which were selected from a list such that your hit chance is between 40% and 90%. Assume the selection is random and uniform. Your average equivalent bonus from Advantage will be 4.05.
Finally, when using the Passive Checks rule, Advantage provides a +5. [EDIT, thanks LukasPrism]
Now, whether you prefer effective bonus or equivalent bonus, how big is that bonus, compared to other sources ?
- Ability Scores + Proficiency Bonuses can go from a starting +5 up to a +11 at level 17, meaning you gain a +6. Standard enemy difficulty, and notably the suggested AC of the Generic Monster (DMG p274), progresses with you, at the same speed. So I feel these are not real bonuses, but more the baseline of the arms race. Although, if your level 17 characters come back to the goblin punks who bullying you at level 1, then you did get a +6. This comes in very slowly though.
- As far as magic weapons go, it seems that the strongest Attack Roll bonus you can get is a +3.
- A spell like Bless gives a bonus of 1d4, which means an average of +2.5.
- The Archery Fighting Style gives a +2 to attack rolls (with ranged weapons). [EDIT]
So Advantage is pretty dope.
(And this is only thinking about hit chances. Rolling two d20 also increases the probability of landing a 20, which on average doubles the damage, so the impact of Advantage in terms of average damage output would be a bit better. But it's not always all about damage.)
