Learn to DM: Skill DCs

[Charon]

I've taught a few people to DM over the years and one question pretty much every new DM inevitably asks is "How do you determine skill DCs?".
This can be especially difficult for newer DMs when they need to come up with a DC on the fly, often leading them to simply pluck a number at random and usually meaning it's either way too easy or way too hard and rarely how they intended, especially when they're reluctant to go over DC 20.

Welcome to my guide on how to easily and reliably come up with a skill check DC in seconds.

Cheat. Well not exactly, but you need to learn that DC targets are an abstract. By default, the rule in DnD is that you do not tell the players the DC. That's partly to avoid metagaming and partly because you don't need to work out the actual DC.

What you should be focusing on is the chance of success. Let's use an example.

A wizard is making a Knowledge Arcana check. What's the DC?

Well, let's say you think this is something the wizard has a pretty good chance of knowing. Let's say roughly an 80% chance of knowing. Let's say this wizard is level 4 and has maxed out Arcana for 7 ranks. He has a +3 int mod giving him a +10 total bonus to Arcana.
That's the important information. He has a +10 to Arcana.

We want him to have an 80% chance of success and luckily, we use a d20 in DnD that measures success in 5% increments. Simply put, if we allow the wizard an 80% success rate then he passes if he rolls 5 or higher on the d20. The +10 doesn't matter (it kinda does and we'll mention why later) but for our calculations, it doesn't matter! He simply needs to roll a 5.

If we wanted him to only have a 20% chance of success he would need to roll a 17 or higher.

Note that we haven't calculated the DCs, but if you did want to, you simply add the wizard's modifier to the roll needed. DC 15 and 27 respectively.

This is literally how you do it. The player within you might be wondering, if it's just success based, then what's the point putting points in the skills? The bonus doesn't matter?

Well it does. Because we are subtly using that bonus to determine the success rate in the first place. The only reason we are giving the wizard an 80% success rate is because he has that +10 in Arcana. For an example, let's compare with the party's barbarian who somehow, don't ask how, has managed to acquire 1 rank in Knowledge Arcana. More amazingly, he has a 0 int mod, leaving him with a +1 to Arcana.

If he attempts the same roll as the wizard against the DC 15, he would need a 14 or higher to pass, a 35% success rate vs the wizard's 80%. Against the DC 27 he can't even succeed as he would need to roll a 26.

Likewise, if you'd worked it out the other way, reckoning the barbarian has a 10% chance of success (DC 20), the wizard would have a 55% chance on the same roll. Which is where the last piece of advice comes into play.

Players do not have to roll against the same DC! :O
That barbarian is illiterate. We gave our wizard an 80% success rate because he's a wizard who has maxed out his arcana across decades of study. Our barbarian can't read and has minimal knowledge of magic. We can give him 10% and have him roll against DC 20 while the wizard rolls against DC 15, further validating that skill point expenditure.

Even better, we can adjust these relevant to the circumstances. If the roll was related to reading something in a journal we can set the DC too high for the barbarian to achieve to represent the fact that he is illiterate and his knowledge of magic is more to do with mysticism.
Or, we can factor the other way. What if the roll was to do with mysticism, specific to the barbarian's own tribe? Suddenly the chances that the barbarian knows the answer as to what might ail the spirit of the land is 75% (DC 7) whereas the wizard with his highly book related knowledge might not be familiar with such a backwards view of magic and might be rolling with a 30% success rate against DC 25 on the off chance he has heard about it.

In summary. To easily come up with realistic DCs. Look at the skill being rolled. Determine how much that character has invested in that skill and identify any specific circumstances that might make it easier or harder for them and then decide what their chance of success might be based on that.

If you know what the chance of success is, then you know what they need to roll and you're done. If you insist on giving the PCs a DC to roll against then simply add their skill modifier to that roll to turn it into a DC.
If multiple players are rolling, decide if they are rolling against the same DC or if their chance of success needs adjusting. Now you're no longer plucking numbers from the air and being surprised when your players easily beat every check (or fail every time) because, as we've seen, that level 4 wizard can hit DC 20 55% of the time... or 100% of the time if he simply takes 10.