ThinkDM

AoE Spells vs. Mobs

Added 2020-01-15 02:22:34 +0000 UTC

This is a calculator I made for an upcoming article. It helps DMs adjudicate how many creatures die when you use an area of effect spell against a mob or horde. I've preloaded it with a certain number of low-CR creatures that you might typically see in a swarm/army. However, you can check any other creature by plugging in their HP and DEX/CON saves.

Here's how it works:

Determine how many monsters are within the range of the spell. For guidance on making this determination, check:

Adjudicating Areas of Effect in the Dungeon Master's Guide (p. 249)
Areas of Effect in the Player's Handbook (p. 204)
Areas of Effect on a Grid in Xanathar's Guide to Everything (p. 86)

Input the spellcaster's DC and number of targets within the AoE. This will calculate the adjusted HP of the monsters based on the save the spell targets, and assign an HP total per target. You can also add a custom monster in the last row of the chart by adding its HP and saves.

Roll damage.

If the damage rolled is more than the total mob HP, everything in range dies.

If the damage rolled is less than the total mob HP, consult the chart for the HP per target. Calculate how many targets the rolled damage could kill. An easy way to do this is to subtract "safe" targets from the total mob HP until you go under the damage total. Any mob not safe is dead.