What is THAC0?

THAC0, or To Hit Armor Class 0, is a fundamental mechanic in thed20 System, a role-playing and tabletop game system. It represents the roll needed on a twenty-sided die (d20) to successfully hit a target with an Armor Class (AC) of 0. The lower the AC, the easier it is to hit the target. Understanding THAC0 is crucial for determining the success of an attack in the game.

Definition of THAC0

THAC0 is a measure that determines the roll needed on a d20 to hit an opponent. It is based on the attacker's level and class, representing the skill and experience of the character. The lower the target's AC, the higher the chance of hitting the target with an attack in the game.

How to Calculate THAC0

The process of calculating THAC0 involves a few steps:

1. Determine the Attacker's THAC0

The THAC0 for a character is determined by their class and level. Each class has a specific THAC0 value that improves as they gain levels. This value is crucial for determining the success of an attack.

2. Identify the Target’s Armor Class (AC)

Armor Class is a measure of how difficult it is to hit a character. The higher the AC, the harder it is to hit:

AC 10 No modifier AC 5 -5 modifier AC 0 -10 modifier AC -5 -15 modifier

3. Calculate the Required Roll

The formula to determine the roll needed on the d20 is as follows:

Roll needed THAC0 - AC

For example, let's say a level 5 fighter has a THAC0 of 15 and is attacking a creature with an AC of 3:

Roll needed 15 - 3 12

To hit the target, the player must roll a d20 and get 12 or higher.

Additional Notes

Critical Hits: If the player rolls a natural 20, it is typically considered a critical hit, regardless of the calculated THAC0.

Modifiers: Other factors such as bonuses from magic items or situational modifiers can also affect the roll needed. These bonuses may increase or decrease the roll needed to hit.

Improving THAC0: As characters level up, their THAC0 will decrease, making it easier to hit targets.

Understanding THAC0 and AC

THAC0 and AC together help determine the success of an attack in the game. By understanding your THAC0 and the target's AC, you can easily calculate the required roll on a d20 to see if your attack hits. This mechanic provides a straightforward way to determine the outcome of combat in the game.

A Pre-calculation Method with THAC0

The THAC0 system is still used in pre-calculation of the minimum number needed on the d20 to hit a creature with an AC of 0. All bonuses are precalculated, and you can add these to the d20 roll to get the AC you can hit, along with all ACs below that. This method simplifies the process of determining hit or miss.

For example, if you are attacking an enemy with an AC of -5 and your THAC0 is 15, you need a minimum roll of 7 (15 - 5) to hit, as any roll of 7 or higher on a d20 will hit an AC of -5 or higher.

Here's a step-by-step breakdown:

1. Identify THAC0 and AC

For a level 5 fighter with a THAC0 of 15, attacking an enemy with an AC of -5.

2. Calculate the Required Roll

Roll needed 15 - 5 10

To hit the target, the player must roll a d20 and get 10 or higher. Rolling 9 or lower means the attack misses.

Summary

THAC0 is a core mechanic that helps players determine the success of an attack in the d20 System. By understanding your THAC0 and the target's AC, you can easily calculate the required roll on a d20 to see if your attack hits. This system simplifies combat and helps in strategic planning during gameplay.