Homebrew University: The Best Defence [5E Armour & AC Revisited]

Getting Hit never felt so good!

Join Demiplane Community Member Kyle Taylor (Stormchaser) in Homebrew University: a recurring blog that explores practical frameworks and perspectives to evolve popular Tabetop Roleplaying games to best fit you and your table’s tastes!

Hello Game Masters!

When Homebrew University first debuted here on Demiplane, I put some of the abstractions made by modern d20 games into context by discussing more explicitly what hit points and armour class really represent. Today, we’re going to take that a little further, focusing on the latter and the question of: How can we make the armour system deeper and more intuitive?

Under the helmet

Now, when working with armour while homebrewing for a classical d20-based system, you’re tweaking a very fundamental part of the game’s mechanics. Namely, the “to-hit” calculation. As such, it’s important to understand the balance implications of any changes that you make and, in order to do this in Dungeons and Dragons 5th Edition (5e), it’s vital to consider a design axiom known as “bounded accuracy”. Have you ever wondered why handing out +3 studded leather at the wrong time can unbalance your game? Let’s dive in.

Bounded accuracy is one of the foundations of 5e’s design philosophy and is, in fact, probably the single biggest thing differentiating 5th edition from its recent predecessors. In order to understand why bounded accuracy is so important to 5e, we have to look back about 10 years to the days of DnD 3.5 and its spiritual successor, the Pathfinder Roleplaying Game. This edition of Dungeons and Dragons is infamous for armour classes and attack bonuses that scale unlimitedly based on how well a given build could stack modifiers. Of course, this meant that it very quickly became impossible for less skilled combatants to hit a powerful enemy, short of rolling a natural 20.

Enter 5th edition.

5e sought to eliminate this problem by changing the way in which encounter difficulty scales. Instead of assuming a given progression of attack bonus and armour class, 5th edition’s design represents differences in power level by the number of hit points and the amount of damage dealt. It becomes possible to fight tougher monsters, because your character now deals enough damage to remove a significant chunk out of their hit points. Similarly, your character can now take a few hits from a powerful monster without being killed, thanks to your character’s own increased hit points. This shifts 3.5’s arms race of attack and AC to damage and hp. Encounters cease to be gated by whether or not a given character can finally hit the monster in question.

This concept is central to the balance of combat in 5e and is one of the most important considerations when homebrewing armour and defensive abilities.

Knowing your limits

So how does bounded accuracy affect how we homebrew? To understand this, we’ll look at some examples that illustrate where the important thresholds and “soft caps” of AC lie. To do this, we consider two important benchmarks:

  1. The 55% success chance around which Dungeons and Dragons is calibrated.
  2. The 10% hit chance that 5e aims to have as a minimum. Below this, an attack would only hit on a natural 20, which is what the principle of bounded accuracy seeks to avoid.



Based on this, player character armour class should almost always be 19 or below and should never exceed 21. This is our “design space”; the boundary conditions within which a balanced homebrew can operate. Note that magic items are not considered in this calculation for two reasons.

  1. 5e is intended to be balanced even without magical items.
  2. +1 armour and a +1 weapon cancel out, meaning that their influence can be neglected.

High level barbarians are a notable edge case here. A barbarian with a shield, +5 constitution and +5 dexterity has AC 22. A level 20 barbarian with +7 constitution can reach AC 24. However, this is a rare, high-level occurrence and, frankly, an unbalanced one that the game’s designers themselves should probably have considered more carefully.

Armour Revisited

Now that we understand the range of allowed armour classes and why this is the case, we are in a position to redesign the mechanics of how AC works. I previously explained that, in order to make armour function more intuitively, I change it to provide damage reduction (DR) instead of increasing armour class. How do we do this without unbalancing the game? We change AC to be a function of proficiency. Instead of a baseline of 10, AC is calculated similarly to saving throw DCs: 8 + proficiency bonus.

Of course, this still starts at 10, but it scales up to a baseline of 14 at tier 4. Thus a tier 4 character with a +5 dexterity modifier has an AC of 19 – our soft cap. In addition to this, shield proficiency is removed and replaced by light (+1), medium (+2) and heavy (+3) shields, each of which comes with the associated armour proficiency. Note that medium and heavy shields limit your maximum dexterity bonus to armour class, just as armour would. Thus, a light armour build can reach AC 20 at tier 4, while a heavy armour build can reach AC 17. Of course, heavy armour comes with other advantages…

In order to calculate the DR provided by a given armour type, simply subtract 10 from the AC values listed on page 145 of the Player’s Handbook. Thus, a suit of full plate armour provides a DR of 8! This is enough to completely ignore the damage from a d8 weapon, unless the attacker has a damage bonus or scores a critical hit. However, light studded armour only provides a DR of 2 and such characters rely more on evading hits than absorbing them.

These mechanics can be deepened further still by introducing a layering system that breaks armour down into a modular system of interacting components. In my homebrew compendium and Grit and Glory these are the underlay, mesolay, overlay and accessories. The overlay provides the bulk of the DR, but the other components can be especially effective at repelling a particular kind of damage. Thus, it becomes possible to customise your armour configuration to your needs, rather than everyone wearing the same generic ‘breastplate’ or similar.

I hope you’ve enjoyed this episode of Homebrew U! Find me on the Demiplane discord if you have more questions about homebrewing armour. Enjoy your games and I’ll see you next time on Homebrew University!


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