Saving throws by HD

A shortcut that I sometimes use for monster saving throws is 1d20+HD >= 16 (because who wants to be bothered looking at the fighter save matrix all the time?). The number 16 may seem arbitrary, but it is easy to remember one constant, and using that value has some nice properties with regard to the resulting probabilities by HD. You can also grant specific monsters bonuses or penalties on the fly as appropriate without jeopardizing impartiality, as long as the bonus is set before the roll happens, though I personally do not go more than +2 or -2.

Recently, I have been considering using ability checks directly for saving throws (I know, this is not new, Castles & Crusades, etc). The way I usually do ability checks is a d20 roll less than or equal to the stat in question, as described by Moldvay (page B60). Rolling under a single target number with no modifications in the common case is about the simplest resolution system possible (even though it does mean that rolling low is better, which some people find confusing). One potential downside to this method is that the target numbers are hugely variable based on chance during character creation.

Thinking about these two systems together (1d20+HD >= 16 and just using ability checks as saving throws) made me think that perhaps I could combine these two approaches without relying on improvised DCs. What I came up with does require using ability score modifiers, rather than just relying on the full score, but that is probably more familiar to most people by now anyways. It would probably work well with both the B/X bell curve progression (13-15 = +1, 16-17 = +2, 18 = +3) or the 3E linear progression (12-13 = +1, 14-15 = +2, 16-17 = +3, etc).

Here is how it works:

  • PC save versus monster: ability check vs. 10 + monster HD
  • Monster save versus PC ability: 1d20 + monster HD vs. 10 + PC level
  • PC save versus dungeon hazard: ability check vs. 10 + dungeon level

For the purposes of these rules, HD values with additive components are always collapsed (so HD 1+1 is treated as 2). Using the ability modifier rather than the full ability score makes the chances less variable, addressing one of the issues with using the older roll-under style of ability checks as saves. The way I am thinking about this right now, the saves (as ability checks) would not improve with level (because I am interesting in seeing how that affects play), but level (or half level) could easily be factored in if you don’t mind one extra term. It might also be reasonable to have PCs gain some bonus for checks directly relevant to class (magic-users versus magic, thieves versus traps, etc). This could be operationalized as a flat bonus (perhaps +2), adding level to the roll (for games that have significant power gain with level increases), or rolling two checks and taking the best result (for games that prefer less numerical inflation).

Some examples. A magic-user casts a sleep spell on a group of orcs, which have 1 HD each. The monsters roll 1d20 + 1 (from the HD) versus a target number of 11 (10 + PC level). An adventurer touches a yellow mold, which squirts out a deadly cloud of spores. As written in the rules, a successful saving throw versus death ray is required for the adventurer to survive. Using this system, the save would be dexterity versus a target number of 12 (10 + yellow mold HD). As you can see from these examples, weaker monsters and lower level PCs have slightly improved chances compared to the traditional saving throw progressions, but that does not bother me.

At first glance, this looks something like the Third Edition approach to setting DCs, but I think this method is a worthwhile simplification, and it opposes known, objective values (ability modifiers, levels, hit dice) rather than needing to make any kind of determination about whether something should be a “hard” DC or an “easy” DC. This is a system that you can use entirely based on the HD value listed for a monster and the ability scores and level written on a PC’s character sheet. As such, it is easy to graft on to virtually any traditional-style fantasy RPG, as long as the game has ability score mods and hit dice listings for monsters.

This leads me even further toward the (perhaps slightly idealized) position that all you really need is HD and AC to specify a monster (along with a few interesting abilities, of course). And for monsters that are not humanoid, 10 + HD is a reasonable guideline for ascending AC as well.

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8 thoughts on “Saving throws by HD

  1. Aaron

    Similar to what I do except for a few minor things. I use dungeon level x 2 since monster HD tend to increase faster than dungeon level. I use a PC’s HD when the save is combat related or Level when not. Finally, I use 11+HD for the target number instead of 10+. This makes it 50%/50% for when monster and PC levels match which also means that either side can roll the check and the probabilities are identical. So a wizard can make a casting roll at d20+level vs 11+HD or the monster can make a save at d20+HD vs 11+caster level.

    1. Brendan Post author


      I like that very much. I probably have an irrational attachment to the number 10 (the LotFP unarmored AC being 12 drives me crazy, for example), but the identical probabilities that you describe are elegant.

      I’m curious, do you have a max PC level, either by explicit rule, or informally in practice?

      1. Aaron

        No max level. I make a conscious effort to try not to think too far ahead of where I currently am (the highest PC IMC currently is 4th). That tends to lead me to analysis paralysis. The highest level player I’ve ever had was 12th which is right where the +level stuff tends to break. But that took more than 3 years of weekly gaming (which I can’t replicate now that I have kids) so I figure I’ve got time.

  2. checkmarkgames

    I have been doing something similar as well. Every PC/Monster has a stat called “Expertise” which is equal to 10 + (level / 2) + Ability Score Bonus. Which is then used as the target number for saves against abilities/actions of that character.

    The Ability Score Bonus used is that of their class Prime Requisite (using the lowest for classes that have more than one), as I figure everyone is using their key abilities for the task (for a save vs being disarmed, a Fighter is relying on Strength to knock your weapon out, the Thief is using more finesse (DEX) and the Mage a bit of trickery or a minor Cantrip (INT))

    I also like the use of tying the lowest prime requisite as classes with multiple PRs typically have more options/tricks that apply to more situations, and this tones down their power level a tad, and makes them a little bit more uncommon.

    I am not a fan of calling it Expertise though, and I am always trying to find a better term for the stat. I am also using a slower save bonus progression, so if you are adding full HD to save rolls, I would do the same here instead of dividing the level by 2.

    1. Brendan Post author


      I’ve used the word “competency” before for a similar construct, but was similarly displeased with it. This is really a stat used in something like opposed checks, so presumably it represents something like strength or capability, neither of which work well in practice as game terms. Puissance perhaps?

    1. Brendan Post author


      Yeah, level doesn’t help as written here. It easily could if you wanted it to, though. Recently I’ve been thinking that not as many aspects of PCs need to improve with level, mostly because I like the traditional lower level experience more than the mid or high level experience. Who knows though, this might be a phase.

      Say more about d20+stat > 20 as an implementation of roll-under. Isn’t that basically target 20, which is a roll high approach?


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