Splitting up Hit Dice: Mass and Fighting Ability

D&D, and through descent, ACKS, uses Hit Dice that combine both the actual capacity to take physical damage (“massd”) and the ability to avoid taking physical damage (“fighting ability”). The proportion which HD represent for any given creature are not always self-evident, but, e.g. a 9th level fighter’s HD surely represent fighting ability and a 9HD elephant’s HD surely represent mass.

Hit Dice also determine a creature’s accuracy in combat. Always keeping in mind that armor makes the target harder to hit, accuracy in combat represents both fighting ability (hitting the target) as well as armor-penetration (breaking through a target’s armor) from mass. The proportion which HD represent are, again, not self-evident, but presumably a 9th level fighter’s attack throws have improved by means of fighting ability, while a 9HD elephant’s attack throws are superior due to its ability to smash through puny man-sized armor.

The fact that Hit Dice include both mass and fighting ability is a masterful simplification but it creates many oddities that have plagued all D&D-like games since inception.

  1. High HD animals have Olympian accuracy, even when attacking unarmored targets whose AC is derived from high DEX.
  2. Lost hit points on creatures where they represent physical injury (e.g. elephants) recuperate at the same rate as lost hit points on characters where they represent mere fatigue and erosion of fighting ability.
  3. When attacked by surprise or back-stabbed, characters can absorb lethal blows through superior fighting ability, defending themselves (as it were) against attacks they were unaware of.
  4. Curative magic will restore morale and fatigue when cast on some creatures, physical damage when cast on other creatures; small, weak creatures can never benefit from a cure of serious or critical wounds as they are incapable of being dealt such wounds (too few HP).

The following house rules are intended to differentiate Mass and Fighting Ability.

*Please let me know what you think! I have broken each section of the rules into its own post so that you can reply to the specific rules section that’s relevant to you.

Calculation of Mass HP and Fighting Ability HP.

  1. For man-sized creatures (including most PCs)
    Mass HP = CON / 2

  2. For creatures of varying size with a known CON score,
    Mass HP = (0.2)*(weight^1/3)*CON

  3. For creatures of varying size without a CON score,
    Mass HP = 2*(weight^1/3)

  4. All remaining HP are Fighting Ability HP.

  5. If a character has more Mass HP than he has actual HP (as might be the case for a high-CON mage) then reduce Mass HP to his actual HP. He will accumulate his remaining Mass HP as he advances in level. This represents a character who is out-of-shape, unhardened to battle, fearful of pain, etc.


  1. Marcus is a 7th Fighter with 18 CON. He has 54hp. Marcus has (CON/2) 9 Mass HP. He has (54-9) 45 Fighting Ability HP.

  2. Theog is an Ogre with 11 CON. A 4HD+1 monster, he has 22hp. He weighs 1,000lb. Theog has (0.2)*(1,000^1/3)*11 Mass HP, or 21 Mass HP. He has (22-21) 1 Fighting Ability HP.

  3. Dumbo is an Elephant with 9HD and 40hp. He weighs 9,000lb. Dumbo has 2*(9,000^1/3) Mass HP, or 41 Mass HP. Dumbo has 40 Mass HP (using the lower of actual and mass) and no Fighting Ability HP.

  4. Simba is a Lion with 5HD and 25 HP. He weighs 500lb. Simba has 2*(500^1/3) Mass HP, or 16 Mass HP. Simbas has 16 Mass HP and (25-16) 9 Fighting Ability HP.

Differentiating Dodge Class and Armor Class

  1. Armor Class is sub-divided into Dodge Class (DC) and Armor Class (AC).

  2. Armor Class is improved by armor, shields, magical adjustments to armor and shields, Weapon & Shield proficiency, magical items “of armor”, and spells that create magical armor or shields or render the subject invulnerable.

  3. Dodge Class is improved by DEX, Bladedancer’s Graceful Fighting, Swashbuckling proficiency, magical items and spells “of protection”, and spells that improve AC through luck, speed, or divine favor.

EXAMPLE: Aurelyn is a 7th level bladedancer (Graceful Fighting +2). She has DEX 16 (+2) and Swashbuckling proficiency (+2). She wears Bracers of Armor AC 3 and a Ring of Protection +1. Her AC is 3 (from the bracers). Her DC is (2+2+2+1) 7.

