by Michael Strauss

Created on: April 04, 2011   Last Updated: April 07, 2011

Generally associated with Dungeons & Dragons (D&D), the d20 system is a method of challenge resolution used in various tabletop role playing games.  As the name suggests, the d20 system resolves challenges by rolling a 20-sided die.  The result of this die roll, plus any applicable modifiers, is compared to a target number.  If the result equals or exceeds the target number, the challenge is surpassed.  Otherwise the challenge is failed.

The biggest advantage of the d20 system is that it is simple to learn and perform.  A single die roll is the simplest form of random number generation that exists in tabletop role playing games.  In addition, unlike challenge resolution in other games, the target difficulty is usually a static number, which makes success or failure easy to determine.

But, simplicity also comes with a consequence.  Because the d20 system only uses a single die roll to resolve most challenges, all odds in the game are multiple of 5%.  While this may seem like this allows for a nice spread of odds, in actual practice this isn’t true.

Depending on the system, most difficulties result in a relatively consistent percent chance of success.  For example, in 4th edition D&D, the percent chance of a character successfully hitting is usually around 60%.  Because the percent chance remains generally constant, bonuses and penalties to a roll can quickly overwhelm the system.  For example, just a +2 bonus to a roll with a 50% chance of succeeding increases the likelihood of success by 20% (compared to the original likelihood of success).

In addition, the d20 system has another noticeable disadvantage.  Because difficulties are static and a d20 only has 20 numbers on it, it can be possible for a challenge to be impossible to succeed on or impossible to fail.  Some versions of the system resolve this issue by treating a natural roll of 1 as an automatic failure and a natural roll of 20 as an automatic success.  But, this solution trades one problem for another, because now all challenges always have a 5% chance of failure, no matter how trivial or a 5% chance of success no matter how impossible.

Despite this, the advantages of the d20 system are usually considered to outweigh the disadvantages.  One of the greatest advantages of the d20 system is that the simple system can be easily injected into more complicated challenge resolution mechanics.  For example, opposed

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