Bringing d20 Poly-Dice to 2d6

My favorite game system BITS uses at its core 2d6 (two six-sided dice). With 2d6, monsters are slayed, gold plundered, and crowds wooed.

2d6 is virtually unseen in the most popular roleplaying games (i.e. the Don and uncontested king of roleplay, Dungeons & Dragons), only showing up in well received though still pretty niche engines like Powered By the Apocalypse.

Yet, where 2d6 does show up, the dice are used in mechanics that are nearly untranslatable to the bread-and-butter d20 and poly-dice systems in use by mainstream games a la D&D.

BITS fixes that by using similar modification and resolutions to D&D (the crunchier part) while using only 2d6 instead of an entire rock-quarry of *d* rolls.

Here’s how:

Roll %s

First, a comparison of percentages in rolls between D&D (which uses a d20 at its core) and BITS 2d6.

D&D uses various difficulty levels that a player has to roll at or above to succeed depending on context. The player can add different modifiers to their rolls to help them get the number they want. However, as a general guideline, challenges can be divided into the following:

    • Roll at or above.
    • 5 – Very easy, 80% success rate.
    • 10 – Easy, 55% success.
    • 15 – Moderate, 30%.
    • 20 – Hard, 5%.
    • 25 – Very hard, cannot be accomplished without some value boost.
    • 30 – Godly, cannot be accomplished without major value boosts.

The percentages above seem really low. That is, until you consider they take into account adding everything from -5 to +10 to the rolls based off the the six abilities a game character has.

Further, “natural” criticals are when a player rolls either a 1 or a 20 (ignoring all modifiers). These crits have a 5% each to give a player something especially harmful or helpful, relatively.

Now 2d6, both with and without D&D‘s heavy use of modifiers.

    • Roll at or above.
    • 5 – BITS has this as easy, 83.3% success chance. D&D would have this as very easy.
    • 7 – BITS moderate, 58.3%. D&D easy.
    • 9 – BITS hard, 27.8%. D&D moderate.
    • 11 – BITS very hard, 8.3%. D&D hard.
    • 13 – BITS very, very hard, and can’t be done without some help. D&D very hard.
    • 15 – D&D‘s god-tier difficulty needing top-level characters and lots of luck.

BITS also has criticals when “natural” doubles are rolled (1-1, 2-2, etc.) above or below the target difficulty number. This means criticals scale with the difficulty of the challenge encountered: easier targets offer more opportunities to really wallop ’em.

However, if the linear scale of D&D roll probability needs to be kept, natural 1-1 and 6-6 (both a 2.8% chance) can be adopted for BITS, no problem. But why? 2.8% does not equal 5%…

Take a look again at those percentages. 55% and 58.3%, 30% and 27.8%, even the 5% and 2.8% for criticals! The conversion from d20 to using 2d6 as a core mechanic is never more than 4%, a sneeze of a difference in gameplay. Fundamentally, swapping 2d6 for d20 has no noticeable effect on outcomes.

Therefore, as a core mechanic, 2d6 can substitute for D&D-like d20. Though, there are still modifiers to add 🙂


D&D has six abilities that have both a base number and a modifier that slowly scales with the base. These six abilities are Strength, Constitution, Dexterity, Charisma, Intelligence, and Wisdom. Each ability’s modifier applies to challenges that are primarily in those abilities’ wheelhouse.

BITS has three abilities that are the modifiers added to the kinds of challenges that best fit their use. They are Body, Interaction, and Thought.

Now, the BIT of BITS has a 1:1 correlation with D&D: Body (Strength and Constitution), Interaction (Dexterity and Charisma), Thought (Intelligence and Wisdom).

If given a D&D character, the modifiers of that character’s abilities translate into a BIT value. By adding together the D&D modifiers, dividing by 2, and rounding down, new BIT values are found.

For example, let’s use the level 1 Fighter, Mage, Rogue, and Cleric starting characters for D&D.

Their stats (including health, aka HP, for later discussion):

Fighter+3+2+1-1+0+113 (d10)
(BITS value)B =+2I =+0T =+05 / 7
Mage-1+2+2+0+3+18 (d6)
B =+0I =+1T =+23 / 3
Rogue-1+2+3+2+1+08 (d6)
B =+0I =+2T =+03 / 3
Cleric+2+2-1+1+0+310 (d8)
B =+2I =+0T =+14 / 6

BITS expects starting characters to have no more than 1 or 2 in any given BIT, so the numbers above for abilities work brilliantly. Not all classes in D&D are created equal, so in exchange for a BIT value perhaps a BITS specialty (the S in BITS; describes history or role and gives advantage when that context applies to a challenge) is gained, or unique equipment acquired, or HP gained (more on these things next).

