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In published adventures (or even homebrew ones), we usually find things like "DC 15 Wisdom (Insight) check tells if the NPC is lying" and so forth and so on. Of course, some checks such as climbing a wall section to bypass an encounter do require every character involved needing to succeed (or perhaps the "working together" rules apply), so there's no adjustment needed for party size. But some tasks, like busting doors open, may become easier because every single character gets to have a shot at it, and an extra number of characters does give the party (statistically speaking) better chances of success.

I know the DMG addresses scaling encounters, but I couldn't find any mention of scaling DC for unusual party sizes (not even here on RPG.SE). Do DMs usually adjust the DC for the size of the party, or leave them alone? Do people just shun away from the calculations involved? Or do they just apply the "working together" rules almost everywhere?

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  • \$\begingroup\$ This question is asking if there is rules for scaling DC, and if so, what are they. Even though it is couched in the idea that there should be rules for it, and the value judgement of such rules being apparently missing is opinion based, that is not the question being asked. \$\endgroup\$ – Tritium21 Sep 15 '16 at 18:38
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    \$\begingroup\$ I look at it slightly differently: they're partly asking if there are rules, but also for best practices. "What's the best way to handle X" is extremely common and answerable on RPG.SE. It's just the word "should" that sounds opinion-based, which it is, but not so much so that experts can't agree on a good answer. \$\endgroup\$ – SirTechSpec Sep 16 '16 at 5:25
  • \$\begingroup\$ @SirTechSpec Should is a bit of a flag, would you like to edit the question title to better capture the content of the question? I find that a lot of questions have poor titles, and some need an edit. \$\endgroup\$ – KorvinStarmast Sep 16 '16 at 12:32
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The DMG doesn't mention scaling DCs for party size.

If you find yourself wondering "how high should I set the DC so that this group of \$N\$ characters with modifiers \$m_1, m_2, \dots, m_N\$ will succeed with probability \$p\$?"...

...then you've missed the point.

Only call for a roll if there is a meaningful consequence for failure.

When deciding whether to use a roll, ask yourself two questions:

  • Is a task so easy...?
  • Is a task so inappropriate or imposible...? (DMG . 237, "Using Ability Scores")

Let's take your example of breaking down the door. Your question is posed in terms of how many characters are in the party, but it's not the number of party members tripping you up: it's the number of attempts you're thinking of allowing. You've implicitly stated that you're going to allow many attempts at this door.

If you've conceded that "every character gets a shot at it," then what's the consequence of any particular check's failure? Six more seconds? Are you going to let them keep going until they get it? In that case you shouldn't be rolling many checks in the first place. The interesting questions are likely "do they burst through on the first try, surprising whatever's on the other side," vs. "how long does it take them to burst through," vs. "can they burst through?" In that case, I'd recommend you have one check: success='through on the first try'; failure='through after d8 rounds' (or d4 minutes, or whatever makes sense to you) or failure='oops, this door's actually steel-cored, and you aren't forcing your way through, ever."


Related Reading

5 Simple Rules for Dating My Teenaged Skill System, by AngryGM (with the usual caveat that Angry's excellent advice and analysis is liberally salted with rude language)

How many times may a character attempt a skill roll for the same test?

I failed to open a lock. Now what?

@dkaeae also recommends to your consideration Angry's Passive Skills, Active Skills, Perception, and Knowldege; it's certainly on-point, though with a different 'take' than I'm providing above.

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In most cases, allowing multiple attempts is a poor idea. If you'd let every character try disarming that trap, why would you not let one member in a party of five try it five times? In other cases, you can allow multiple attempts regardless of the amount of people, such as breaking down a door. In some cases, like attempting to discern someone lying, a larger group has a natural advantage, and I feel that it makes very little sense to try to artificially remove that. If you started fiddling with that, you'd also have to compensate for the natural disadvantages a larger group has, like difficulties fighting in close quarters, loot being divided to more people, etc. They also have more advantages, for example they can have more casters meaning they get more spell casts per day than a smaller group would, but it doesn't make any sense that their mages would be any weaker just because there are more than one.

