Development Blog - Dice Pools

A controversial mechanic perfectly suited for the table top.

There is one mechanical decision in Breaking Iron that has drawn a lot of raised eyebrows and some amount of outright scorn. Dice pools are sometimes loved and often hated as a core mechanic in RPGs. I find them to be exceptionally well suited for action adventure role playing games on the tabletop.

Illustration: Fist Full of Sixes - Alexander Rask

Part One: Tactile Advantage

An intuitive dice pool has physical weight throughout the roll. All bonuses and penalties should be discussed and applied before the test. The dice pool grows and shrinks based on the skill, equipment, and conditions of the character with a real presence on the table.

As the player prepares to roll, the heft of the pool communicates their chance of success or precarious odds by the weight of the dice in the hand. This may seem a small thing, but it should not be underestimated. A high skill reduced to a small pool by challenging conditions will feel ominously light. Strong bonuses from magical effects or artifacts will communicate their power with a notable heft. A high pool and low pool can even communicate dramatic information with the sound of the roll from behind the GM’s screen.

With all modifications handled before the roll, success or failure is immediately obvious once the dice settle. So long as the pool does not grow too large, players can mentally tally the successes on the table at a glance. No confusion exists after the dice are cast, and most disagreements should have already been settled. All that remains is success or failure.

Taken together, these advantages help maintain dramatic tension and pacing. All discussion and argument that might deflate the drama of the result is pushed to before the roll. The player feels anticipation or confidence by the weight of the roll in the hand. The result is communicated quickly and the game moves on. Placing discussion about the test before the roll encourages player engagement with the rules. This discourages a player from the practice of blind rolling a die, reporting the value, and then expecting the GM to calculate success or failure.

Part Two: A Good Roll

Dungeons and Dragons is famous for its motley collection of dice, but it is not simply the odd shapes and geometric vocabulary words that make a bag of dice such a satisfying artifact of the game. A high stakes roll of the dice sits at the center of almost every memorable RPG story. I write Breaking Iron for the table top, which I consider the heart of the hobby. With that in mind, what makes a good roll on a table?

A roll needs to be simple to set up, even easier to resolve, and satisfyingly tactile.

To the first point: if too much time is spent determining what and how to roll a test, a lot of the drama can leak out of the game. Players want to be able to declare their action and roll. Game Masters need to be able to move quickly through many actions at a time.

The same issue presents even more strongly after the roll is made. It is best if the result is obvious at a glance, not after a calculation. Disagreement about the roll made after the result is visible hinders play an order of magnitude more than the same discussion resolved before the dice are cast.

Finally, the physical act of rolling is important. This is may not apply as strongly off the table top, but even virtual play often make efforts to mimic real dice with animated polyhedrons rattling around the screen to the accompaniment of simulated plastic striking a 1980s era wooden dining table. Even many computer RPG games include simulated dice for the nostalgia and drama that they evoke. The feeling of rolling dice is central to the experience for many players, and should not be undervalued.

Part Three: Considering Dice Pools

Can dice pools meet my standards of a good roll outlined above? Many assume they cannot, and not without reason. Roll a handful of dice in a dice pool and the test could be modified by:

• Increasing or decreasing the target number for a success

• Counting successes or totaling results

• Increasing or decreasing the size of one or more of the dice in the pool

• Adding or subtracting dice from the pool

• Exploding on certain results, allowing additional dice to be rolled

• Cancelling out successes if other dice roll a 1

This is a problem for players. If too many modifications are used, the roll can be extremely difficult to set up without complete system mastery. Worse, without a deep dive into the probability of the roll, it can be very difficult to determine the expected success rate. The formula needed to calculate the probability of success is simply too complicated for a dice pool where the size of one or more of the dice can change and/or results explode on different thresholds for different characters. It’s not worth trying to calculate and impossible to figure on the fly.

This is not an insurmountable problem. It is not a problem that needs innovative mechanical structures to fix. It is a problem of excess. You can modify dice pool rolls in a lot of creative ways. In my opinion, the simple answer is: you should not. The solution is not complicated design, but a clean use of the mechanic. Dice pools in Breaking Iron are kept simple for this very reason.

• All dice pool rolls use 6-sided dice

The classic d6 is not only highly available, but has long been at the core of classic mechanics. Dungeons and Dragons has used the d6 for detecting traps and secret doors, checking for surprise, and more. It is granular enough that a 1-in-6 chance can be used for a “lucky chance” roll without being either too rare or too common. Alternatives such as the d4, d8, or d10 are more difficult read as a handful cast upon the table. A pool of d6 dice can be rolled and the number of successes counted at a glance.