  1. Armor Class: Well-armored targets are harder to hit than lightly-armored ones. The target’s Armor Class is added to the attack throw value necessary to hit it.

  2. Dodge Class: Highly dexterous targets are harder to hit than sluggish ones. The target’s Dodge Class is added to the attack throw value necessary to hit it.

Attack Throw and Armor Penetration

  1. Since armor makes a target harder to hit, some portion of a creature’s improvement in its attack throw actually represents its ability to break through armor. This factor will now be separately represented as “Armor Penetration”.

  2. Find a monster’s Attack Throw by dividing its Fighting Ability Hit Points by 4.5 and using that value as its HD on the “Monster Attack Throws table” in the How to Attack section of ACKS.
    a. If the monster has 4 or fewer Fighting Ability Hit Points, it attacks as a “Less than 1HD” Creature.
    b. If the monster has effectively negative Fighting Ability Hit Points (e.g. its Mass HP exceeded its base Hit Points), then it attacks as a “Normal Man”.

  3. Find a monster’s Armor Penetration by first dividing its Mass Hit Points by 4.5 and using that value as its HD on the “Monster Attack Throws table” in the How to Attack section of ACKS. This will give an attack throw value. Armor Penetration is equal to 11 minus this value.

  4. Armor Penetration: When attacking, a character may ignore a number of points of the target’s Armor Class equal to his Armor Penetration. Dodge Class is not affected by Armor Penetration.

Theog the Ogre has 21 Mass HP and 1 Fighting Ability HP. With 1 Fighting Ability HP, he attacks as a 1HD or Less monster. 21 Mass HP/4.5 = 4.67. Consulting the Monster Attack Throw table for “4+ to 5 HD” yields an attack throw of 6+. Theog’s Armor Penetration is (11-6) 5 points. Theog is not very accurate (10+ attack throw), but he can smash through chainmail and shield (AC 5) like cloth.

Dumbo the Elephant has 40 Mass HP and no Fighting Ability HP. Dumbo attacks as a Normal Man, because his base HP were actually lower than his Mass! Dumbo’s Armor Penetration is very good, though. 40/4.5 = 8.9, putting him on the 7+ to 9 HD line for a 3+. That yields (11-3) 8 points of Armor Penetration. Dumbo is something of a wild and imprecise attacker but no mundane armor or shield is going be useful against his titanic strength.

Simba the Lion has 16 Mass HP and 9 Fighting Ability HP. Simba attacks as a(9/4.5 = 2) 2HD monster, with an attack throw of 9+. Simba’s Armor Penetration is (16/4.5 = 3.6; 3+ to 4HD attack throw 7+; 11-7=4) 4.

(The net effect of these rules is to make lightly-armored, dexterous heroes more advantaged versus slow-moving and clumsy monsters. A shirtless barbarian hero like Conan can actually fight a monster and win through virtue of his high DEX and Swashbuckling proficiency making him hard to hit. Armored characters are not worse off, of course.)

Very interesting. Going to take me a couple reads to fully process this.

Given this reminds me of the same, did you get a chance to read through the Conan d20 you Ebay’d?

Combat, Mass HP, and Fighting Ability HP

  1. Except where otherwise noted, damage is first dealt to Fighting Ability HP. When Fighting Ability HP are reduced to 0, remaining damage is dealt to Mass HP.
  2. When a creature is reduced to 0 or fewer Mass HP, it is incapacitated and must roll on the Mortal Wounds table.
  3. If a creature takes 1/4 its Mass HP in one attack, it suffers a -2 penalty on all attack throws next round due to pain. (?)
  4. If a creature takes 1/2 its Mass HP in one attack, it must make a saving throw versus Death. If it fails, the creature collapses from shock. (?)

Critical Hits

  1. An attack throw that exceeds the target value by 8 or more is a critical hit. The target must make a saving throw versus Death. On a failed saving throw, the attack deals double maximum damage to the target’s Mass HP. On a successful saving throw, the attack deals maximum damage to the target’s Fighting Ability HP.

