The Five Variables of a Core Dice Mechanic That Matter

By JimmiWazEre

Opinionated tabletop gaming chap

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TL;DR:

A core dice roll is a bundle of variables that shape how a game feels, how players assess risk, and how much work the GM has to do. This post breaks down the five big factors inside the dice loop: who sets the target, what shape the randomness takes, how odds get modified, how outcomes are interpreted, and when you should actually roll in the first place. Each of these choices changes tension, pacing, tone, and player responsibility.

Introduction

Last time we talked about what a core dice mechanic is, so today we’re going to dive into the five variable elements of a dice loop to dissect what happens between the expression of player’s intent and the result, and then why so many systems seemingly reinvent it.

Get yourself a cup of tea for this one, there’s a lot to unpack here.

The Dice Loop

Quick refresher first for those at the back: what do I mean when I talk about dice loops? Well, I’m talking about the four stage process that happens whenever you engage in the core mechanic. Specifically:

1. Action declared

2. Variables Applied

3. Dice Rolled

4. Consequences Interpreted

This post drills into everything that happens between declaring the action and reading the outcome. That is the variables that shape odds, difficulty, and tone.

Variable 1 - What Are We Rolling Against, And Who Decides?

Whether you call it the DC, TN, AC, or number of successes, this variable is simply the target you need to beat. Different systems decide that target in different ways, and that choice carries a lot of weight.

Some games put the decision in the GM’s hands. In 5e, for example, the GM sets the DC, which gives them fine control over difficulty but also creates a subtle conflict of interest to manage: you want the players to succeed, but you also want the challenge to feel meaningful. On top of that, every judgement call adds to the GM’s mental load, which is already stretched thin.

Other systems remove that burden entirely. In GOZR, for instance, the GM doesn’t set a difficulty at all — the “target” is simply the relevant character stat. That strips out GM bias and keeps the load light, but also means the GM can’t tune difficulty moment to moment.

Therefore this choice changes the feel of the game. Rolling against fixed values gives players more meta-knowledge and puts responsibility for risk firmly in their hands, encouraging calculated decision-making. However, when the GM sets the target instead, players may feel the GM shares responsibility for success or failure — which is why you often hear GMs say things like “I killed my player last session.”

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Variable 2 - What Shape Does Randomness Take?

You can also radically impact the game by what the game has you roll, due to the way different combinations of dice affect results distributions.

Single Dice

A single dice, take a d20 for instance, has a 5% chance of landing on any given result. For this reason, single dice rolls can feel swingy as the range of possible outcomes is equally likely. This is part of why 5e feels “heroic” - massive rolls are not uncommon in comparison to other results, and there’s a good chance of getting a stunningly high roll at any given time. In isolation it also contributes to comedy goofy moments where massive failure is also a very realistic prospect.

Multiple Dice

However, if you roll multiple dice and combine the score, then you’re in a bell curve distribution situation where the final result will heavily favour the median possible outcome, and outcomes at the extreme success and failure ends will be significantly rarer. This has the effect of making the game more predictable, and therefore gives the player more ownership over the outcomes they generated. It works well for games with high lethality because players can predict odds more reliably:

2d6 result | Odds

2 or 12 | 3%

3 or 11 | 6%

4 or 10 | 8%

5 or 9 | 11%

6 or 8 | 14%

7 | 17%

Dice Pool

If you have a dice pool system, like in AlienRPG where each result of 6 is a success, in that circumstance each dice added to the pool increases your chance of obtaining at least one success, but the impact on the odds that each new dice added to your pool shrinks massively with each new dice added via diminishing returns:

Dice | Odds | Increase

1d6 | 17% | +17%

2d6 | 31% | +14%

3d6 | 42% | +11%

4d6 | 52% | +10%

5d6 | 60% | +8%

6d6 | 67% | +7%

7d6 | 72% | +5%

8d6 | 76% | +4%

This type of system is good for capping the ability of player characters within a certain range, keeping abilities grounded which is important for systems where you want your players to never feel invulnerable.

It’s undoubtedly true that rolling big handfuls of dice is not only fun, but also that fraction of a second you spend sorting through the results hoping for a success is tense.

Variable 3 - How Do We Modify The Odds?

This works very closely with variable 2, because different dice methods of generating RNG present us with different options for modifying those rolls.

Additive Modifiers

A common method to change the odds of a roll is to use your character’s derived stats and “add your modifiers” to the result of the roll. This is clean and intuitive from a simplicity standpoint, but in doing so, it takes certain low results off the table completely. For example, a 5e Rogue with a +9 in stealth is never going to score less than 10 on their roll (we’ll talk about critical fails in a bit) and that’s a problem, because now our modifiers have moved beyond adjusting the odds toward creating certainty and in doing so risks undermining the purpose behind having a core dice mechanic in the first place.

If you want to create a game where the players can indulge in a power fantasy, this is the route to take.

Dice Chains/Step Dice

Rather than giving players a bonus of an absolute value to add to their dice roll, dice chains and step dice elect to give them a different sized dice instead. Let’s assume that you want to roll high - in this case, a character rolling a d6 is capped out at 6, vs. a character rolling a d12 is capped out at 12.

The potential of the d12 character is therefore twice that of the d6 character, but we’ve avoided creating certainty, as a d12 can still roll low. So instead of narrowing the result range as with additive modifiers, step dice grow or shrink the entire results band upward or downward.

This works well where we want to give players meaningful variances in ability without turning their characters into unbeatable demi-gods, but it does make dice rolls slightly less intuitive. Who hasn’t had that player that asks every time about what dice they need to roll, even when it’s always a d20? This will probably exacerbate that problem!

