Dice & Probability

How Dice Checks Work

When deciding what happens next, luck and skill both play a part - just like reality. The GM will ask you to roll a die when you try something that might fail or when chance is involved.

A D20 (twenty-sided die) works well because each increment maps to a 5% probability on a scale from 0 to 100%. The core formula is:

If you have any skills or abilities you think would help, ask the GM. Does your character or organisation have special traits that would assist? Are there circumstances or items that would change your chances?

The DC Difficulty Guide

Use this quick reference when setting or interpreting a Dice Check:

Step-by-Step

When an event occurs that could succeed or fail, follow these steps to resolve it:

1. Flag the event. Identify that something has happened which has a chance of failing. Confirm it is not something that would automatically succeed or fail. Decide which ability or skill is being tested.
2. Set the DC. Consider how challenging the task would be and set a threshold number for success using the difficulty guide above.
3. Identify modifiers. Any skills, traits, circumstances, or items that help or hinder?
   1. If the character or organisation sheet defines relevant modifiers, add or subtract them before the roll.
   2. In some circumstances the GM may grant advantage or impose disadvantage on the check (see below).
4. Roll the die. The player rolls a D20 and adds any modifiers.
5. Compare to the DC. If the total meets or exceeds the DC, the action succeeds. If it falls below, the action fails.
6. Describe the outcome. The GM narrates what happens as a result of the roll.

Advantage and Disadvantage

Sometimes circumstances make a task notably easier or harder than normal. When this happens, the GM may grant advantage or impose disadvantage.

• Advantage: Roll two D20s and take the higher result.
• Disadvantage: Roll two D20s and take the lower result.

If circumstances grant both advantage and disadvantage simultaneously, they cancel out - roll a single die as normal.

Worked Example

Full Probability Table

Use this table to convert between a DC value and the probability of success on a single, unmodified D20 roll:

Probabilistic Thinking

The dice mechanic is also a training tool. Asking players to estimate probabilities before a roll makes them practise calibrating judgement and checking bias.

When a player says "I think there is about a 30% chance this works," they are using the same probabilistic reasoning that underpins sound decision-making in real organisations. Across a session, this builds the habit of thinking in likelihoods rather than certainties.

Invite players to weigh probabilities and suggest DC levels. Every dice check becomes a small calibration exercise.