Monday, August 18, 2008

A Simple Algorithm for Generating DCs

As has become something of a habit for me lately, I wrote an adventure on the plane ride home from GenCon. Lately, it's rare that I have 3 to 4 consecutive hours to do anything other than (sometimes) sleep.

I'm a big believer that laziness is the mother of insight. Many folk see a tendency toward sloth as a bad thing, but if not for sloth, we'd still be living in caves, hunting brontosauruses, and hiding from thunderstorms. It's the human capacity for laziness that drives us to make short term sacrifices (inventing science was no easy feat) for long term pay offs (computers for everyone!).

Thus, I found myself writing an adventure while wedged into seat 24D with a pencil box, a notebook, and two Forgotten Realms sourcebooks (the new 4e one and the DM's book from the old gray box). I had put my D&D books into my luggage because, frankly, I didn't want to carry them around.

The monsters were pretty easy to generate for the adventure - I either remembered their levels, or I built them from scratch (the formulas are easy peasy to memorize, and again my commitment to laziness pays off - I don't need to carry the DMG).

However, I was a little annoyed when working on traps, terrain features, and just general stuff that needed DCs. I couldn't remember the DC progression. I puzzled over it for a few minutes, then realized that my laziness had saved me once again. In an insight so obvious that I feel like punching myself, here's Dr. Mearls' Patented, Super Easy, Instant DC Generator:

1. Start with 10.
2. Add half the trap, encounter, or challenge's level.
3. Do you want only people trained in the skill to have a shot at success? If so, add 5.
4. Think of the stat a PC would need to have a 55% chance of success, assuming the PC is trained if you want only trained guys to have a chance of success. Add that stat's modifier.

Voila! Instant DC.

Here's an example:
In a dungeon aimed at 6th level PCs, there's a locked door. It's a really good lock, so only a rogue or someone trained in Thievery has a chance to open it, but someone who is trained doesn't need much natural talent to open it.

+ 3 (half of level 6)
+ 5 (assume training)
+ 1 (a Dex 13 character who is trained should be able to handle this one)

Is a total DC of 19. Voila! An untrained character with a good Dex still has a 30% chance to open the lock.

If you know the PCs' stats, you can achieve a fine control of the DC by using the process above, plugging in the PC's stat mod, then adding 1 to the DC for every 5% you want to drop the success chance.

The nice thing about this method is that it pushes you to think of what the PCs can do, and it's simple enough (IMO) that you can use it on the fly.

OK, back to taking care of all the chores that have piled up since I've been away. I'll talk about GenCon later this week.


Anonymous said...

*sniff sniff* So intuitive it makes me cry. Plane trips should be a required part of the tail end of your development process.

Anonymous said...

Anyway, I kind of feel cheated. The better my PC becomes, the higher the DC of a Dungeon Door gets.

Geek Gazette said...

good post. I will have to use your idea.

Mike Mearls said...

Cyberpunk - Only if that's what you, as DM, want to do. In my example, consider the lock to be a level 6 lock. A better lock would be level 8, and so forth. The idea isn't that the lock becomes tougher as you become better - the lock is tougher because you are taking on a more challenging situation.

Level is a function of the challenge level posed by an environment, not the PCs' skills.

ChattyDM said...

Plus, why should doors still pose a challenge after a few levels?

You probably feel cheated because you are analyzing it outside of a game itself where you forget numbers because of the Rules of Cool and Fun.

I crunched the numbers and PC competence advances faster than DCs if you go by page 42. Mikes trick is great for plane ride authors who must have a game ready when they land...

Speaking of which Mike, I created the Adventure Project Kobold Love on my plane ride and it will be featured on my side of the Blogsphere over many weeks.

Having talked to you for some time, I think you will love it.

Anonymous said...


And really, your specific comment isn't something new anyways. As you go up in level the monsters get harder, the traps more deadly, and the doors stronger... because when they don't it becomes trivial. Which, yes, there are times to use trivial encounters but for the most part, trivial= wasted time rolling dice and to me is better as narrative.

DNAphil said...

You had a similar algorithm for the DC's for Zones in the Mastering Iron Heroes book.

I took those algorithms to heart, and have been using them for my IH game for the past three years.

I love these kinds of tips.

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