Advantage/Disadvantage

Advantage/disadvantage is one of the simplest difficulty tools you can give a GM: roll twice, keep the better or worse result. It shows up in different forms across systems, but the core idea is always the same.

There’s a lot to like about it:

• It’s clean. No maths, no modifiers, no lookup tables.

• It’s emotional. Players immediately feel the stakes when the dice leave their hand a second time.

• It’s universal. You can bolt it onto almost any core mechanic without breaking anything — d20, roll-under, step dice, dice pools, whatever.

But it’s not flawless.

Firstly, the maths isn’t intuitive. Rolling twice feels like a small nudge, but in a d20 system advantage is worth roughly a +3 to +5 bonus depending on the situation which can be bigger than many GMs intend. The reverse is true too. If you don’t know the underlying probability shift, you may end up modifying odds more aggressively than you realise.

Secondly, it adds friction. You’re doubling the number of rolls, and while that sounds trivial, groups who rely on this mechanic heavily might notice the slowdown, especially at tables where players already hesitate or re-check dice.

For those reasons, the only time I’d avoid using advantage/disadvantage is when the system already has too many levers to pull. If you’ve got static modifiers, step dice, DC adjustments, and situational tags all competing for attention, adding another knob to twist just dumps more cognitive load onto the GM and makes it harder to stay consistent.

Variable 4 - How Do We Measure Outcomes?

Binary

Essentially we have three options. Firstly, we could argue that it is purely binary - the roll resulted in either a success or failure. This is clean and simple for sure, but it does not lend itself well to interesting outcomes, or keeping the game moving forwards. We’ve all heard the advice that as a GM, you should try to avoid saying “no”, well that’s what a failure is in this circumstance - it’s “no”. The problem is that it’s shut down an avenue of progress without opening up an alternative.

On the plus side though, it’s light on GM load. There’s nothing difficult about interpreting a binary result, and it’s clean and fast, and there’s less chance of the players feeling like they’ve been victims to some unanticipated gotcha.

GM Fiat

The second option creates GM load in the extreme, and opens you right up to conflict of interest: There is no codified success or failure - the GM simply interprets the strength of the result and assigns a suitable outcome to it based upon fiat, circumstance, and vibes.

GM fiat isn’t an official mechanic, but it becomes a de facto one when rules don’t specify degrees of success - you’ve seen it in action when the GM calls for a roll, you score a 4 and everyone at the table understands intrinsically that you’ve failed, yet the GM sort of awkwardly goes on to award you a success of sorts because failure wouldn’t have made sense.

It’s only really an option for non dice pool mechanics though. No one would be able to get away with witnessing a dice pool result of zero successes and then contorting that into a limited success!

Degrees of Success

This option is a middle ground. In this system the game has some codified way of defining outcomes more than yes or no. Typically opening up to:

• yes and

• yes

• yes but

• no but

• no

• no and

Now different mechanics will allow this in different ways. With a dice pool, it might be that you strengthen the outcome with the more successes you roll. With other systems they break possible dice results down into ranges, either according to absolute values (such as 1 below TN) or percentages (such as 10% below TN) and then transpose the list above to those ranges.

We’ve seen this applied to great effect in games like Call of Cthulhu where the ranges regular, hard, and extreme are mapped to a percentage over or under your stat, or Powered by the Apocalypse, which favours absolute values.

Critical Hits

Critical hits are a wildcard baked into many core mechanics; that sudden spike of drama when the dice explode, double, or land on that one special result. In design terms, crits are a way to break the expected curve, injecting moments of swinginess into systems that might otherwise feel predictable.

The simplest form is the classic natural 20 in D&D: roll the highest face on the die and you get a bigger, flashier result. What’s important is that this happens regardless of modifiers. Even a clumsy novice can occasionally land a perfect blow, and even an expert can fumble catastrophically. Crits flatten the power curve in tiny unexpected moments, and as a consequence they’re exciting.

Different systems spin this idea in different ways, and they tell you what sort of game you’re playing:

• Linear dice systems (like d20 games) produce crits fairly often because all outcomes are equally likely. This reinforces the “heroic swinginess” the d20 is known for.

• Bell-curve systems (like 2d6 or 3d6) make crits rare and meaningful, because the extreme ends of the curve hardly ever come up. You’ll still occasionally roll a double six or triple six, but it’s much rarer and less reliable.

• Dice pool systems handle crits by counting multiple successes, matching numbers, or converting high results into special effects. This lets crits scale with character competence: more dice rolled equals more chances to spike, but still without guaranteeing it.

• Exploding dice create a different flavour of critical entirely: every max roll triggers another roll, allowing theoretically infinite results. I use them when I play D&D because it kind of represents the lowly peasant hitting the dragon in his eye with an arrow.

Under the hood, critical hits interrupt the normal flow of risk and reward. They’re a “spike of possibility” that keeps players hoping, even when the odds aren’t in their favour.

Variable 5 - What Justifies A Roll?

Of all the variables in the dice loop, this one is the most misunderstood: when should you roll at all? It sounds trivial — “roll when there’s uncertainty” — but in practice, this decision shapes the entire pace, tone, and feel of a system far more than most people realise.

I’ve written about this before in my older post (Do You Call For Too Many Rolls?), but it’s worth pulling back into this series, because it turtley belongs on the list of core variables.

• A game that rolls sparingly feels empowering, deliberate, investigative, even cinematic.

• A game that rolls constantly feels random, procedural, or punishing.

• A game that leaves it vague risks becoming muddled, inconsistent, and exhausting for the GM.

Conclusion

Well done, you got to the end! Honestly that one was a lot of work and took ages to write up, so I hope it proves useful to all the TTRPG dice nerds, academics, and designers out there. If you didn’t catch the first post in this series, you can check it out here, and stay tuned for the next piece on what mechanics work well with different tones and genres.

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