In most cases, if you feel the need to balance the campaign to account for a larger group size, do it by adding enemies, forcing them to fight in close quarters, using AoE spells and abilities against them, or adding other obstacles. Don't do artificial difficulty hikes. RPGs are made to model real life at least to an extent and changing DCs just to make things harder is defeating that purpose. Always remember that you're running a fun game, not a tabletop version of Dark Souls or a lesson in statistical mathematics.

To aid this, I often run my campaigns with the idea "does this make any sense?" What it means is that let's say my player wants to do something that has no direct rule, but that makes sense within the context of the game and there is at least a theoretical possibility of success. An example off the top of my head was a large half-orc character wanting to throw a very small-sized (half-?)elf over the enemy battle line to wreak havoc from behind. Now, I could just say "no, you can't do that" because there isn't a ready-made rule for that or a convention on what the skill check should be, but instead I chose to allow them to try, the orc was strong enough that he could, at least in theory, easily throw the small elf over the heads of the goblins they were fighting. It makes sense, therefore I have no reason to not let them do it. I cooked up a roll on the fly, making the orc roll throw with some penalty because he was throwing something a lot bigger and heavier than a javelin or a rock, and the elf roll a landing with some penalty since she wasn't in control of her trajectory. I could easily have told them no, but instead it became one of the more memorable moments of that dungeon.

TL;DR: If possible, let your players play with as few limitations and artificial difficulty hikes as you can. The gameplay is a lot more entertaining and it will be a good exercise for your creativity.

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In situations where only one person can attempt something at a time, see nitsua's answer.

For situations where it makes sense that everyone would try something at the same time, use passive ability checks.

"DC 15 Wisdom (Insight) check tells if the NPC is lying" would work much better as "any character with a passive Wisdom (insight) can tell if the NPC is lying." If you want an element of randomness involved, have the person acting (in this case, lying) roll a check (Charisma (persuasion)) against the passive skills of the PCs.

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    \$\begingroup\$ This complements nitsua's answer perfectly by covering the more specific part of my question. I'm going to do exactly this in my game, thanks! It also kind of makes you wonder why passive checks are so underrated with most DMs out there (and even published modules, for that matter)... \$\endgroup\$ – dkaeae Sep 15 '16 at 22:51
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Evidence by Absence: DC scaling was not a desired game element

Given that the design team took the trouble to present encounter scaling guidance, but not DC scaling guidance, there is no reason to juggle DC's based on party size rather than on how difficult a task is judged to be.

D&D 5e tries to get away from "it's got to be in the book or I don't have a tool." For DC's in particular, the game design explicitly delegates the difficulty(DC) of a given attempt or task to the DM.

Use the 'working together' feature where applicable

Without getting into a litany of supercrunch, a larger party has a deeper talent pool. It's OK that having more party members eligible to help with, or to attempt, a task will improve the party's chances for success. In the D&D 5e design philosophy, they simplified that team play benefit down to the advantage accruing when using the Help action.

Working Together

Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action (see chapter 9). A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help. (Basic Rules p. 59)

Real life analogy: Sometimes, it is better to have 6 planes out searching for survivors in the open ocean than 4 planes. The task is already hard. Having more assets makes it more possible to achieve. (Personal experience from a few search and rescue operations).

So what do I do?

Assign a DC and play on. You are the one assigning DC's in the first place, and choosing when a roll is needed.
Using Ability Scores

Only call for a roll if there is a meaningful consequence for failure.

When deciding whether to use a roll, ask yourself two questions:
Is a task so easy ...?
Is a task so inappropriate or impossible ...? (DMG p. 237)

It doesn't need to be complicated, nor does it require number crunching.
Do what makes the most sense to you in assigning a DC, and press on.

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  • \$\begingroup\$ What exactly is a "litany of supercrunch"? \$\endgroup\$ – Tim Grant Sep 15 '16 at 17:04
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    \$\begingroup\$ @timster digressing into an exposition with anydice examples on how many rolls versus a given DC will increase or decrease the chances of success for an ability check. \$\endgroup\$ – KorvinStarmast Sep 15 '16 at 17:14
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This answer's mostly a combination of opinion and stats:

Based on the info you provided, I'm not sure how large your party is beyond the standard 4 or 5, but I often caution DMs in 5e from deviating from the recommended DCs because the system's improved bounding on numbers can result in you accidentally pushing the DC into impossible for the characters' level.