EXAMPLE: Brad (10th level Explorer) is attacking Smog, a huge venerable dragon (65,000lb; Mass HP 40, Fighting Ability HP 50). Brad has 18 DEX and Missile Fighting proficiency, wields a +2 bow, and has been Blessed and Inspired with Courage. His attack throw is (4-3-1-1-2-1-1) -4. His +2 arrows will deal 1d6 (+1 +4 +1 +1) +8 damage. Smog has AC 12. Brad needs (-4+12) 8+ to hit and 16+ to critically hit. He rolls an 18, and critically hits! Smog must make a saving throw versus Death. Sadly Smog rolls a 2, and fails. Smog takes 6x2+8=20hp to his Mass Hp. This is 1/2 Smog’s Mass HP in one attack. Smog must make a saving throw versus Death. He fails again! Smog collapses from shock. Brad calls his friend, the bard Talkin, to write this crazy outcome down.

Surprise, Ambush, and Back-stabbing
1a. Any attack by surprise deals damage to Mass HP. (Hardcore/Gritty)
1b. An attack by surprise deals 10% damage to Mass HP and 90% damage to Fighting Ability HP. If the character has Ambush or Backstab, he deals a greater percentage to Mass HP, equal to 10% times his multiplier. (Heroic)

Yes, I did! I quite liked many of its rules. I also recently read MERP, Rolemaster, GURPS, and a bunch of other systems. I’ve been trying to parse through some of the assumptions of ACKS to get deeper insights into the structure of the game and work out how it might vary.


  1. Mass HP heals at a rate of 20% of maximum per week of rest. This rate can be increased by Healing proficiency.
  2. Fighting Ability heals at a rate of 20% of maximum per turn of rest.
  3. Cure Light Wounds and similar magic instantly heals 20% of Mass HP. It can also heal certain permanent wounds: 26/3, 21-25/4, 16-20/5, and 11-15/6.
  4. Cure Moderate Wounds instantly heals 40% of Mass HP. It can also heal these permanent wounds: 26/2, 21-25/3, 16-20/4, 11-15/5, 6-10/6.
  5. Cure Major Wounds instantly heals 60% of Mass HP. It can also heal these permanent wounds: 26/1, 21-25/2, 16-20/3, 11-15/4, 6-10/5, 1-5/6.
  6. Cure Serious Wounds instantly heals 80% of Mass HP. It can also heal these permanent wounds: 21-25/1, 16-20/2, 11-15/3, 6-10/4, 1-5/5.
  7. Cure Critical Wounds instantly heals 100% of Mass HP. It can also heal these permanent wounds: 16-20/1, 11-15/2, 6-10/3, 1-5/4.

Healing permanent wounds using cure spells takes time equal to that listed by the Condition on the Mortal Wounds table. If there has been a limb loss, the lost limb must be available as the spells cannot regenerate, merely re-stitch.

Using a higher-level spell than the permanent wound requires increases the rate of recovery by one condition level.

Healing proficiency can “surgically” repair permanent wounds at the appropriate level through successful proficiency throws. Only one throw is permitted per wound, and the surgery must take place within one hour of the wound.

(In the genres of fantasy which these rules are intended to emulate, Restore Life and Limb will be unavailable, and critical hits will make permanent wounds more likely. These rules offer an alternative means by which such damage can be repaired).

Healing Fighting Ability HP
Presumably spells might exist to restore lost Fighting Ability HP, perhaps representing inspirational morale. Perhaps Bards can restore FAHP through song. TBD.


  1. Poison, paralyzation, and energy drain - or “how are you suffering these consequences if you didn’t actually get hit”
  2. Smashing through armor, or “shouldn’t two-handed swords get a bonus to hit”
  3. Strength and attack throws, or “should strength apply to hit or to armor penetration”
  4. Magic item bonus and attack throws, or “should magic weapons provide a bonus to hit or to armor penetration”

[NOTE: Due to the awkward acronym FAHP, I am changing the term Mass HP to Life Points and Fighting Ability Hit Points to Hero Points, or LP and HP].


  1. Special Maneuvers: Special maneuvers (such as sunder or disarm) ignore Armor Class. Defense Class still applies. The target’s Defense Class against the special maneuver is increased by +1 per 5 Hero Points [Fighting Ability HP] remaining. The target receives a saving throw v. Paralyzation to avoid the effect.

