Spread out AC or consolidate?

Question

Imagine a Party with 4 characters. Each one has different AC:

• 14 AC/48 HP (squishy caster)
• 15 AC/50 HP
• 16 AC/50 HP
• 17 AC/54 HP (the tank)

The Party picks up a Ring of Protection +1. Who "should" get it for Party survivability?

• The squishy caster to even things out?
• The tank to make them tankier?
• One of the others, to make them generally tankier/hardier?

I know that it ultimately becomes situational (Does the squishy caster run into danger all the time? Does the tank get distracted with picking flowers on the side of the battlefield?), but assuming a "typical" marching order and battle roles, who should get it?

I'm torn between making the tank tankier or bolstering the caster.

Is there general wisdom or is there a model that applies? Does it purely come down to: 'who will be the target of weapon attacks more often?'

Answer 1

Generally-speaking, AC is stronger the more AC you have.

For instance, an enemy that has to roll a 19 to hit has a 1/10 chance to hit, and would have a 1/20 chance if they needed a 20. A single +1 AC can effectively double your defense (assuming it's high enough).

You'll probably get more value adding AC to those that anticipate being attacked, while your other members should get the non-AC benefits (THP, save bonuses, etc).

Of course, if your DM is the kind to ignore the 20 AC Fighter, it's probably better to spread out those items or have the Fighter pick up things that make him a better target (like the Sentinel feat or the Cavalier subclass).

Answer 2

This can probably only be determined empirically

Because of the wide variation in play styles among both players and DMs, I don't think it's possible to give a general recommendation. The best approach I can think of is the empirical approach: pick a reasonable objective to optimize, and dole out the defense buff based on who has historically been most lacking in that area. The objective should be related in some way to survivability, such as:

• which character gets hit the most (total number of hits, not fraction)
• which character takes the most damage
• which character gets knocked unconscious the most often

Additionally, the objective should be something the ring can influence. In this case you should consider hits/damage/KOs from both attacks and failed saving throws, since the Ring of Protection affects both AC and saving throw modifiers.)

Let's say you choose the first one. Now ask: among the party, who takes the most hits in practice? If the tank with 17 AC is doing a good job of soaking up attacks, maybe it's them. If the tank is not doing a good job, or if the party has been frequently put in situations where the tank is unable to do so (e.g. swarms of enemies or big AOE spells), then maybe the squishy caster is taking too many hits due to their low AC. Maybe the 2 characters with the highest AC (16 and 17) have both ended up tanking equally, but the 16 AC character is getting hit more due to their lower AC, and they need the buff most. You don't actually need to count up every hit in every past fight; you probably have at least a vague sense of the answer without having to do so.

In summary:

1. Choose the empirical statistic you most wish to optimize.
2. Decide who is doing the worst in this statistic, and give the item to them, since they need it most.

I believe this will have the greatest impact because ideally, you want to spread out incoming damage exactly in proportion to each character's hit point maximum (i.e. the opposite principle from focusing one enemy down at a time). In the most ideal case, if damage is spread out so that every character takes 99.9% of their hit point maximum (rounded down) in damage, they will all still be standing while having taken the maximum amount of damage possible. Shoring up the defenses of whoever is taking too much damage gets the party closer to this ideal case.

Lastly, I'll note that this all assumes that future combats will be similar to past combats, since you are using past combat performance to make a decision that will affect future combats. If you have reason to believe that future combats will be substantially different, then you will need to adjust accordingly.

Answer 3

Below is a table that lays out how increases in armor class affect relative survivability of a character. For each Armor Class, we assume the character has 100 hit points, and we calculate (on average) how many rounds it takes for a level 1 character with 16 strength, wielding a greatsword, to knock them down to 0 hit points. We will then use this to compare (again, on average), how much a 1 point increase in Armor Class improves their survivability.

