[RPG] ‘Advantage’ or ‘Disadvantage’ on damage rolls

advantage-and-disadvantagedicednd-5estatistics

I am looking at creating a few new categories of weapons just to add a bit more reason to spend money on mundane equipment in the early game. Poor, and Masterwork.

My initial idea is that poor weapons will use the disadvantage mechanic on damage rolls, that is rolling twice and taking the lower, and masterwork weapons will use the advantage mechanic.

They won't be magic, and I will probably make poor and masterwork versions of magical weapons as well, but I don't want a masterwork weapon to overshadow a magical +1 weapon.

When rolling a D20 the accepted average seems to be that advantage equates to a +4/+5 bonus, so what would that average bonus be on damage rolls? Given the different damage dice it is probably best to stick with a 1d8 weapon in this case, but I would assume the advantage is less for small die, and more with larger die.

A good answer might include an anydice table or some kind of maths, but I find all that really hard to get my head around, so an answer along the lines of 'advantage when rolling a d8 equates to roughly x' is the best for me, with explanations in words rather than maths if you need to add anything else.

A really good answer will include anything else that I might need to consider but haven't added to the question.

Note: I have seen this question, which is similar, but if the answers there happen to cover my question I don't understand them.

Best Answer

Here's a quick AnyDice program to show the effect of applying the D&D 5e (dis)advantage mechanic (i.e. roll twice, pick lowest/highest) to dice of various size. Based on the summary results, here are the average rolls in each case:

Die Normal Advantage Disadvantage Difference
d4 2.5  3.125 1.875 ±0.625
d6 3.5 4.4722 2.5278 ±0.9722
d8 4.5 5.8125 3.1875 ±1.3125
d10 5.5 7.15 3.85 ±1.65
d12 6.5 8.4861 4.5139 ±1.9861
d20 10.5 13.825 7.175 ±3.325

From this table we can see that — just considering the average damage per hit, and ignoring the distribution of results — (dis)advantage is equal to a bonus of about ±0.6 on a d4, ±1.0 on a d6, ±1.3 on a d8, ±1.7 on a d10, ±2.0 on a d12 and ±3.3 on a d20.

(This is lower than the often quoted "advantage ≈ +4 or +5" rule of thumb for d20 since it's based on the average roll rather than the chance of meeting a target DC that's typically somewhere near the middle of the 1–20 range. Due to the way the advantage mechanic works, it has the biggest effect compared to a flat bonus for mid-range target numbers.)


Addendum based on comments: For damage rolls with multiple dice, like a 2d6 greatsword, the magnitude of the effect depends on how you choose to generalize (dis)advantage to rolls using multiple dice. Off the top of my head, I can think of at least the following options for rolling NdX with (dis)advantage:

  1. Roll N pairs of dX, choose the highest/lowest die of each pair and add them up.
  2. Roll two sets of NdX, choose the set with the highest/lowest sum.
  3. Roll (N+1)dX, choose the N highest/lowest out of N+1.
  4. Roll 2dX and take highest/lowest, then add N−1 normal dX rolls.

This AnyDice program shows the average result of each of these methods of rolling NdX with advantage. For example, for 2d6 the averages look like this:

Roll Average Difference
normal 2d6 7.00
2 rolls of d6-with-advantage 8.94 +1.94
highest of two 2d6 rolls 8.37 +1.37
highest 2 of 3d6 8.46 +1.46
d6 with advantage + d6 7.97 +0.97

For method #4 the average difference between a normal roll and one with advantage is of course the same as for a single dX roll, while for method #1 it's N times that. Methods #2 and #3 tend to be somewhere in between. In fact, for N = 2 method #3 seems to give results exactly halfway between methods #1 and #4, although this does not continue to hold for higher values of N.