How to convert AC in oldschool B/X dnd

[lugaid]

Originally Posted by Fred Brackin

I'm pretty sure plain leather was AC 8 and it was "studded" leather that was 7

Nope, that was AD&D. The other D&D lineage used a scale of AC going from 9 as "no armor", 7 as "leather/light armor", 5 as "chain/medium armor", and 3 as "plate/heavy armor", with a shield providing a one-point AC benefit.

[Polydamas]

In D&D THACO improves automatically with Hit Dice, in GURPS I would assign skill levels based on what seems plausible. A giant might not have higher Melee Weapons skill than an orc, but a Wight or Wraith probably does.

Originally Posted by Agemegos

The OP might need to convert monsters with natural armour, and magic items that give a bonus to AC without constituting any IS.

It is still tricky though because one creature might have a better-than-10 AC because of its thick hide (DR), and another because it is quick and nimble (Dodge and the Range/Speed bonus against ranged attacks). A third might have it because it is rapidly phasing between the Prime Material and another plane.

I would be inclined to convert magic item bonuses at +1:+1 and stat out monsters ad hoc using Dungeon Fantasy stats and armour stats as a guideline. Something with AC 3 might have DR 5-7 if it seems 'thick skinned.' Armour Class in old-school D&D does not correspond to a single concept in GURPS.

[Fred Brackin]

Originally Posted by Polydamas

In D&D THACO improves automatically with Hit Dice,.

But not on a one for one scale for all classes. Fighters and monsters improved THACO by1 with every level/hit die. The other classes had different and slower improvement schemes.

Really, direct numeric conversions should probably be avoided. The differences in basic and underlying theory will distort those sorts of numbers and game balance (even no more than D&D had) will not automaticaly transfer..

[Rupert]

Originally Posted by Fred Brackin

But not on a one for one scale for all classes. Fighters and monsters improved THACO by1 with every level/hit die. The other classes had different and slower improvement schemes.

Just to confuse matters, that was different between AD&D and B/XD&D too. AD&D had warriors at -2 THAC0/2 levels (actual fighters could get a bonus on even levels giving them effectively -1/level, but that was optional), and the rest at varying (and changing) rates (cleaned up in AD&D2 to Warriors getting -1/level, Priests getting -2 per 3 levels, Rogues -1 / 2 levels, and Wizards getting -1 / 3 levels).

Basic had fighters getting -2 per 3 levels, clerics getting -2 per 4 levels, and magic users getting -2 per 5 levels. This may have been different in the first version of B/X, as there were quite a few changes between that and the later edition.

[mr beer]

AC doesn't convert well using simple formulas. In GURPS terms it represents a combination of DR, DB, Active Defences, a factor which roughly means "more HD = more powerful = better AC", and more esoteric things such as Homogenous, Damage Reduction and so on.

Like most D&D conversions, translate the flavour, don't try to map using an objective formulaic standard.

For people that wear armour, just use the armour. For monsters, I tend to eyeball it.

A primitive Ogre will have a couple of levels of DR (Tough Skin) and smelly Furs or Leather for DR 4 on the Torso and 2 elsewhere.

A small to medium Dragon will have equivalent of Platemail, so DR 6 or 7 with Nictating Membrane (DR 1) on the eyes.

A spirit doesn't have DR, it has Insubstantial and requires special measures to attack.

A Quickling (small hyperspeed fae) doesn't have DR, it has Enhanced Time Sense, Altered Time Rate and Enhanced Dodge.

An Iron Golem has DR 7 similar to Platemail, but also Homogenous.

And so on.

[maximara]

Originally Posted by kimjongfun

I am having a headache at attempting to convert AC to DR in gurps.

So, in B/X dnd the higher the AC the worse the score.
Therefore, to make things simple, I have made it so that any score up to AC4 is 0 DR. Any score up to 0 AC has a 1:1 conversion, meaning AC4 is DR1, ac3 is DR2 etc.

AC-1 and further on, is a 2:1 conversion, and so every -1 gives you a 2 number increase in DR.

Now, the issue is the THAC0 tables.
Do I make the THAC0 table the creature's skill level with their attack???
or do I implement something else???

I tried to translated AC and THAC0 to GURPS and it never worked. Especially if you are looking at AD&D1 which had an insanely complicated Armor Class Adjustment table no one sane used (Players Handbook 1e pg 38)

The combat systems are just too different for such a conversion to be worth the time and actually work. Even in modern D&D a combat "turn" is 6 GURPS combat turns and if you are converting AD&D 1 you are looking at insanely long 1 minute ie 60 (!) GURPS combat turns

The "doesn't convert via formula" is hammered home when you compare things that exist in both systems.

Take Bear from the MM1e for example, it has either an AC of 6 or 7 and HD ranging from 3+3 to 6+6. But in GURPS all bears (Black, Grizzly, and Polar) all have Speed 6; Dodge 9 with HP 14-20.

This why the step by step conversion of the D&D Lich to GURPS states "First, we forget about things like AC, HD, and HP and look at the basic concept of the monster. "

[maximara]

Originally Posted by mr beer

AC doesn't convert well using simple formulas. In GURPS terms it represents a combination of DR, DB, Active Defences, a factor which roughly means "more HD = more powerful = better AC", and more esoteric things such as Homogenous, Damage Reduction and so on.

