by Mike Shea on 21 February 2017
Note: This article has been updated since the original written in February 2015.
Below is a summary of the numbers you can keep in your head to help you run Dungeons and Dragons 5th Edition roleplaying games. These numbers can help you adjudicate challenge, traps, encounters, and environmental effects without needing to look it up in a chart or table. Though it looks like you could build monsters on the fly with these numbers, you're probably better off reskinning monsters in the Monster Manual instead.
Here are the numbers to keep in your head:
The flat math system of Dungeons and Dragons 5th Edition makes it easier to calculate mechanics on the fly. We've talked about customizing monsters by tweaking attributes as one such example. Unlike the damage per level cheat sheets we used back with 4th edition, 5e's flat math gives us more flexibility when determining the right number for the right challenge.
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 a roll. Likewise, 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. Instead, we can set a range of numbers useful for quantifying challenges for just about any situation in D&D.
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.
Whenever a situation comes up, consider how hard the situation is from easy (DC 10) to very hard (DC 20) and choose a number that feels right. No tables needed!
This number also works for an improvised armor class and save DC if needed. If you happen to improvise a trap or an effect of some sort, or the characters start attacking a stone statue, you can use this range to set the AC of the statue or the DC of the trap's saving throw.
Example: The Icebolt Trap
Say you've decided on the spot that a particular room has an icebolt trap in it. 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 14 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 likely for most situations. When you have an improvised attack, choose a bonus based on the severity of the attack.
Example: The Icebolt's Attack
Going back to our example from before, let's look again our icebolt trap. If a character fails to detect it or disarm it, it fires an icebolt at the one who triggered the trap with a +6 to attack.
If you need to throw in some improvised damage, 6 damage per challenge level is a good rule of thumb. You can roll 1d10 per challenge level if you prefer dice. This challenge rating isn't necessarily based on the level of the PCs but instead the level of the challenge they face. It's roughly the challenge faced by four characters so a challenge 6 is the equivalent of four level 6 characters. If this damage would affect more than one creature, you might want to reduce it to 3 (1d6) challenge level. 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, we'll have to decide how dangerous this icebolt really is and choose 6 damage per challenge level. You can always use the characters' level 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 a challenge 2 evil goblin wizard? Let's do 12 (2d10) damage. Was it Xathron the challenge 16 Lich Lord? Let's try 90 (16d10) damage. The power of the villain will give you the idea how much damage to dish out.
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 and requires very little work.
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, 15 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 rather than use 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.
This one is a little complicated and I wish to the gods I could make it simpler but I can't. I've tried all sorts of zany attempts to create simpler encounter building guidelines and this is about as simple as I can make it. Building encounters requires two steps:
Step 1: Choose the right monster or monsters to fit the story.
Step 2: Determine the number of monsters for the encounter:
If the monster has a challenge rating of roughly 1/3 the characters' level, use one monster per character.
If the monster has a challenge rating of roughly 1/4 the characters' level or less, use two monsters per character.
If the monster has a challenge rating of roughly 2/3 the characters' level, use one monster per two characters.
If the monster has a challenge rating equal to or up to two higher than the characters' level, use one monster per four characters.
To keep the number in your head, just remember the rule of 1/3. If a monsters' challenge rating is roughly 1/3 the characters' level, use one monster per character. If the monster's CR is higher than that, use fewer monsters. If it's lower than that, add some monsters. That's pretty much it. The 1/3 rule is a good rule of thumb to help you determine if a monster is roughly balanced to a single character.
Of course, keep in mind that many other variables will determine the challenge of an encounter such as the number of characters, the skill and experience of the players, the environmental terrain, how long the characters have gone without a short rest, and many other variables. The rule of 1/3 is a loose guideline at best.
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.