	Base AC	+1AC from Magic Item	% Difference	"Effective HP"	"EHP" w/+1AC
AC5	10.771	10.771	0.000%	100	100
AC6	10.771	10.771	0.000%	100	100
AC7	10.771	11.349	+5.366%	100	105
AC8	11.349	11.992	+5.362%	105	111
AC9	11.992	12.713	+5.671%	111	118
AC10	12.713	13.526	+6.011%	118	126
AC11	13.526	14.449	+6.388%	126	134
AC12	14.449	15.508	+6.829%	134	144
AC13	15.508	16.735	+7.332%	144	155
AC14	16.735	18.172	+7.908%	155	169
AC15	18.172	19.879	+8.587%	169	185
AC16	19.879	21.941	+9.398%	185	204
AC17	21.941	24.479	+10.368%	204	227
AC18	24.479	27.682	+11.571%	227	257
AC19	27.682	31.848	+13.081%	257	296
AC20	31.848	37.491	+15.052%	296	348
AC21	37.491	45.564	+17.718%	348	423
AC22	45.564	58.066	+21.531%	423	539
AC23	58.066	80.019	+27.435%	539	743
AC24	80.019	128.569	+37.762%	743	1194
AC25	128.569	128.569	0.000%	1194	1194

Obviously, at unrealistically low armor classes, the +1 does nothing; the greatsword-wielding character cannot roll lower than a 6 on their attack roll, so they'll hit 95% of the time against any character with an armor class of 7 or lower. Conversely, as the armor class goes up, each point of armor class represents a greater and greater relative survivability, not just compared to the lowest armor classes, but relative to the next highest armor class.

This tells us that, in general, each point of Armor Class becomes more valuable proportional to how much a character already has. A paladin with 20 armor class gains a much larger survivability boost from going to 21 armor class than a wizard at 15 armor class gains from going to 16.

This also doesn't take into account the raw hit point differences between a highly armored paladin and a lesser-armored wizard, although we could apply the "% Difference" column from the table to work that out.

So it's transparently obvious from the example at your table that the "tank", with 54 hit points and 17 armor class would receive the most substantial boost in survivability by going to 18 armor class.

Of course, that isn't to necessarily say that they are the one who should receive the ring. If the weakest character is frequently vulnerable to attacks, then that +1AC might be the difference between them going down or staying up at a critical moment, in a way that the "tank" is already not particularly vulnerable. That is a judgement that your party will have to make for themselves.

But if the goal is raw survivability of the party, then any item that boosts armor class should always be given to the party member who already has the highest armor class, up until the point where their armor class exceeds what their enemies can roll on a natural 20.

Answer 4

There will never be a "one size fits all" rule

That being said consider extra survivability options different classes, feats and spells may give beyond the standard HP and AC survivability of the group. Some general things to consider:

• Barbarian's Rage: This provides resistance to damage from mundane attacks (piercing, slashing and bludgeoning) - this means a boost to AC isn't blocking as much damage on this character as it may on a different one with equal AC
• Druid's wild shape: Wildshape gives a great pool of HP and, whilst using it, this class won't benefit from the extra AC so the ring would be better on a different character to improve survivability of the group
• Healers: You want to increase the chance these survive to bring back the rest of your party - this would be my safest bet for the increase to AC. From experience if you've got a cleric who is worried about stepping into the fray to cast their cure wounds or is the first to fall then the survivability of your party drops drastically.
• Sentinel: This feat is amazing for that front line fighter. If you have a party member with this feat I'd give them the ring even over the healer - if no one can get through to your healer then you don't need to worry so much about them taking damage.
• Spells: Anything that ensures your tank is center of attention for the BBEG (i.e. Compelled Duel) is a tactic that requires the tank to have the highest AC possible.

At the end of the day, however, a ring is great for situations like this. You don't need to know before going in who is most in need because the ring can change hands on long rests. Just play with your party, see what sorts of encounters are coming your way, and switch the ring around when needed.

Answer 5

My inclination would generally be to:

a) if the party mostly adventures in confined spaces where one person can defend the others, it best goes to the existing tank, or

b) if the party adventures in more open areas, and/or fights enemies that are more mobile, it best goes to the third character, the 16 AC/50 HP guy, to create a second good tank to help hold the line.