Spell Hit Chance is an attribute that can be found on items. In general, +1% spell hit chance increases the base chance for a spell to land against a target by 1%. If the target is the same level as the caster, a spell has a base chance to hit of 96%.
- If the target is +1 level compared to caster: 95%
- If the target is +2 levels compared to caster: 94%
- If the target is +3 levels compared to caster: 83% if the target is a monster, 87% if the target is a player.
- If the target is +4 levels compared to caster: 72% if the target is a monster, 80% if the target is a player.
- If the target is +5 levels compared to caster: 61% if the target is a monster, 73% if the target is a player.
If wearing a +1% spell hit chance item, the above percentages will increase by 1.
For binary spells only, there is an additional modifier for resistance to particular spell schools: fire, frost, shadow, nature, and arcane. That modifier is multiplied by hit chance to get the actual chance to land. This is done with binary spells only, because they never do partial damage.
Eyonix the Mage (level 60) fires a frost bolt at Yeti of Doom (level 63). Eyonix is also wearing a total of +6% spell hit gear. Yeti of Doom has frost resistance such that he takes 50% from level 60 frost attacks. So, here's the hit calculation:
0.83 (83% for +3 levels monster) + 0.06 (+6% spell hit) = 0.89 0.89*0.5 (50% damage from frost) = 0.445.
The game will roll a number between 0 and 1, and if it's less than 0.445, the frost bolt will hit for full damage. Otherwise, a resist message will appear.
After the resist, Eyonix decides to fire a fireball at Yeti of Doom. Eyonix still has +6% spell hit. Fireball is not a binary spell. Here's the calculation:
The game will roll a number between 0 and 1, and if its less than 0.89, the fireball will hit. Otherwise, a resist message will appear. After the fireball lands, the game will then apply spell resistance to determine a partial resist, if any. Assuming the yeti also has 50% fire resistance, on average, 50% of the damage will be resisted.