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| Roger Moore battles to the death with the AD&D XP calculation rules, Dragon #89 |
Update 20 July 2025 to fix various math errors and add a few missing creature entries.
I've been working on a little project for my home game which has had me taking a deep-dive into the AD&D experience point system. I thought I'd share some of the system's strengths, errors, weaknesses, and other interesting bits (presuming you're the type of weirdo system wonk that finds this sort of thing interesting, I should say).
On average, monster XP was always secondary to treasure XP in old-school D&D. That doesn't mean that some thought wasn't put into how to generate XP values for monsters, however. To see how 1st edition handled XP, you'll need the DMG: specifically, p. 85 (Experience Points Value of Monsters table) and pp. 196-215 (Appendix E: Alphabetical Monster Listing, which gives all canon XP values for the monsters in the Monster Manual).
So, you look up the creature's Hit Dice, apply the appropriate Basic XP Value, and then start adding less expensive SAXPBs and more expensive EAXPAs, each representing some sort of special ability the creature has, until you arrive at the final total. The Hit Dice for the creature also determines how much XP per hit point the creature is worth, an amount added on top of the base creature value. So, a 6-HD creature isn't just worth, say, 350 XP, but instead 350 + 6 xp per hp--on average, more like 377 XP.
Calculation Issues
If you dive in and start trying to reverse-engineer the XP values given in the DMG Appendix, you'll soon start running into trouble. In particular, the guidelines given on p. 85 for assigning ability XP don't seem to be all that closely followed. As a result, I've seen some arguments that the Appendix doesn't actually use the p. 85 method: that the method is merely a means of calculating XP for your own monsters, and that the Appendix does its own thing. However, having gone through all the monsters in the Monster Manual and Fiend Folio, I can guarantee that Appendix E definitely uses the p. 85 rules: the number of values impossible to calculate using that method is a mere handful, versus the dozens that work out correctly. You just need to take a very careful look to work out some of the kludges the designers used.
Arbitrary Values
One place where confusion and natural skepticism creeps in is that the Appendix often arbitrarily assigns SAXPBs and EAXPAs to arrive at its monster totals, despite (or in addition to) the footnoted guidelines for those on p. 85.
A good example is the hydra. Look at its breakdown:
|
5 HD/IV |
SV: D11/P12/R13/B13/S14 |
165+5/hp |
|
6 HD/V |
SV: D11/P12/R13/B13/S14 |
275+6/hp |
|
7 HD/V |
SV: D10/P11/R12/B12/S13 |
400+8/hp |
|
8 HD/VI |
SV: D10/P11/R12/B12/S13 |
650+10/hp |
|
9 HD/VI |
SV: D8/P9/R10/B9/S11 |
1,000+12/hp |
|
10 HD/VII |
SV: D8/P9/R10/B9/S11 |
1,500+14/hp |
|
11 HD/VII |
SV: D7/P8/R9/B8/S10 |
2,150+16/hp |
|
12 HD/VII |
SV: D7/P8/R9/B8/S10 |
2,850+16/hp |
(Interestingly enough, the DMG gives values for up to 16 heads, but by the Monster Manual a hydra tops out at 12 heads, even if using the Lernaean variant that grows extra heads.)
If you compare the 11-HD version and the 12-HD version, you get a noticeable XP value increase. However, by the p. 85 table there's no reason to do this. They're both in the same HD category (11 to 12+), and no new special abilities are gained at 12 HD that, by the footnotes, would appear to justify a higher XP value. They even have the same saving throws. However, a 12-HD hydra does get an extra attack (hydras have 1 attack per head), and it uses the next higher attack matrix column for all 12 attacks (because it's 12 HD). Neither is specifically listed in the special abilities section, but if you compare the 11-HD and 12-HD values, the difference in their XP values is exactly 700. This might seem rather arbitrary, but checking p. 85, you can see that 700 is the SAXPB value for the 11-12 HD range. So, the designer didn't pick any random number, but selected the precise listed modifier for the creature's HD range to represent this combat capability, and more importantly to make it so that the two variants didn't have the exact same base XP value despite being noticeably different. As such, an 11-HD hydra can be calculated out as:
1,300 Base + (0) SAXPB + (1 x 850) EAXPA = 2,150+16/hp
And a 12-headed hydra works out like so:
1,300 Base + (1 x 700) SAXPB + (1 x 850) EAXPA = 2,850+16/hp
So it's not that the math doesn't work, but that the designers just used the math as they felt the need to. But, it's still the math given in the system. The fault lies more with the incomplete footnotes than a true inconsistency, since the additional cost is clearly warranted and follows the established pattern.
Composite Values
The DMG does not provide XP values for every possible iteration of every creature. The idea I'm sure was to save space (and time), since otherwise the Appendix would be noticeably larger. For example, boars in the DMG have three XP entries, but need eleven to do the whole range as presented in the Monster Manual. Unfortunately, when one or a handful of XP values are given to represent a wider range of possibilities, how those summational values are generated is inconsistent, using one of two different methods, which might lead people to think again that the p. 85 method isn't being used. But it still is: just in arbitrary fashions.
The first method is when the Appendix just takes one valid value and uses it to represent the whole range. For example, the Barracuda is given a single 20+2 XP value, which perfectly represents the XP for its middle-range example: a Barracuda can have from 1 to 3 HD, and the given 20+2 XP value is what the 2-HD example would have (the others, unstated in the Appendix, would be 10+1 for 1 HD and 35+3 for 3 HD).
The second method is when the Appendix creates a composite value, a rough average for the whole creature range. These tend to have a base value fitting one element of the range, coupled with a XP-per-hp value from a different part of the range. For example, the Sea Horse's singular 20+4 XP value is impossible using p. 85, but when taken as a stand-in for all three of its HD possibilities it has the base 20 XP value from the low end (2 HD) and the 4 XP-per-hp value of the high end (4 HD).
The DMG doesn't explain which method it uses for any given entry: the only way to figure this out is to calculate XP for individual subentries yourself, using the p. 85 methodology. It doesn't even say that these are averages of one kind or another: you need a copy of any of the rare Monster Cards sets for such a note ("Average value only, see DMG p. 85"). But regardless, the values are consistent with p. 85.
HD Pluses
The p. 85 table says you have to take HD pluses into account. For example, there's a "6+1 to 7" category, so we know that 6+1 is treated different than a straight 6 HD. However, the table doesn't appear to say precisely how. Creatures can have some pretty high pluses at times (e.g. the Type VI demon at 8+8, or the all-time champ mezzodaemon at 10+40). And, if you know your 1st edition, you know that the DMG treats Hit Dice pluses differently depending on the situation.
For example, for saving throw purposes, pluses in hit points move a creature steadily upwards in Hit Dice. +1 hit point raises it one HD level for saving throw purposes. Every 4 full points beyond that raises it another Hit Die: e.g. +5 raises it two HD, +9 raises it three HD, and so on.
For attack purposes, however, while any plus of 4 or higher equals an additional Hit Die, this doesn't scale. So, if you've got a monster with 4 HD, 4+1 to 4+3 does nothing, while anything from 4+4 to 4+1000 gets you the same result: treat as a 5-HD creature. And as a 5-HD creature uses the same column as a 4-HD creature on the attack matrix, it's pointless. So, the only creatures that benefit are those with a) odd HD values (since advances on even ones always just bump you up within the same attack matrix column), and b) with a bonus of +4 or higher. Only a handful of canon creatures actually have this.
This does make one naturally ask which of these methods the monster creation system uses, or if it uses either of them at all. Checking out the creatures with high HD pluses in the Appendix as test cases, we have:
