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:

Dice Check difficulty levels and their corresponding DC values
DC	Difficulty
1	Trivial
5	Easy
10	Moderate
15	Hard
18	Very Hard
20	Nearly Impossible

Step-by-Step

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

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.
Set the DC. Consider how challenging the task would be and set a threshold number for success using the difficulty guide above.
Identify modifiers. Consider whether any modifiers help or hinder this check. Does the character or organisation have special skills or traits that would assist? Are there circumstances or items that would change the odds?
1. If you are playing with stat blocks or character sheets that define abilities and attributes, apply the relevant modifier as an addition or subtraction before the roll. Use these if applicable.
2. Under certain circumstances, the GM may determine that a character has advantage or disadvantage on a check (see below).
Roll the die. The player rolls a D20 and adds any modifiers.
Compare to the DC. If the total meets or exceeds the DC, the action succeeds. If it falls below, the action fails.
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

Alternate Universe — Media Scenario

GM: “…It appears she has been embezzling money all along.”

P1: “I want to see if we can get ahead and positively respond to the press.”

GM: “OK, how are you going to do this?”

P1: “We have a big media and marketing arm. Perhaps they can help design some releases and notes for us?”

GM (thinking): Hmm … pretty tough challenge. I think it would be difficult, so I will set the DC at 20, but they do have a big marketing team …

GM: “OK, it is going to be challenging, but your big marketing team can help. Roll a D20 and add +2 to the roll.”

P1: Rolls a 4.

GM: “The press is not buying it. You can tell they are going to run an unfavourable story on this tomorrow.”

(Later in the game …)

P1: Rolls a 19.

GM: “It appears the press is going to run your framing of the events! 19 + 2 = 21 — you manage to pull this off!”

Full Probability Table

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

DC values mapped to probability estimates and percentage chance of success
DC	Probability Estimate	Probability of Success
1	Almost certain	99%
2	Highly likely	95%
3	Very likely	90%
4	Expected	85%
5	Good chance	80%
6	Probable	75%
7	Likely	70%
8	Quite possible	65%
9	More often than not	60%
10	Maybe	55%
11	Even odds	50%
12	Perhaps	45%
13	Somewhat unlikely	40%
14	Unlikely	35%
15	Not often	30%
16	Probably not	25%
17	Rarely	20%
18	Remote	15%
19	Highly unlikely	10%
20	Extremely unlikely	5%

Probabilistic Thinking

The dice mechanic is not just a game system — it is a training tool. By asking players to estimate probabilities before a roll, you encourage them to practise calibrating their judgement and checking their bias.

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

Encourage players to think about the probability of outcomes and invite their input on DC levels. This collaborative approach turns every dice check into a learning moment.