Calculating Encounter Challenge Rating

[SailorLovins]

Ok I am embarrassed to say I am not 100% sure how an Encounter's CR is calculated. I know you don't just combine all of the NPC's CR together. So what do you do? What is considered? For example if I had 9 lizardmen (CR 1/2 each), 1 lizardman shaman (CR 2) and a Lizard Queen (CR 4) what would be the overall encounter CR and why?

[Zacchaeus]

There’s a table in the DMG that shows you the factors. It depends on the party level, the XP for all the NPCs and the number of NPCs.

https://dhmstark.co.uk/rpgs/encounter-calculator-5th/

[Three of Swords]

Note that if you do a search online, you'll find that many DMs don't feel the 5e encounter calculation system is very good. And lots of creatures with the same CR are pretty different, in terms of difficulty. Plus the DMG doesn't do encounter CR. It calculates Easy, Medium, Hard, Deadly.

Something else to note is that when you create an encounter using the 5e ruleset, it will calculate an Encounter Level for you. It's not how the 5e DMG works, but it's pretty good. If the EL matches your PC's level, it should be medium difficulty. If the resulting EL is one or 1 or 2 levels higher, it can be hard. 3-4 is usually pretty tough. 5+ can be pretty deadly.

[SailorLovins]

Yea I have been using this online generator for my random encounters.

https://donjon.bin.sh/5e/random/#type=encounter

[ProNobis]

The CR scale is based on total XP in the encounter. The DMG difficulty scale also factors in the number of enemies in the encounter. Personal experience (322 documented hours running games https://explorers-guild.github.io/Fatigue) says that for a tier 1 (levels 1-4) party of 4 that Three Swords is pretty close. As we start moving up through the tiers it becomes much more difficult to tell. Of note, there are a few outliers that I highly recommend banning as they are way mis-placed in CR (Specter is a CR 1 that damages Max HP and deals 3d6 damage, ie with a good role it can one shot kill [no death save rolls] a level 1 barbarian or a level 3 wizard)
Personal recommendation:
Tier 1 (1-4) start with the average party level CR for a party of 4, +1/2 per additional player
Tier 2 (5-10) start with the average party level + 1 CR for party of 4, +1/2 per additional player
Tier 3 (11-16) start with the average party level +2 CR for party of 4, +1/2 per additional player
Tier 4 (17+) start with average party level +3 CR for party of 4, +1/2 per additional player
Then
if they crush it (few spell slots used, little damage taken) increase CR by 2
if they make it look easy (half or less spell slots used, moderate damage taken) increase CR by 1
if it was a good fight (one of them knocked down but the healer got them up quick. Some spell slots left) leave the CR alone
if they had difficulties (knocked down 2 or 3 times but healer still has some spell slots left) lower CR by 1
if they had very big challenges lower the CR by 2
if you have a tpk on the first fight apologize and eat crow about how you underestimated the enemies, give them everything back and set the next fight at CR -3

Believe it or not, most players understand the GM making mistakes. If you own them, and learn from them then it will all be okay. If you try to bull through, they will have a bad time and that is harder for them to forgive.