• Results of 5 or 6 are counted as successes

A 1-in-3 chance of success is intuitive to grasp. In a perfect world, only one face of the die would count as a success, but a d4 is too difficult to quickly read in a group. Counting 5s and 6s in a set of d6 dice for most players is more easily done at a glance than counting 4s in a set of rolled d4 dice. The is also a significant tactile difference between a handful of d6 dice and d4 dice. Most players find a pool of d6 dice more pleasant to roll.

• The game limits the maximum size of dice pools

Using only results of 6 as successes could be an option, but it grows the size of the dice pool to a point where it can become cumbersome to roll and count. A 6-only pool requires a size of 4 dice to have a 50% chance of success. A 5-or-6 pool has roughly a 50% chance of success at just two dice. This may not be a problem with starting characters, but more experienced characters who might roll 8 dice in a 5-or-6 pool would require 16 dice in a 6-only pool. Somewhere over 10 dice, it becomes increasingly difficult to count successes at a glance, and successes must be picked from the pile and counted manually, significantly slowing down the rolls.

• Bonuses increase the number of dice rolled

Bonuses are handled first. Specialization, skilled techniques, equipment, and magical effects are the most common source of bonus dice. Most of these will not change from roll to roll. Conditions that are more subject to frequent change are calculated as penalties if possible. Most bonuses should be plainly listed on the character sheet, limiting clarifying questions from players in this step.

• Penalties reduce the number of dice rolled

Penalties are subtracted from the pool. Visibility, tactical disadvantage, or environmental conditions remove dice in the final step before the roll. This is the step of the process most likely to generate discussion. At the conclusion of the conversation, the dice are ready to be cast. The discussion of conditions is important to establish stakes, limiting it to the last step as much as possible helps maintain pace of play.

• Overall success requires one success to be rolled

Multiple successes can increase the degree of success, but overall success or failure should depend on one success when making an isolated roll. In an opposed roll, two characters roll their dice pools at the same time. The character with more successes wins. Isolated rolls, such as skill use, become increasingly reliable as the pool increases. This is by design. The reason should be familiar to the OSR crowd: for most uses of a skill we don’t actually want to roll. Allow players to use their characters’ skills in reasonable ways without a roll. Only in dramatic situations or within impactful procedures should the roll be made. In those cases, conditions will be extreme, the test will be opposed, or the player will be trying to maximize their degree of success to save time or resources.

Part Four: Personal Preference

Most of the arguments for dice pools given above apply generally, but there are mechanical advantages that come down to personal preference. Breaking Iron uses a condition monitor rather than hit points. Combat is an opposed test of skill. Each success in the dice pool is significantly harder to achieve than the last and improves the effect of a strike. Armor is rolled to soak damage. Heavily armored characters are significantly more resistant to damage, but inventory is limited, even for the strong.

Dice pools work very well for highly lethal action oriented combat. They allow the skilled to act reliably well against lesser opponents but require tactical acumen when engaging equal or superior foes.

Outside of combat, dice pools are well suited to detailed crafting and extended skill tests. Opposed social tests have the same advantages are combat tests. While not every roll in Breaking Iron is a full dice pool test, they fit very well with many mechanics that contribute to the aesthetic of the game.

Appendix: Probabilities

I have included a table outlining probabilities for the most common range of dice pools in Breaking Iron, but it can be summed up in a few observations.

You can expect a good chance of a success for every three dice rolled. To be exact, it’s a 71% chance of at least one success on three dice and a 65% chance of two successes or more on six dice. There’s no guarantee, but your odds are good.

Dealing with single successes, you have a decent chance at just two dice, a very good chance of success at four dice, an excellent chance of success at six, and are nearly guaranteed success above that point. As long as you are rolling at least two dice on an isolated test, there is still incentive to make the attempt unless you absolutely cannot accept the consequences of failure.

In opposed tests, it is important to remember that an advantage or disadvantage of a single die may not seem a large thing, but it translates into significant changes in probability.

In the defense, an equal dice pool to the attack is not always a comfort as most types of attack count a tie as a victory for the aggressor. An attacker with more dice than the average player character should be directed to attack the tank who can stack as many dice as possible into the defense roll and rely on toughness and armor to absorb any damage that is taken.

If the opponent rolls two successes on an attack, a block defense at five dice has a 20% chance of achieving three or more success. A counter attack at four dice has only a 10% chance.

In higher level play, an opponent rolling eight dice may roll two successes. A block rolling eight dice would have a 53% chance of beating the attack. A counter attack rolling seven dice would have only a 45% chance.

In both cases, being able to make a defense roll is vastly superior to allowing the attack to go uncontested.

The final and most important rule of thumb in the system is, of course:

More dice is better.