I must create a system, or be enslaved by another man's. I will not reason and compare: my business is to create.

- William Blake

Friday, November 25, 2016

D&D 5e: Advantage/Disadvantage converted to modifiers

Here is a quick comparison between advantage/disadvantage and flat bonuses. Lots of people have written about the subject, but since I couldn't find the exact table I was looking for (the last two columns in the table below), I thought you might find it useful.

Here is how it works: if you need to roll 5 or more in the d20 (column a), you have 80% chance of succeeding (column b), or about 96% if you have advantage, or 64% if you had disadvantage - which means that in this case ad/disad is equivalent to a 3.2 bonus or penalty (rounded to 3 in the last column).
d20 roll needed
Chances
Advant. 
Disad.
d20 bonus/
pen.
 Rounded 
1
100.00
100.00
100.00
0
0
2
95.00
99.75
90.25
0.95
1
3
90.00
99.00
81.00
1.8
2
4
85.00
97.75
72.25
2.55
3
5
80.00
96.00
64.00
3.2
3
6
75.00
93.75
56.25
3.75
4
7
70.00
91.00
49.00
4.2
4
8
65.00
87.75
42.25
4.55
5
9
60.00
84.00
36.00
4.8
5
10
55.00
79.75
30.25
4.95
5
11
50.00
75.00
25.00
5
5
12
45.00
69.75
20.25
4.95
5
13
40.00
64.00
16.00
4.8
5
14
35.00
57.75
12.25
4.55
5
15
30.00
51.00
9.00
4.2
4
16
25.00
43.75
6.25
3.75
4
17
20.00
36.00
4.00
3.2
3
18
15.00
27.75
2.25
2.55
3
19
10.00
19.00
1.00
1.8
2
20
5.00
9.75
0.25
0.95
1

As you can see, as long as you need to roll something between 8 and 14, the bonus/penalty of ad/disad is equivalent to +5/-5, or a bit less. But if you need a natural 20 to succeed, having advantage is only equivalent to a +1 bonus - which is not that bad really, since it doubles your chances of success. On the other hand, a Bless spell is often better than having advantage when you need a natural 19 or 20! 

The average bonus/penalty is 3.5 - which is equivalent to rolling a d6 and adding it to the d20 (notice I disregarded the "1" line, since there is no point in rolling if you need to roll 1 or more - and if you have a different solution, please explain your reasoning in the comments).

It is also worth remembering that this comparison doesn't take the benefits of a natural 20 (such as a critical hit, etc.) into account.


These numbers are not that hard to remember (but not that easy too). You may find it easier to square your chances of failure, when using advantage, or your chances of success, when using disadvantage. For example, you know you have 20% chance of rolling 17 or more - with disadvantage, this falls to 4% only (20% times 20%). If you have advantage, on the other hand, your chance of failure falls from 80% to 16%, which is 80% of 80% (source). Or just memorize the table above.

I'm not advocating replacing the advantage/disadvantage for something else*. But this knowledge can be useful in many ways: quickly calculating your chances when rolling two d20s, comparing advantage/disadvantage to various bonus and penalties (cover, bless, etc), translating advantage/disadvantage to other forms of D&D (with the OSR Rosetta Stone, for example), and so on.

More than anything, you can now roll ten d20 if ten goblins are attacking the party with advantage**, instead of rolling twenty times!

* In fact, I really like this mechanic. The coolest thing about it is that advantage is ALWAYS very relevant to you chances. Even at the extremes (when you need a natural 20, for example), remember that +1 bonuses will double your chances! Which falls within the realm of reasonable granularity to me.

** Example from the same source - thanks, Rob Conley! Also, every 19-20 should be a crit if you use this system instead of advantage.

2 comments:

  1. You should factor in rolling a 1 somehow, since rolling a 1 is always a failure and advantage, unlike a flat bonus, allows a 're-roll' of that, basically making 'critical failure' much harder.

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    Replies
    1. Well, rolling a 1 is always a failure (in combat only), but there is no critical failure in the rules.
      If you have +10 to hit and the target has AC 10, 11 or 8, the result is the same.
      Advantage does improve your change of rolling a natural 20, though.

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