How to calculate D&D 5e challenge ratings when party has improvised weapons and no armor

Question

I'm running a game where the players are planning to attend a grand ball. At this ball, the players will not be able to wear their armor with their finery or smuggle in their weapons. The players are informed and agree that they'll likely be going in without weapons or armor, and know the risks of offending the Duke by trying anyway. That's all kosher.

So now, when I'm sketching out possible encounters when they'll be without armor, how do I figure out the challenge ratings? We use narrative-based leveling, so I don't need to worry about calculating exact XP based on challenge rating. I use Challenge rating calculators (like this one) to get a sense for how hard encounters will be for the party, and scale most of the encounters appropriately to what the players be able to handle. While our social contract includes the fact that encounters CAN sometimes be too hard to deal with head-on and that character death is a possibility, I still don't want to TPK an unarmed party because I calculated the CR wrong.

Are there a rules of thumb about calculating CR when the party doesn't have armor or weapons?

While I'm looking for guidance about combat without weapons and armor that will be generalizable to future scenarios and future parties at a different level, here's some specifics about the party.

• 4 players at level 7: 2 casters, 2 (mostly) martial
• While the spell slingers will probably have their arcane focuses etc. the fighting/arrow-shooting players will have to scramble to grab fire pokers, heavy vases etc. depending on the room. I've asked the everyone to review the rules on improvised weapons, and we'll have a little group refresher prior to the ball.

The top answer to this similar question (How do I balance combat encounters for an unequipped group?) is excellent, but it's very specific to a prison-break situation and that party's particular composition. In addition, most of the answers argued convincingly that that the prison-break situation needn't be a combat encounter; I'm still looking for guidance about balancing combat encounters. What I took away from this previous discussion is that it's going to be important to ensure a) that there are multiple ways for the players to address the encounter without fighting, b) that there are a couple off-ramps to let them decide to break off combat with a creative solution, c) that I remember that the players still have a lot of abilities even without their full equipment/spell component sets, and d) figure out some failure conditions that aren't just "everyone dies. the end." I'll definitely do all that.

BUT IF the players DO decide to stand and fight against a monster or monsters that actively want them dead, I want the fight to appropriately balanced so that they feel the effects of their decision to fight without armor etc. but that they don't feel like defeat was inevitable.

Answer 1

Roughly halve the number of enemies.

Martial types without armour in 5e (barring the barbarian and monk) lose a good two-thirds to three quarters of their AC. That means level appropriate enemies will hit them around twice as often.

Casters caught without buffs will often spend a round buffing, and their best effects vs groups of enemies are not going to be usable in a room filled with panicked nobles and guards.

No weapons on the martial characters means they will need to get their hands on a weapon. This might be an improvised weapon, a noble's rapier, a guard's polearm, or an enemy's weapon. At best, this is movement and object interaction. At worst, this is one or more turns.

It's hard to say specifics without knowing the level, stats, spells chosen, and player tactical ability of your PCs.

But in general with these negative environmental factors, you could roughly halve the number of enemies and still have a challenging fight. The casters will likely carry the day, but not immediately.

If you want to present them with a more typical encounter, have guards or nobles weigh in on their side of the fight (assuming they aren't being ambushed by the guards or something). NPCs to take enemy attacks or provide a bit of damage if lucky will help the party (turned into glass cannons by their lack of buff spells and armour) stay alive long enough to end the fight.