Need Armour class explained

[Xsio]

Can someone explain for me how armour class (AC) works?

As i understand when attack happens d20 +modifiers is rolled, and if result is higher than AC, hit is scored?

So why does it syas 30% hit chance against 30+ AC?

[zero]

lets say you have 20 ac

to "hit this person" (and then do damage) you need to roll a d20+ whatever bonus you have.

IE if you have a +2 to str and nothing else, you need to roll an 18 or above.

and then you roll for damage.

"hit it to win it" or "attacker wins in ties" is the general rule of thumb.

you do however always hit on a 20, and this game does have conditional bonuses, which provide advantage (roll twice, take better result)

[faffy]

If your targets AC is 15 and you roll 16, you the target.
If your targets AC is 15 and you roll a 14, you miss the target.
AC is the variable that's used to mediate whether an attack hits or misses. So when it says 30% chance to hit against a target of 30+AC, it means that with an average roll of 10 on a d20 and with +hit modifiers you will hit the target an average of 30% of the time.

[Xsio]

So If i understand correctly it's useless to add AC after you got 20, since on roll 20 the attacker always scores a hit?

[SpaceNvader]

100%/20=5%. Hence 20 sided dice. Its an abstraction with special cases of 1 (5%) always being a miss and 20 (100%) always being a hit.

[zero]

Originally posted by Xsio:

So If i understand correctly it's useless to add AC after you got 20, since on roll 20 the attacker always scores a hit?

no, because enemies will still have modifers, an enemy with a +5 can hit you on a 15, yet if you had 25AC, they could only hit you on a 20.

[medicinezombie]

Originally posted by Xsio:

So If i understand correctly it's useless to add AC after you got 20, since on roll 20 the attacker always scores a hit?

No, because of bonuses added for the attacker - a "natural 20" is always a hit, and a "natural 1" is always a miss, but you can wind up with values like that from modifiers, where the normal rules apply (need to EQUAL or EXCEED AC to hit)

So, AC 21, attacker has a +3, rolls a 17 - that equals 20, so a miss
AC 21, attacker has a -3, rolls a 20 - ignore the modifiers because a rolled 20 always hits

edit: more AC for your characters will always be a good investment. It's hard to improve after you've already put armor on - magic armor can only possibly add +3, stats max out at 20/+5, most characters can't use a shield, etc

So every point of AC is good to have

[SpaceNvader]

Originally posted by Xsio:

So If i understand correctly it's useless to add AC after you got 20, since on roll 20 the attacker always scores a hit?

No, you fail to take into account attack bonuses.

[Kyutaru]

Originally posted by Xsio:

So If i understand correctly it's useless to add AC after you got 20, since on roll 20 the attacker always scores a hit?

No because your roll is modified by Attack bonuses and Proficiency and Stat bonuses and other things, so your roll can have +12 to it, allowing people to hit ACs up to 32 more frequently.

A 20 is always a hit yes, but the bonuses make even lesser rolls able to hit (10+12= 22, so roll of 10 can hit AC 22 with enough bonuses).

[Xsio]

Thanks got it. I wonder what's the biggest modifier NPC can have

[cire87]

if i go full barb am i restricted to only medium armor ?

[Ettanin]

30% hit chance means that you need to roll at least a 15:
15, 16, 17, 18, 19 and 20 mean you hit (6 of 20 possible results = 30%).
Your total attack bonus should be
30 (the AC, that means the minimum number for AB + roll)
- 15 (min roll to hit)
= +15 attack bonus.

The higher your AC, the more proficient the enemy in their attack accuracy (that is the attack bonus) must become to hit you.

THAC0 from AD&D which is what Baldur's Gate 1 and 2 as well as Icewind Dale use is more complex: That's the dice result you need to roll to hit an enemy with an AC of 0. That means a positive AC number was bad while a negative one was good and you wanted to have a lower THAC0 than a higher one. You add the defender's AC as a modifier to the attacker's attack roll and compare the result to the THAC0 of the attacker.