During a recent dungeon, my character received wounds that were filled with a magical metal. The GM ruled that these wounds gave my character +1 AC and -9 Max HP until cured. My question is: should I bother to cure this status effect? Some related follow-up questions might be: what is the break-even point for -MaxHP/+AC? Is there a general-case way of thinking about this sort of status effect?

Character-specific details: the wounded character is a level 7 battlemaster fighter who wears platemail but no shield. The character has 14 CON and (unmodified) 66 Max HP. In other words, before the wound the character had 18AC/66HP; after, the character has 19AC/57HP. I'll gladly edit with any additional requested details.

Additionally, I know there are plot considerations and so on when something like this happens; for scope's sake, this question is strictly interested in the number-craft.


Yes, it's a very good trade-off.

Every level-up, you will gain additional hitpoints. However, +1 AC is hard to get once you maxed out your armour options (which you most likely already have at level 7). Especially as a STR-based fighter and considering the way lower AC totals high-level 5e characters can achieve.

The +1 will put you above other fighters, and even with magical armour (which is way harder to get in 5e than previous editions) you will be ahead of the curve.

In addition, every time you 'need' the +9 HP it means you need to spend resources to replenish them, either your own or your party healer's. The +1 of not getting hit doesn't cost you anything.

And finally, consider advantage/disadvantage. The +1 is even more valuable if you can get attackers to attack you at disadvantage, which in a full party is likely given spell-casters, maneuvers and other special abilities, especially when battling big baddies with high attack bonuses.

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Here's how to figure this out.

First, let's think about monster attack bonuses at your level. I looked at three example CR7 monsters. The giant ape has +9 to hit. The oni has +7 to hit. The shield guardian has +7 to hit. So let's say that the average monster has a +8 to hit you.

With an 18 AC, you'll be hit 55% of the time; with a 19 AC, you would be hit 50% of the time. The +1 AC will prevent 5/50=10% of incoming damage.

This is before accounting for all the things that would damage you without going through your armor class. Most damaging spells, for example, will ignore your armor class entirely.

If you assume that 75% of incoming damage is hitting your AC, then a 10% decrease in incoming damage is worth a 7.5% reduction in HP -- so, around 5 hit points.

Of course this is only an approximation. There are lots of things that could happen that would make this calculation different:

  • You could be attacked with disadvantage, so that improving your AC would be worth more
  • You could be attacked with advantage, so that improving your AC would be worth less
  • You could have a dedicated healer standing next to you, so that you can take more damage than your max HP
  • You could gain levels, so the -9hp would be a smaller fraction of your total

But this calculation will help you get a feel for what's correct in the general case.

Note that this math changes significantly depending on what your AC is right now. If monsters were hitting you only on a 19-20, and you got a +1 AC, that would reduce damage by a lot! (You might think it would reduce damage by 50%, but the natural 20 result deals more damage, so the actual number might be closer to 33%.)

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Personally I'd cure it. While a +1 AC might be good, you're losing almost a level's worth of HP. There are plenty of ways to improve your defense, especially with the battle master abilities. Plus, fighters need the HP: depending on your party, you're the guy I imagine gets targeted the most.

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