Like most D&D conversions, translate the flavour, don't try to map using an objective formulaic standard.

For people that wear armour, just use the armour. For monsters, I tend to eyeball it.

A primitive Ogre will have a couple of levels of DR (Tough Skin) and smelly Furs or Leather for DR 4 on the Torso and 2 elsewhere.

A small to medium Dragon will have equivalent of Platemail, so DR 6 or 7 with Nictating Membrane (DR 1) on the eyes.

A spirit doesn't have DR, it has Insubstantial and requires special measures to attack.

A Quickling (small hyperspeed fae) doesn't have DR, it has Enhanced Time Sense, Altered Time Rate and Enhanced Dodge.

An Iron Golem has DR 7 similar to Platemail, but also Homogenous.

And so on.

Well said. Also don't forget that thanks to their size dragons are easier to land a blow on in GURPS (SM+5 is +5 your weapon skill) but they can still dodge and you have to get through all that DR to actually hurt them.

Something with Unkillable 1 will stay down once you kill it but until then damage doesn't slow it down. Unkillable 2-3 (Lich, demons, etc) are even worse as they can come back and boy they are not going to be happy you "killed" them.

[Varyon]

Originally Posted by maximara

Well said. Also don't forget that thanks to their size dragons are easier to land a blow on in GURPS (SM+5 is +5 your weapon skill) but they can still dodge and you have to get through all that DR to actually hurt them.

That's a big part of the difference in how armor (natural or otherwise) is presented in the systems. In D&D, armor makes you harder to hit. In GURPS, it makes you harder to hurt. I can't speak to the early editions being discussed here, but I do know that at least as of 3e they had Size Categories that function similarly to GURPS' Size Modifiers, giving a penalty to AC (or in the older versions where a low AC was good, I guess it would have been a + to AC), which in D&D is mathematically the same thing as a bonus to hit.

Originally Posted by maximara

Something with Unkillable 1 will stay down once you kill it but until then damage doesn't slow it down. Unkillable 2-3 (Lich, demons, etc) are even worse as they can come back and boy they are not going to be happy you "killed" them.

Minor quibble - Unkillable 1 doesn't prevent a foe from slowing down below 1/3rd HP, and they still have to make HT checks to stay conscious below 0 HP, it just prevents them from having to make Death Checks at -1xHP, -2xHP, etc, and also shifts the "insta-death" threshold from -5xHP to -10xHP. Unkillable 2/3 is similar, except once they hit the "insta-death" threshold of -10xHP, they go into a kind of torpor (UK2) or leave their body (UK3) and don't take further Injury, being rendered unconscious until they heal up (for UK2 though you can just hack away at their body every so often to keep them permanently at or around -10xHP). If you want an enemy who basically ignores wounds until said wounds kill them, you'll need a different build. I think there's a form of Injury Tolerance in Monster Hunters that helps (Unstoppable, maybe?). Supernatural Durability can also be worthwhile. Alternatively, you can come at things from a completely different angle - an enemy with HP 1, Fragile: Unnatural, and a huge pile of Ablative DR (or, better, Vitality Reserve) is going to be more-or-less completely unphased by whatever you throw at it, up until it runs out of DR/VR at which point it falls over dead (technically, a "just right" hit could reduce the foe to only 0 HP, but just 1 more point will drop it to -1, which is also -1xHP, killing it). Given these are enemies rather than characters/allies, you can also just fiat things - I believe there's a DF-focused Pyramid article that suggests allowing large, dumb monsters to basically keep fighting without much issue into negative HP, but results in them dying outright much sooner.

[Ulzgoroth]

Originally Posted by Varyon

That's a big part of the difference in how armor (natural or otherwise) is presented in the systems. In D&D, armor makes you harder to hit. In GURPS, it makes you harder to hurt.

It's messier than this, at least in late editions, because there's the touch AC derivative. At least some hits (since everything is case-by-case) that only need to make contact rather than wound effectively are not hindered the presence of armor. One might infer that more conventional blows that would hit touch AC but don't hit against actual AC are hitting the target but failing to have an effect. (One might find places where that inference makes the fiction very confusing because everything is case-by-case and not every case is well thought through.)

[maximara]

Originally Posted by Varyon

That's a big part of the difference in how armor (natural or otherwise) is presented in the systems. In D&D, armor makes you harder to hit. In GURPS, it makes you harder to hurt. I can't speak to the early editions being discussed here, but I do know that at least as of 3e they had Size Categories that function similarly to GURPS' Size Modifiers, giving a penalty to AC (or in the older versions where a low AC was good, I guess it would have been a + to AC), which in D&D is mathematically the same thing as a bonus to hit.

Actually with the Unearthed Arcana (or I liked to call it The Munchkin Manual) plate armor did both - made you harder to hit and harder to hurt. While it is true D&D 3e and later did things more intelligently we are talking about "oldschool B/X dnd" or BD&D/AD&D.

BD&D generally stands for Basic Dungeon & Dragons or more properly "Basic Dungeons and Dragons" which came after OD&D (1974) and saw several changes from 1977 to 1994 and is a very different beast than the D&D most people are familiar with.

xD&D is shorthand for AD&D which is 1e and 2e. The "A" was dropped when 3e came out in 2000. So this is about versions of D&D that WoTC released rather than stuff they themselves came up with ie 20+ years old (1977-1999).