In traditional d20 combat, Armor Class operates as a strictly binary gate: you either take full damage or walk away completely unscathed. From a pure game-speed standpoint, this abstraction is legendary—one roll against one static target number resolves the entire exchange.
From a tactical and verisimilitude standpoint, however, it leaves a lot on the table. A high-Dexterity rogue and a full-plate fighter can easily share an AC of 18, yet mechanically, the game treats dodging through an open window of air identically to a battleaxe deflecting off a hardened steel breastplate. Furthermore, high-AC stalemates often lead to repetitive rounds where nothing happens except players and GMs taking turns announcing, "That misses."
To bridge the gap between fast-paced combat and the tactile reality of armor, here is a proposal for a Split-AC / Glancing Blow system that replaces the all-or-nothing threshold without adding complicated math or extra dice pools.
The Core Concept: Split Armor Class
Every combatant tracks two defensive numbers on their sheet: Evasion AC / Total AC (for example, 10 / 18).
Evasion AC (The Dodge Floor): Represents a creature's baseline capability to clear the line of attack entirely through agility, instinct, and footwork.
Total AC (The Armor Ceiling): Represents the creature's full protective package, incorporating physical armor plates, shields, natural carapaces, or deflection wards.
Calculating the Split
Player Characters:
Evasion AC = 10 + Dexterity modifier (plus innate unarmored traits, if applicable).
Total AC = Standard armor calculation (10 + Dexterity modifier + Armor + Shield + Magic).
Monsters & NPCs (Zero-Prep):
Evasion AC = 10 + Dexterity modifier.
Total AC = The standard AC printed in the stat block.
Examples in Play:
Knight in Full Plate (Dex +0): Evasion 10, Total 18 -- 10 / 18
Rogue in Leather (Dex +3): Evasion 13, Total 14 -- 13 / 14
Monk (Unarmored, Dex +3, Wis +2): Evasion 15, Total 15 -- 15 / 15 (no glancing band)
Adult Red Dragon (Dex +0, Natural AC 19): Evasion 10, Total 19 -- 10 / 19
Attack Resolution
When an attack roll is made, compare the total against the target's split threshold:
Below Evasion AC: Clean Miss. The blow misses entirely or is cleanly parried into empty space. Zero damage.
Between Evasion and Total AC minus 1: Glancing Blow. The weapon strikes armor, shield, or thick hide. Roll damage with disadvantage.
Equal to or Greater than Total AC: Solid Hit. The attack penetrates a joint, slit, or weak point. Roll standard damage.
Natural 20: Critical Hit. Maximum damage or your table's standard critical rules.
Resolving Glancing Damage
Instead of dividing damage by two or introducing flat damage-reduction subtractions at the table, the attacker simply rolls their damage die pool twice and takes the lower result:
Weapon Dice: Roll the weapon’s damage dice twice and keep the lower result.
Single-die weapons (d8, d10, d12): Roll two dice and keep the lower (for example, rolling a 7 and a 2 on two d8s results in 2 base damage).
Multi-die weapons (2d6): Roll 4d6 and keep the lower pair.
Modifiers: Flat modifiers (Strength, Dexterity, weapon enhancement) are added as normal. A glancing blow deals a minimum of 1 damage.
Handling Combat Riders
Burst Damage (Sneak Attack, Divine Smite, Superiority Dice): These effects require clean penetration to exploit anatomy or channel raw mystical power. They do not trigger on a Glancing Blow; they require a Solid Hit.
Ongoing Hexes (Hunter's Mark, Hex): The extra die is rolled with disadvantage alongside the weapon die.
On-Hit Conditions (Poisons, Grabs, Knockdowns): The defender makes their saving throw with advantage, as the blow lacked full purchase.
Magical Armor & Dexterity Offsets
A common point of debate with split AC is how magical enhancement bonuses (+1, +2, +3) should function.
Rather than bogging the game down with conditional math or tracking zero-caps on negative Dexterity penalties, apply the magic bonus directly to both numbers:
Rule: Magical Deflection
A magical bonus from armor or shields adds equally to both Evasion AC and Total AC.
A knight with Dex 8 (-1) in standard plate has a split of 9 / 18.
Putting on Plate +2 shifts both numbers upward by 2, resulting in a split of 11 / 20.
This keeps the width of the glancing band consistent while raising the floor, modeling how magical armor actively guides the wearer and subtly turns incoming hits into clean deflections before physical contact occurs.
Caveats & Mechanical Trade-Offs (Full Disclosure)
No house rule exists in a vacuum. Changing the core resolution engine introduces significant downstream shifts that every GM and player should consider before adopting it:
1. Combat Becomes Far More Attritional
Because the space between your dodge and your armor is no longer safe, damage lands far more frequently. Against a target with a split of 10 / 18, an attacker with a +4 attack bonus who used to hit 35% of the time will now deal damage (glancing or solid) 55% of the time.
Even with disadvantage on damage dice, static ability modifiers still add up. Front-line tanks who rely on high AC to stay upright will suffer steady chip damage over an adventuring day, which can alter the survival economy of long dungeon crawls.
2. Low-CR Mobs and Swarms Become Deadly
In standard play, a high-level paladin can walk through a dozen goblins without taking a scratch because their attack bonuses cannot clear the armor's high static ceiling. Under this system, those same goblins will land glancing blows regularly on rolls of 10 through 17. The horde becomes dangerous through sheer dice volume.
3. Concentration Checks Spike
For spellcasters, the frequency of damage is far more punishing than the size of the damage. A cleric in chain mail on the front line will be forced to make DC 10 Constitution saves nearly every turn.
(Table Fix: You may want to rule that Glancing Blows do not trigger Concentration saves unless the damage exceeds a set threshold, such as 5 or 10 damage).
4. Symmetrical Acceleration: Monsters Die Faster Too
The flip side of increased party attrition is that combat slog evaporates. Monsters with massive natural AC pools (dragons, golems, heavily armored bosses) will no longer cause round after round of frustrating party whiffs. Low attack rolls from secondary martial characters will steadily chip away at the monster's health pool, noticeably shortening combat length in terms of rounds.
Opening It Up for Feedback
This system is built to sit in that sweet spot between binary simplicity and crunchy simulation, delivering the verisimilitude of armor deflection while using existing d20 speed.
For those running gritty or low-magic campaigns: Does the constant chip damage feel like an asset that makes weapons dangerous, or does it overly penalize heavy-armor players?
How would your table prefer to handle flat ability modifiers on glancing hits—keep them flat, halve them, or drop them entirely so glancing blows deal only the low weapon die?
Is stripping rider effects (like Smite or Sneak Attack) on glancing hits a fair trade-off to keep heavy armor relevant against burst-damage threats?
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