Party in, monsters in, difficulty out, using the 2014 DMG maths. Plus the thing those tables never tell you: whether the fight is going to take all night, which is a different question from whether it is dangerous.
They are not predictions of death. They describe how much of the party's resources a fight is likely to cost, and they assume a party that has already spent some of the day.
Six monsters worth 200 XP each are much more dangerous than one monster worth 1,200. They get six turns to your four, they spread damage across the party, and they keep acting after the first two are down. The multiplier is a rough correction for that. The raw XP total on its own will badly under-rate a swarm.
It also cuts the other way. A party of five or six gets more actions than the tables assume, so the multiplier steps down; a party of two or three gets fewer, so it steps up.
Difficulty is about risk. It says nothing about time, and time is what actually ruins sessions. Twelve goblins against four characters may only be a Hard encounter, but it is sixteen initiative turns per round, and the table will feel every one of them.
That is why this calculator flags the ratio separately. If you want a long fight, that is fine and you are choosing it. The problem is choosing it by accident and losing the second half of your session to it.
These are the 2014 Dungeon Master's Guide thresholds and multipliers, which is what most tables and most tools still use. The 2024 rules take a simpler approach with a single XP budget and no multiplier. If you are running 2024 strictly, treat the numbers here as a sanity check rather than the rule.
Encounters sized to your actual party, from a DM tool that also records your session, writes the recap, and keeps your campaign wiki up to date.
See how it works