Balancing GURPS Combat: Odds to Hit

Odds to Hit for Effective Attack vs Defense Skills

One of the keys to being a confident GURPS GM is to understand how to create combat encounters that are balanced to the party’s abilities. This post will look at a critical element of combat balance: how likely each side is to land blows. GMs should consider the defensive abilities of the combatants when evaluating how challenging an encounter will be.

Start With Defense

It doesn’t matter how much damage an attack can deal if the blow never lands. Unless the attacker rolls a critical success, the defender gets to attempt a defense roll. As a result, defensive skill is extraordinarily important.

The table below shows the chances of landing a hit based on the effective skills of the attacker and defender. The first thing you should notice is that the attacker’s skill makes a big difference when the defender’s skill is low. However, once the defender’s effective skill gets above 10, most attackers will eek out a hit only one out of three attempts, at best.

Odds to Hit for Effective Attack vs Defense Skills
Odds to Hit for Effective Attack vs Defense Skills

So, when judging how difficult a combat encounter is, the first question should be: how well can each side defend? If the player characters are facing an opponent that has a base active defense skill above 10, they need to have ways to lower the effective defense skill in order to expect to land blows with any regularity. Conversely, if the players have a high active defense skill, the opponents should have ways to lower the effective skill in order to put the characters into jeopardy.

When Defense Isn’t Enough

Defense skills are important for balancing combat, but there are some situations in which even the best defensive skills aren’t enough:

  • Lethal damage: if an attack can do lethal damage with a single blow, then a high defense skill doesn’t eliminate the risk—it just makes the outcome a high-stakes gamble. A single good (or bad) dice roll can radically change the result, so the GM needs to be prepared to handle the worst.
  • Area effects: when an attack targets a large area, there may be no active defense possible. Area effects may give PCs a way to take down NPCs with extremely high active defense skills, but opponents can also use these effects to circumvent player defenses.
  • Mental attacks: Physical defenses are useless against mental assaults. If a character can terrify an opponent, manipulate their senses, or otherwise get inside their antagonist’s head, the GM should think about how that will play out in combat.
  • Surprise attacks: Finally, active defenses are useless if the target never sees the attack coming. The players (or NPCs) can make tactical decisions to give themselves the element of surprise during the combat, but the GM should also think about whether one side can set up an ambush before the battle is joined.

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