Since some ability modifiers can be negative, thus resulting in a negative BITS value, what should be done?

Well, BITS could flatline the BIT value as “-“, meaning whenever a challenge would be solved by that particular BIT, the roll has disadvantage. Or maybe a “anti-specialty” where if certain situations come up, all failures are critical failures.

Even though BITS on principle refrains from using negative numbers, a conversion of d20 to 2d6 doesn’t need to use that tenet of BITS, maybe in this one case negative modifiers can remain 🙂


Health, the lifeblood of player characters, the timer of how long a fight can possibly last.

The chart in the previous section has two numbers for BITS HP: the average of the die used in D&D (specified by class; d6, d8, d10, etc.) divided by 2, and that die average with the Body BITS value added.

That’s done because in D&D, HP is a certain die roll (d6, d8, d10, etc.) plus the Constitution modifier. For BITS, Body can be added to the average of the die for a class to achieve the same result.

Including the Body value in HP calculation can lead to HP bloat. While this may give more a feeling of heroic superiority to the player characters, it also leads to longer fights, less caution, and more flippant actions when the consequences aren’t that, well, consequential.

Depending if the Body value is added or not, and if there is any cap on HP (BITS typically likes to aim to cap at 12 HP), that changes the way combat and the use of equipment as a mechanic occur in the 2d6 conversion.


BITS divides both fictional beings and their equipment into six tiers:

    • BITS Fantasy Weapon Tiers
    • 0 – Fists, unarmed combat.
    • 1 – Knives, small swords, cudgels, sticks, brass knuckles, hatchets, throwing spears.
    • 2 – Swords, axes, clubs, maces, short bows, light crossbows.
    • 3 – 2-handed mauls and bastard swords, pikes, longbows, flails, heavy crossbows.
    • 4 – Ballistae, claymores, halberds, tree-trunks.
    • 6 – Especially heroic or magical devices, such as Excalibur or Hercules’ club.
    • BITS Armor Tiers
    • 0 – No armor, clothing, robes, a buckler used as a shield in hand.
    • 1 – Leathers, round shields.
    • 2 – Mails, full-body shields.
    • 3 – Partial plates (a mix of mail and plate), 2-handed massive shields.
    • 4 – Full plate, a rolling barricade of treated wood used as a shield.
    • 6 – Heroic suits that are probably enchanted.

Gear can play into the BITS 2d6 conversion two ways. Either A) gear does nothing to a roll and gives its value as damage or reduced by 1 to negate damage, or B) gear adds to the roll value and the difference between the roll value and the target value is the damage given to a target (in the case of armor, it increases the chances of deflecting damage).

Let’s call option A the realism option, and B the heroic option.

Realism ought apply when a character’s HP is limited, either capped or very slow growing. Some characters ought die in a hit or two from a sharp object (just like real life!).

Heroic damage comes into play when characters feel overpowered. They smite small threats and can weather harsher punishment because their HP grows to accommodate.

Non-Player Characters

Whichever equipment mechanic is used to match the HP mechanic, non-player characters (NPCs; beasts, town guards, etc.) have their own tiers 0 to 6.

I personally am a =huge= fan of reducing enemy complexity in simulators games like D&D. Thereby in BITS, NPCs have HP equal to their tier and do damage equal to their tier. Players have to roll at or above the tier equivalent if wanting to either act against or defend against an NPC (e.g. a tier 1 may require a 7+ roll).

No rolling extra damage, no having to calculate HP, no having to figure out what every goon is wearing and carrying!

Keeping it simple like this should remain balanced between d20 and 2d6 implementations. Since I can’t vouch completely for it, if taking a D&D NPC into 2d6 territory, determine its abilities, HP, and equipment the same way done for characters detailed above.

(It does help that there are a plethora of NPC creation and balancing tools for D&D available, each ripe for conversion to BITS!)

And that’s it! Just about all that’s needed to convert a poly-dice d20 system into a 2d6 BITS-like.

The joy of having the tools to do this means a lot of games can be converted into a concise system shared between multiple fictions and titles for faster-yet-still-hefty play.

What’s your take? Any sections of d20 and poly-dice mechanics from games like D&D missing here?

Hit me up and let me know! If you’ve any other suggestions or would like to see a test IP get converted into 2d6 and BITS (even if the IP doesn’t have a widely-recognized game with it!), I’d be happy to walk through the challenge of the conversion.

In any case, do well! Cheers ~

Published by

Jimmy Chattin

Processor of data, applier of patterns.

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