Yes, this is in the Help file for RPG Maker XP (F1 > RPG Maker XP > Battle Formulas (also found under the Help menu)), but I figured it should be posted here as well for reference, since I always know that I forget to check the Help file for random little things and end up googling it half the time...oopsies.

Also, a handy tool if you want to figure some of this stuff out a faster way. Algorithmate at RPG Palace:
While not all users may find a use for this little utility, those who are in the middle of custom systems could live by this tool. It simplifies your workload by calculating some algorithms for you. Things such as finding the physical damage dealt to an enemy with attack power and enemy defenses in mind. It's a small download, so take a look.



BATTLE FORMULAS

RPGXP's default battle system uses the following formulas.
The active battler is represented by A, where the battler who is the target for A's actions is represented by B.


First Hit
Normal attacks:
First hit rate = Hit rate of A's state

Skills:
Attack power F is 1 or greater (physical attack)
First hit rate = Skill's hit rate × hit rate of A's state ÷ 100

Attack power F is 0 (non-physical attack)
First hit rate = Skill's hit rate

Items:
First hit rate = Item's hit rate

- If A's condition is normal, the state hit rate is 100.



Critical Hit
Normal attacks:
Critical hit rate = 4 × A's dexterity ÷ B's agility

Skills:
Critical hit rate = 0

Items:
Critical hit rate = 0



Calculating Damage
Normal attacks:
Power = A's attack power - (B's physical defense ÷ 2)
Rate = 20 + A's strength
Variance = 15
- Minimum force: 0

Skills:

Skill's force is positive:
Force = Skill's force
+ (A's attack power × skill's attack power F ÷ 100)
- (B's physical defense × skill's physical defense F ÷ 200)
- (B's magic defense × skill's magic defense F ÷ 200)

- Minimum force: 0

Skill's force is negative:
Force = Skill's force

Rate = 20
+ (A's strength × skill's strength F ÷ 100)
+ (A's dexterity × skill's dexterity F ÷ 100)
+ (A's agility × skill's agility F ÷ 100)
+ (A's intelligence × skill's intelligence F ÷ 100)
Variance = Skill's variance

Items:
HP recovery amount is negative:
Force = - Amount of HP recovered
- (B's physical defense × item's physical defense F ÷ 20)
- (B's magic defense × item's magic defense F ÷ 20)

- Minimum force: 0

HP recovery amount is positive:
Force = - Amount of HP recovered

Rate = 20
Variance = Item's variance

Damage = force × multiplier ÷ 20 × elemental modifier × critical modifier × defense modifier (± variance %)

- Elemental modifier: The weakest of B's effective elements corresponding to the action's element(s).
A: 200%, B: 150%, C: 100%, D: 50%, E: 0%, F: -100%
Reduced by half if B's armor or state has a defending (opposing) element.
When there are more than one of the same defending elements, the damage may be halved multiple times.
- Critical modifier: Equals 2 when the damage is positive and a critical hit is made.
- Defense modifier: Equals 1/2 when the damage is positive and B is defending.



Second Hit
Damage is positive:

B's condition is normal:
Evasion = 8 × B's agility ÷ A's dexterity + B's evasion modifier

Normal attacks:
Second hit rate = 100 - evasion

Skills:
Second hit rate = 100 - (evasion × skill's evasion F ÷ 100)

Items:
Second hit rate = 100

B has "Can't Evade" state:
Second hit rate = 100

Damage is negative (recovery):
Second hit rate = 100



Successful Escape
Successful escape rate = 50 × actors' average agility ÷ enemies' average agility