Conversions from AD&D to Hackmaster [Hackmaster]

This is something I developed on the Kenzerco Message Boards, as "How do I convert material from AD&D to Hackmaster" is a not-uncommon question. These are not perfect guidelines by any means; they're a means to get you "in the neighborhood", balancing the different mechanics.

General Conversion Rule For Monsters:

Base HP: 10 + size modifier (0 for tiny, 5 for small, 10 for medium, 15 for large, etc), plus normal HD, plus any bonus HP the regular creature has.
Base Attack: HD of original creature
Base Defense/DR: Look at the AC of the original creature; decide how much is agility and how much is toughness. For agility, double that and make it a defense modifier. For toughness, apply the equivalent armor as DR (if they have a 2 AC just from toughness, don't give them an 8 DR, but the DR of plate)
Base Damage: Double what it was in AD&D.

As others will say, there's more art to it than science... while those guidelines above WORK, they need to be looked at and tweaked.

Sample Conversion using these guidelines:

The Ogre.  (They're a simple creature, and available in the free Hackmaster Basic PDF)

An Ogre in 2nd edition AD&D had 4+1 HD, an AC of 5, and did 1d10 damage (or "By weapon +6"). They are size L.

So, we know its HP are going to be 10+15+4d8+1, or 26+4d8. It's AC is 5, but that's going to be all toughness... the equivalent of Chain Mail. Looking at Chain Mail, they have a DR of 5, but -5 defense, +2 initiative, and +2 speed.

Their base attack is going to be +4 (for their HD). We can either call them 2d10p or give them a big weapon and add +12. I prefer the second method for ogres, so we know their strength will be prodigious... assuming the PH chart continues, they have about a 25 strength, meaning a +23 FOS.

A few other things... It's 8 intelligence, so we'll stick its wisdom there, too, giving it a -1 to attack, +3 to intiative, -1 to Defense and Mental Saves. No particular reason to Buff Dexterity, so we leave it at 10.5, for another +2 to initiative. The starting HP assume a Con of 11 (10, +1 because of it's 4+1 HD). That's low, but we'll leave it for now.

So, arming my ogre with a Bardiche (because, in Phantasie III, my brother's epic Ogre Fighter Fogey used a Bardiche), we get something like this...

Hit Points: 26+4d8
Size/Weight: L/650 pounds
Fatigue Factor: +0

Saves
Physical: +4 (base HD)
Mental: +3 (base HD, modified for wisdom)
Dodge: +4 (Base HD)

Attack: +4
Defense: -2
Damage Reduction: 5
Damage: By weapon +12
Speed: 16
Init: +7
Reach: 6'
Trauma Save: 5

This looks a little puny, so I'm going to buff it up... instead of counting it as 4th level, I'm going to count it as a 4 HD fighter, or 8th level, which will give it better saves and a bit of a reduction in speed and initiative. I'm also going to kick up its assumed Constitution, to 15, which gives it 4 more HP, a better TOP, and a bonus to Physical saves.

Hit Points: 30+4d8
Size/Weight: L/650 pounds
Fatigue Factor: -2

Saves
Physical: +10 (base level, plus con bonus)
Mental: +7 (base level, modified for wisdom)
Dodge: +8 (Base level)

Attack: +4
Defense: -2
Damage Reduction: 5
Damage: By weapon +12
Speed: 15
Init: +6
Reach: 6'
Trauma Save: 7

A fair bit stouter, my naked ogre with a big axe.

How does he stack up against a HoB ogre? Here's the same numbers, with the HoB number and commentary in parentheses

Hit Points: 30+4d8 (34+4d8; the HOB seems to assume a 19 constitution, not the 15 I went with)
Size/Weight: L/650 pounds (L/550#; mine may have a lower Con since he seems to be a bit of a porker)
Fatigue Factor: -2 (-2/-1)

Saves
Physical: +10 (+8)
Mental: +7 (+2)
Dodge: +8 (+4.... seems my calculations were a bit off, here)

Attack: +4 (+5)
Defense: -2 (-1)
Damage Reduction: 5 (4)
Damage: By weapon +12 (By Weapon +6; I aimed a bit high, it seems)
Speed: 15 (8)
Init: +6 (+4)
Reach: 6' (long; I think I'm on target, there)
Trauma Save: 7 (9)

So, the HoB Ogre assumes about 4 more points of Con than I did, but about 6 less points of strength. My calculations for saving throws a pretty off, which means I need to revisit them... probably make more sense to base those on HD, not level, since my first numbers were closer. Overall, I'm pretty satisfied with my quick-and-dirty conversion of the ogre, following the above principles. It's not perfect, and there are a couple of clear places where I made mistakes (WAY overinflated their strength and saves; my weapon choice leaves this ogre very slow), but close enough that I'd use it without problem.