Hit chance

The PvE hit chance formula
In PvE combat, one's hit chance is the chance of successfully hitting a monster (or the chance of a monster hitting you), given your accuracy, the armour of your opponent and the style you are using.

$$hitchance = weakness~modifier \times (\frac{accuracy}{armour})$$

Weakness modifier
Depending on what style you're using and what type of monster your target is, the weakness value can be one of 4 values.
 * .90 when using the target's specific weakness (eg. arrows, bolts, slash, stab, air, fire, etc.)
 * .65 when using a combat style the target is weak against, but not the specific weakness (eg. melee against a ranger)
 * .55 when using a combat style the target is neutral against (eg. melee against a meleer)
 * .45 when using a combat style the target is strong against (eg. melee against a mage)

Calculating accuracy
There is a single formula that relates level to accuracy bonus for base levels, weapons and monsters. Given a as the level, the formula is: $$F(a) = 0.0008a^3 + 4a + 40$$

For simplicity this formula will be referred to as F, so for example F(99) would be 1212.2, which rounds to the true value of 1212 (the level accuracy bonus of a player with level 99 Attack).

Note that drinking potions and using stat-boosting prayers will increase the input value for this.
 * Using a supreme overload boosts your attack level by 16% +4, or to 118 at 99. F(118) would be a 1826 accuracy bonus.
 * Using turmoil invisibly boosts your attack level by 10. At level 99, this boosts it to 109. F(109) would be a 1512 accuracy bonus.
 * Using both a supreme overload and turmoil brings your attack level to 128. F(128) would be a 2230 accuracy bonus.

Your base accuracy
$$accuracy = F(a)+ 2.5 \times F(weapon)$$

Your accuracy is given by your level accuracy bonus added to your weapon's accuracy bonus. The level accuracy bonus is F(attacklevel) and the weapon accuracy bonus is 2.5 * F(weapon level). Therefore a player with 99 Attack using Drygore weaponry would have F(99) + 2.5 * F(90) = 3670 accuracy.

Equipment penalty
Wearing armour of the wrong class can negatively affect hit chance. Wearing any items in the body, legs, helm, boots, gloves, or shield slot of the class you are strong to (if using Melee, this is Ranged) will reduce your accuracy stat by 3/2 (x1.5) of that item's armor stat. Wearing any items in those slots of the class you are weak to will reduce your accuracy stat by 4/5 (x0.8) of that item's armor stat. This is intended to force players to use items of the same class while fighting.

Accuracy boosts
There are various things that can be used to increase your accuracy rating; these simply multiply your accuracy value by a number, and can be stacked with one another. These include:

Hit chance boosts
Also known as flat or additive accuracy bonuses. Unlike the above mentioned accuracy bonuses; these add the value directly to your final hit chance, rather than multiplying it. These can also be stacked with one another.

An example to better distinguish between both types: If you have 60% hit chance, then a 10% accuracy bonus multiplies it by 1.1, thus becoming 66% hit chance. A 10% hit chance bonus adds 10% to 60% thus becoming 70% hit chance.

This means that hit chance bonuses are generally superior to equal numbered accuracy bonuses