Ignoring boons like Expertise and Guidance, which only provided their benefit to one person, a character's base skill check caps at +7 at level 1 through +11 at level 17. This example conservatively assumes that a character somehow starts with a 20 in the relevant ability score. More likely starting skill checks will be probably +3 through +6 assuming both proficiency and a positive modifier on the relevant skill and ability score. Thus, for those scenarios, it's around a 50-50 success chance for a DC15.

That being said, if you've got 8 players at the table all rolling a d20 for a social interaction, at least one of them is pretty likely to roll over a 15. To remedy this problem, I recommend that you have them pick group representatives for the Charisma (Persuasion) and Wisdom (Insight) checks. Those players make the rolls, possibly with the benefit of Guidance from the others.

You run into a tricky situation with that many players because it can be annoying to be the barbarian when diplomacy breaks out because that player's just hanging back. As DM you can say them's the breaks or allow that character to periodically try and provide advantage in some way, but that will edge the check results higher.

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  • \$\begingroup\$ It's worth mentioning the Rogue's (and eventually Bard's) Expertise feature. It doubles their proficiency bonus, so the maximum you're looking at is a +9 at level 1, and +17 at level 17. It's not uncommon for rogues to have a +7 to some skills at first level, although they're usually DEX based. \$\endgroup\$ – UrhoKarila Sep 15 '16 at 15:51
  • \$\begingroup\$ While I like this answer overall (there's much good in it) I don't see where the problem is. To remedy this problem ... As Ringo Starr said it - "I get by with a little help from my friends." The "provide advantage in some way" is actually covered for ability checks in the "working together rule" that I cite in my answer. You may or may not wish include that in your answer. \$\endgroup\$ – KorvinStarmast Sep 15 '16 at 17:35
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Hi I don't think anyone has added the actual answer, in terms of maths that the DM can do to hide this shift in probablility from the Players. Personally I believe this is important to ensure the game does not feel too easy for the increased party size (always a looming feeling you are cheating)

Firstly if designing an encounter I believe it is incorrect to focus on what Your team can do (what Your team's mods are). Instead think of what a generic team would be able to do in the situation.

The set up arriving at a LVL 1 DC 15 team check as a 4player team vs as a 8player team:

To a generic single adventurer a DC 15 represents a 75% chance of failure. To a 4 player team the DC 15 represents (75%)^4 which comes out to only 30% chance of failure. While to a team of 8 it is (75%)^8 = 10% chance of failure.

So all you need to do is set (75%)^4 = (X)^8 and Solve for X= SQRT(75%)= 8RT(30%)= 87%= 17/20, and so use DC = 17 for your 8 player team check. Where 8RT stands for 8th root.

How to design your DCs:

  1. Decide if only one person is attempting or every person in your group.
  2. If only 1 decide on their chance of failure and directly convert to DC.
  3. If Group check Decide on Group Chance of Failure and take the nth root to get your DC.

If transferring from one group size to another. Group 1 -> Group 2.

  1. Take chance of failing.
  2. Raise to the power of the size of group 1.
  3. Take the Root equal to the size of group 2.

Finally you want to consider the actual power of your group, the math might go a little wrong but the approach I use is taking the average mod of my team, eg Wis check of +1+3+5+4 = 13/4 = +3.25 and so add +3 to the DC of the overall check. This can seem cheap so use it on important checks, eg a door does not increase in difficulty as you level up.

For a quicker reference let me provide a table for you to see this change in group size in action. DCs to probability of failure for Multi Person chacks

If your group is a different size tell me and I will update else use the maths provided above.

PS. I agree with other answers that bring up Lore/Gameplay > Mechanics/Math, but strictly for how the math should be applied see this answer. I would also advise avoiding checks where everyone rolls, instead split the group in these situations. Only best 3 can persuade, only 2 can attack this door in this corridor, only those on watch can notice the Approaching enemy, timelimits on looting the rooms, etc.

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    \$\begingroup\$ For those who like tables, this is certainly helpful and useful. +1 \$\endgroup\$ – KorvinStarmast Sep 15 '16 at 21:34

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