EXAMPLE: Marcus, a heroic warrior with AC 10, DC 0, 18 LP and 50 HP, is fighting a skilled duelist (attack throw 2+). The duelist decides to disarm Marcus. The duelist needs 2+, modified by 50HP/5, 10, or 12+. If the duelist hits, Marcus will have to make a save v. Paralyzation or be disarmed.

  1. Smashing Through Armor: An attacker may reduce his weapon’s damage dice by one die type to gain +1 to hit, or two dice type to gain +2 to hit. Weapons may not be reduced below 1d4.

EXAMPLE: Gallantine, a 1st level fighter (attack throw 10+) is wielding a two-handed sword (1d10 damage). He is fighting Morgex, in plate armor and shield (AC 7). He is having trouble hitting Morgex, so he decides to try to “smash through armor”, reducing his weapon’s damage by three die types (d10->d8->d6->d4) in exchange for a +3 to hit. Now he needs 14+ instead of 17+.

  1. Poison, Paralyzation, and Energy Drain: A blow that misses due to DC has been dodged. A blow that misses due to AC has struck armor. A blow that hits and damages Hero Points has connected in some manner with the target, but it was parried or blocked. If the attacker’s damage deals Poison, Paralyzation, or Energy Drain, the target must make the appropriate saving throw to see if he was knicked/touched in the course of his parry. [This is more-or-less how zombie movies work. The hero tends to make his saving throws and avoid being “bitten” even though he’s clearly being beat-up during the fight.]

EXAMPLE: Lucas, an unarmored survivor of the ghoul-fever apocalypse (AC 0, DC 0, LP 5, HP 10) is attacked by a ghoul (attack throw 9+). The ghoul strikes and misses with two claws but hits with its bite. Lucas loses 4 HP. He must now save versus Paralyzation. If he succeeds, then the bite didn’t break skin - e.g. he actually was hit by the creature’s jaw bone slamming onto his forearm. If he fails, the bite broke skin and he is paralyzed.

  1. Mage’s Hero Points: A mage’s hero points don’t represent fighting ability so much as they represent the magical wards and enchantments that every mage of power surrounds himself with. Narration of the effects of hits versus the mage’s Hero Points should describe sparks of magic, strange deflections in thin air, etc.

  2. LP, HP, and Zero Level NPCs: Normal men have their Life Points. When they earn a level of experience, the hit die is gained as Hero Points. Starting PCs will thus have 1/2 CON LP and a die roll’s worth of HP. Normal monsters have their Life Points and default Hero Points based on their innate ferocity (like lions). Monsters of above-average HD for their type, such as orc chieftains, gain their additional hit points as Hero Points. Heroic monsters (a dragon highly experienced in war, for instance) might also have additional HP in this manner.

  3. Ability Score Bonuses to Life Points and Hero Points: Each level, up to 9th level, a character’s Life Points are increased by 3 points, to a maximum of CON. Each level, up to 9th level, a character’s Hero Points are increased by his WIS modifier. This (a) helps characters be a bit more survivable in the face of nasty critical hits, and (b) makes WIS an ability of more utility, and goes along with the idea that WIS improves saving throws.

EXAMPLE: Marcus has WIS 13 and CON 18. At 1st level he has 9 (one half CON) +3 (for one level) LP and 5 (roll of 1d8 +1 WIS modifier) HP. At 2nd level he gains 3 LP and (roll of 1d8+1 yielding a 6) 6 HP, for a total of 15 LP and 11 HP. At third level he reaches his maximum of 18 LP and gets another 1d8+1 HP, giving him (e.g.) 18 LP and 16 HP.

One of the reasons ACKS stands out to me is that there’s obviously a lot of math going on behind the scenes, but the “end-user” doesn’t have to deal with it directly, unless they want to.

I doubt I’ll use these rules (I highly value ACKS’ simplicity-in-play and how it contrasts with the depth-in-design), although I am a little tempted to. It would require going over every monster and explicitly listing their Atk Thr and APen, and maybe listing HD in a new form, with the prerequisite of knowing their weight…

But just the fact you can do the math to get these results is impressive.

Thank you for the kind words!

If we were to ever publish these rules, I’d provide that data to the end-user, of course. I have all the weights of the monsters worked out in a spreadsheet so it’s as easy as just entering the formula.

