Hit chance

Chance to hit your target is calculated differently for Close Combat and Small Guns/Big Guns/Energy Weapons/Throwing.

Unarmed/Melee attack Hit Chance

Chance to hit your target with an Close Combat attack.

The formula is the following:

Take your Close Combat skill.
Deduct armor class of the target.
Accurate weapons get a 20% bonus.
Aimed Attack gives you a penalty to hit chance depending on the limb you're aiming for.

Also note, that there are other variables ex. perks, traits, quests bonuses, weapon crafting bonuses affecting your current chance to hit.

Chance to hit your target with a ranged attack.

The formula is the following:

Start with weapon skill.
Add (PE-2)x16% for all long range weapons(i.e. with a range of 26+) that were already implemented in FO1, or are upgrades of these weapons, (PE-2)x8% for the rest.
The Scoped Hunting Rifle gets (PE-2)x20% instead of the x16% if you shoot at a target that is at least 5 hexes away, or no PE-bonus at all for closer enemies.
It seems the other newly implemented long range weapons in FO2 are bugged, they only get the (PE-2)x8% while all long range weapons from FO1 had the (PE-2)x16%. (most weapons were implemented in FO1, but if in doubt, Google your weapon of interest)
Deduct 4% per hex between you and your target.
Accurate weapons get a 20% bonus.
Deduct the Armor class of your target after modifying it with the used ammo type (each ammo has AC modifier) - the resulting number cannot go below zero
Obstructions in the line of fire (creatures between you and your target) gives you a certain malus.
Aimed Attack gives you a penalty to hit chance depending on the limb you're aiming for.
Sharpshooter, despite its description gives you 16% to Hit chance.
Weapons with perk Scoped are affecting your chance to hit according to range.

Also note, that there are other variables ex. perks, traits, quests bonuses, weapon crafting bonuses affecting your current chance to hit.