Converting D&D to d6s

As my Curse of Strahd campaign continues, I’m returned from my flirtations with Fate Core to the d20 niche, and it feels like home. The d20 system is familiar and everything about it makes sense to me in a way that some parts of Fate do not–probably because the d20 mechanics more model a simulation of characters adventuring in a fantasy world over modeling dramatic personas telling a story. To be more blunt: when I see a D&D spell that says “make a Dexterity save or be blinded,” I have an idea of exactly what is going on in the fiction. In the case of some sort of radiant light spell, the Dexterity save involves averting your eyes at the last second before the light crisps your retinas. In Fate…I’m not entirely sure how to model such a thing. Is it an attack roll? Is it creating an aspect? Is it a consequence? What do I roll to make it happen? Or take the medusa’s gaze:

Petrifying Gaze. When a creature that can see the medusa’s eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 14 Constitution saving throw if the medusa isn’t incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic. Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can’t see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save. If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.

These are very sensible rules and I understand exactly what is going on. Constitution save to resist being turned to stone. I know exactly how to model averting one’s eyes within the game (disadvantage on attack rolls). In Fate, I don’t know how because Fate doesn’t do modifiers to attack rolls. Moreover, am I allowed to say, “Roll your Physique or be turned to stone”? That seems like a no-no in Fate, which is all about empowered protagonists and all that jazz.

But I am blathering here. My point is that I’m back to playing D&D, and it’s like I’m getting reacquainted with an old friend. As a system tinkerer, I’ve been hemming and hawing about certain changes to the game, one of those being to convert the majority of the game to d6s over non-standard dice. Just for the sake of simplicity and ease of access.

On the surface, the process is relatively simple:

    • 1d4 has an average of 2.5. Minimum 1, maximum 4.
    • 1d6 has an average of 3.5. Minimum 1, maximum 6.
    • 1d8 has an average of 4.5. Minimum 1, maximum 8.
    • 1d10 has an average of 5.5. Minimum 1, maximum 10.
    • 1d12 has an average of 6.5. Minimum 1, maximum 12.

Simple mathematical conversions dictate a few possibilities.

    • 1d4 becomes 1d6-1. Average 2.5, minimum 0, maximum 5.
    • 1d6 stays as-is.
    • 1d8 becomes 1d6+1. Average 4.5, minimum 2, maximum 7.
    • 1d10 becomes 1d6+2. Average 5.5. Minimum 3, maximum 8.
    • 1d12 becomes 1d6+3. Average 6.5. Minimum 3, maximum 9.

As you can see, such changes create some wonkiness in mathematical probabilities. The numbers generated are more predictable, with a higher maximum (save for 1d4), but their maximums are reduced. 1d12 is a particularly tricky case because it could simply be converted to 2d6, but that would give it a lower minimum than 1d10 (2 vs. 3) and a considerably higher maximum (9 vs. 12). An alternative setup might be:

    • 1d8 becomes 1d6+1. Average 4.5, minimum 2, maximum 7.
    • 1d10 becomes 2d6-1. Average 6. Minimum 1, maximum 11.
    • 1d12 becomes 2d6. Average 7. Minimum 2, maximum 12.

That still leaves us with a spot of irregularity: the jump in power from 1d8 to 1d10 is considerably less than 1d6+1 to 2d6-1 (higher average and maximum). Overall, I don’t see resolving this issue as possible within the scope of D&D, wherein we add bonus dice and fling around numbers. No, I think I’d probably just suck it up and do as follows:

    • 1d4 becomes 1d6. 1-6 (3.5).
    • 1d6 becomes 1d6+1. 2-7 (4.5).
    • 1d8 becomes 1d6+2. 3-8 (5.5).
    • 1d10 becomes 2d6. 2-12 (7.0).
    • 1d12 becomes 2d6+1. 3-13 (8.0).
    • 2d6 becomes 2d6+1. Because if 1d12 is getting a boost, the 2d6 weapons need a boost, too.

That is a general and broad rule, mostly applicable to damage. For HD, I would probably change it to:

    • 1d6 stays as-is.
    • 1d8 becomes 1d6+1. Average 4.5, minimum 2, maximum 7.
    • 1d10 becomes 1d6+2. Average 5.5. Minimum 3, maximum 8.
    • 1d12 becomes 1d6+3. Average 6.5. Minimum 3, maximum 9.

If only because HD averages ought to be constrained slightly more. I am not a fan of hit point bloat.

The only thing I would be hesitant to change with such a system is the d20 roll. Oh, I’m certainly fine with converting 1d20 to 3d6, but there are a few considerations to keep in mind:

1. The bell curve of 3d6 throws off the math of the system in a way that is more profound than damage rolls. 1d20 vs. DC 15: 30% chance of success. 3d6 vs. DC 15: 9.26% chance of success. In this same way, every +1 is more valuable and every -1 is more crippling. The numbers would have to be considerably rejiggered to keep things functioning smoothly, otherwise the game will fall to pieces.

2. Rolling 1d20 is much faster to resolve than rolling 3d6. With 1d20 you roll the dice and add one number. With 3d6, you’re adding a minimum of three numbers, usually four. Then if you’re making multiple attacks, you do it again, and again, and again. There’s an argument to made for making combat go faster and require fewer rolls overall, but that is not the purpose of this post.

3. Advantage in 5e would change quite a bit when rolling 4d6, keep the highest three vs. rolling 2d20 and keep the highest one. Disadvantage likewise. The probabilities would, again, be thrown out of whack.

If I were to convert the d20 roll to 3d6, I would make the following changes:

    • On any 3d6 roll, you add half your total bonus, rounding down. A +5 becomes a +2. A +27 becomes a +13. That’s a pain in the ass.

    • Armor Class is equal to 10 + (your current AC – 10)/2. Passive Perception is calculated similarly. That’s complicated.

    • Skill DCs: instead of being an idiot and going from DC 5 to DC 30, skills have a difficulty of DC 10 (average), DC 15 (difficult), and DC 20 (extremely hard). Anything less than DC 10 is stupid and you shouldn’t be rolling for it anyway. Adjust by +2 or -2 if you really want more granularity, but I don’t see it as necessary.

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