5e minus Attack Rolls

There’s a certain house rule I’ve seen a couple times that you can convert typical to-hit armor class in D&D Fifth Edition by subtracting 10 from the original value, and then using the new value as damage reduction (to a minimum of 0?). For example, 10 becomes -0 and 20 becomes -10. This is pretty straightforward, and also interfaces more cleanly with DEX modifiers (e.g., rather than having a base AC of 10 + DEX: your base AC just equals your DEX, and armor provides a bonus to this value).

The issue is that this obscenely decreases the damage output of characters.

Adding Proficiency Bonus to Damage

Let’s take the AC 15 orc, against which a typical starting character might have +5 to attack and +3 damage (the difference due to them not adding their proficiency to their damage roll). Originally, the attacking character has a 55% chance to land a hit, and deals on average 7.5 damage per hit (assuming a weapon with d8 base damage). This results in a complete average of 4.125 damage per attempted attack.

Meanwhile, let’s say that our player uses the new algorithm above. They roll d8 plus their ability modifier of 3, but subtract the orc’s armor reduction of 5. Their total is d8 + 3 – 5, or d8 – 2, which results in an average of 2.63 damage per attempted attack. This poses a significant detraction from characters’ abilities, and even slows down combat—which is not a desired outcome from trying to simplify it!

Here’s an easy solution which also makes the math more consistent across the board: add your proficiency modifier to weapon damage (if applicable). Now our player has a modifier of 5, which against the orc results in a plain d8 roll for damage: an average of 4.5! This is much closer to their original damage output per round, even being slightly higher.

Limiting Possible Armor Values

Another, maybe less problematic issue is that some AC values are higher than the typical character would be able to meet even while adding their proficiency bonus. Most of the ancient dragons have (original) AC values that exceed 20, such as the ancient red dragon with an AC of 22 (which would become -12). The tarrasque has an even higher AC of 25 (which would become -15). These may make sense in the context of rolling d20, and we were able to convert lesser values so they made sense with smaller dice, but now we’re getting pretty excessive.

For this reason, I’d suggest capping damage reduction to -10. Few monsters have an AC greater than 19 in the rules as written, so this would not particularly impact the game across the board; in fact, it would help achieve the desired effect that characters would feel more capable rather than missing half the time (since even a character with an attack bonus of +11 would have only a 35% chance of hitting a tarrasque).

Now, a character with a total bonus of +5 and even a small d6 weapon can deal at least 1 point of damage one-sixth of the time against a target with damage reduction of -10 (orig. AC 20+). Considering how few characters really need to use d4 weapons, which can deal up to 9 points of damage this way, we need not even consider that.