by Mike Shea on 9 February 2015
The flat math system of Dungeons and Dragons 5th Edition makes it much easier to calculate math on the fly. We've already talked about customizing monsters by tweaking attributes. While we can certainly build 5e cheat sheets similar to what we had in 4th edition, we don't really need to. Instead, we can keep a few ranges of numbers in our head and use them as the situation requires.
In most cases, these numerical ranges go from "moderately easy" to "hard". We can omit "easy" numbers because most of the time these don't even require a roll. We can assume that difficulty checks (DCs) of less than 10 are easy enough not to warrant requiring a roll. Upper ranges work this way as well. Anything higher than DC 20 is going to be very hard to reach, so we don't have to bother keeping higher numbers in our head.
Now let's take a look at the numbers.
When a situation comes up that requires a difficulty check, choose a number between 10 and 20 as the target. The harder the challenge, the higher the number. A 10 is considered relatively easy yet still challenging enough to warrant a roll. A 20 is considered hard. You can go below or above it, but anything above 20 is going to be very tough and anything below 10 is going to be so easy its probably worth assuming an automatic success. Almost all reasonable checks will be between 10 and 20.
When in doubt, just use DC 15. It's an easy to remember number that represents a moderate challenge. Highly skilled people will find it relatively easy. Untrained people with a poor related attribute will find it relatively tough but still have a good chance. DC 15 is a nice number to keep in mind for just about any situation.
This number can also work as an improvised armor class and save DC if you happen to need such a number in an improvised situation such as a triggered trap or an animated attacking object or something like that.
Example: The Icebolt Trap
Say you've decided that a particular room has an Icebolt trap in it and you don't really feel like doing the work to figure out the specific math. How tough was the wizard who planted the trap? Was he an apprentice or an archmage? Choose a number between 10 and 20 to determine the difficulty of finding and disarming the trap. For this example, let's say this Icebolt trap has a DC of 15 to detect and disarm.
Likewise, if we ever have an improvised attack score, it's a safe bet that it will be between +3 and +10. Anything lower is going to be unlikely to hit. Anything higher is likely to hit often. There are some situations where the attack is lower or higher than this but this range is 90% likely for most situations. When you have an improvised attack, choose a bonus based on the severity of the attack.
Like DC 15, +7 is a nice easy number to remember and useful in most situations. +7 would be the base attack score of a 5th level character with an 18 attribute and a +3 proficiency bonus.
Example: The Icebolt's Attack
Going back to our example from before, let's consider an Icebolt trap. We'll go with our default of DC 15 to detect and disarm it. If it fails, it fires an Icebolt at the one disarming it with a +7 to attack.
If you need to throw in some improvised damage, 1d10 damage per challenge level is a good rule of thumb. This isn't necessarily based on the level of the PCs but instead the level of the challenge they face. If this damage would affect more than one creature, you might want to reduce it to 1d10 per two levels instead of one. Like the other number ranges, this isn't perfect science, but it will get you close to what you want.
Example: The Icebolt's Damage
Returning to our Icebolt trap example, you'll have to decide how dangerous this icebolt really is and choose 1d10 per level between 1 and 20. You can always use the PC's level as a gauge if you don't feel like doing any work but you're better off choosing the level of the villain who put the trap in place. Was it an evil goblin wizard (1d10)? Was it Xathron the Lich Lord (20d10)? The power of the villain will give you the idea how much damage to dish out.
If you get higher than 6d10 with the above trick, you can add a static number and remove some dice. For every 2d10 you remove from the pool of dice, add 11. This rounds the number out a bit but still has a good random amount of damage to it. For example, if Xathron the Lich Lord had really put that Icebolt in place, it would do 6d10+77 instead of 20d10. This removed 14 dice from the pool and added 11 * 7 to the roll. That's a much easier dice roll than 20d10.
If you need to improvise some hit points for an object, 20 hit points per challenge level of the object is about right. This won't match up perfectly to the hit points of monsters in the Monster Manual or the Monster Statistics by Challenge Rating chart on page 274 of the Dungeon Master's Guide, but it will get you close.
Example: Xathron's Icy Automaton
Let's say the PCs have invaded Xathron the Lich Lord's treasure vault and inside is Xathron's Icy Automaton. This isn't Xathron's best guardian, but it's pretty solid. We'll consider it a level 5 challenge.
The PCs fail to notice the Automaton's danger (failed on a DC 15 perception) and it begins to fire Icebolts at random PCs (two attacks, +7 to attack, 2d10 damage). The PCs can't seem to get it disarmed (failed on three potential DC 15 Arcana or Athletics checks) and now they want to bash it down (AC 15, 100 hps). After inflicting 100 damage to it, the automaton falls apart.
Looking at these number ranges, you may be tempted to use them to build a monster. Instead, consider reskinning an existing monster from the Monster Manual instead of using these numbers to build one from scratch. While you might be able to build a reasonable monster with these scores, the asymmetrical nature of the stats in the Monster Manual makes creatures much more fun to fight than a static box of perfectly aligned scores.
Instead use these scores to build improvised challenges right at the table and worry less about having to prepare everything ahead of time.
In summary, here are some numbers to keep in your head:
With those numbers in mind, you have a simple toolbox for running all sorts of challenges for your D&D 5e group.
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