Heck, if there’s interest, I could publish it on the forums or blog.

There’s a tiny accountant trapped inside me, desperate for that data! I’d love to see it (with the pre-worked-out split HD, etc., if possible).

I like the Mass HP/Fighting HP rules.

Does the Dodge/Penetration system require two rolls to hit (one to defeat the dodge, another to penetrate the armour)?

No, it’s one attack throw. Here’s an example. An ogre (attack throw 10+, armor penetration 4, damage 1d12) is attacking Aurelyn, a bladedancer (AC 2, DC 7, MHP 5, FAHP 15).

The ogre’s attack throw target value is increased by 7 due to her DC. It is increased by (2-2) 0 from her armor. The ogre needs a 17+ to hit Aurelyn (20%).

Under traditional ACKS rules, the ogre would have had an attack throw of 6+ versus Aurelyn’s AC of 9, and would have needed 15+ to hit her (30%). So the chances of Aurelyn being hit have been reduced by about 1/3.

I didn’t mean to add the acronym “FAP” to the game.

Mass HP shall henceforth be called “Damage Points” (DP)
Fighting Ability HP shall henceforth be called “Hero Points” (HP)

OK. I’ll post it at some point, after more folks have given feedback on the thread.

I’ve seen Wound Points / Stamina for similar splits in other games (Traveller d20, SWd20, probably others). I like the idea here, but keeping armor penetration separate runs into the same issue it does in other systems where armor remains AC - limited conditional addition. Determining whether an attack hits now involves a roll, a subtract from Fighting to-hit, a query regarding value of armor worn by target, a subtraction / min / max operation (armor pen - armor value, min 0), an addition (of AP-AV to roll-THAC0), and another comparison with AC. That’s a lot of extra work compared to the basic roll, subtract, report AC hit!

I actually like Golan’s idea regarding separating to-hit and pen rolls. Two rolls is much easier to handle numerically - you roll 2d20 at the same time (in different, predesignated colors), perform two subtractions (your THAC0 and TPAC0), and then report a tuple “Hit AC n and penetrate armor m”. The trick here is getting the constants right to keep the probabilities about the same as they originally were (or to ensure other intended outcomes).

As for TBDs:

  • One could assume scratches and glancing blows; perhaps save at +4 if no body damage was inflicted?
  • This is one place where separating hit and pen rolls makes life easier. Some weapons might have +hit (daggers, fast weapons), some might have +pen (warpicks, pikes).
  • Seems to me that Str should apply to pen rather than hit, but that’s a raw deal for fighters.
  • This is the easy one - all of the above, in various combinations! It’s magic, man. Some magic swords cut through armor like butter, some guide their wielder’s hands. Plenty of room for variety in effects.

Since we’re calling out armor/shields as a distinct object getting hit now, what about armor degradation? Give those Craft proficiencies something to do in the field.

In theory, the Ogre with crossbow will be at a larger disadvantage than expected due to his mass not being able to contribute to his attack throw in this case? His strength contributes nothing to the crossbow’s operation, though he can wield a larger one. Spears would be better, as they’re thrown, perhaps slings and giant high-pull bows…

Would certain ranged weapons due to design, quality, or size add their own Armor Penetration values to use in lieu of the attacking creatures?

Firearms spring to mind…

I am not a fan of splitting attacks into 2 rolls; largely because of the unexpected impact it has on the GM. For instance, when the GM is rolling for 5 orcs to attack a PC, he needs to only do the math once, but he has to roll 2 dice 5 times. Using multiple dice is faster in 1-on-1 situations where the odds are recalculated frequently but much slower as soon as you get into instances of multiple attacks under the same mathematical circumstances. My fights, at least, have a lot of the latter and not much of the former.

(I also don’t think one can get the mathematics to work out the same very easily…or at least I’ve not been able to. The corner cases get weird).

In any event, I think the process would work as:

  1. Player declares attack.
  2. Judge indicates AC and DC.
  3. Player reduces AC by his AP, adds remaining AC and DC to target value and rolls.
  4. Player announces hit or miss.

It would NOT work well if AC is kept hidden, but perhaps that’s a worthwhile trade-off for other interesting game mechanical benefits.