Chance to Cast

% Chance to Cast gives a chance to use a skill when triggered by an event, without costing any mana.

Skill level cannot be increased: however, equipping multiple % Chance to Cast of the same skill, level and event will result in the individual percentages being added together to apply a single % Chance to Cast of that skill, level and event (although >100% Chance to Cast is redundant). [1]
Skills used receive passive bonuses including synergy bonuses, but do not apply them.
The existence of pets summoned will be unstable unless the player invests skill points in the native skill or acquires it from an Oskill. Pets will not be summoned by hirelings.
% Chance to Cast on Attack, on Striking and When You Kill an Enemy are restricted to the weapon on which they spawn (% Chance to Cast on one weapon will not apply to the other). [2]
No % Chance to Cast is applied while using Whirlwind.

Skill level

% Chance to Cast normally specifies a set skill level (slvl), but a few apply a skill level which is dependent on the skill's required level (rlvl) and the item level (ilvl): slvl = min([(ilvl-rlvl)/4]+1,20). [3] The table below specifies the minimum ilvl required for particular slvls, dependent on the skill's rlvl.

Rlvl	Slvl
Rlvl	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
1	1	5	9	13	17	21	25	29	33	37	41	45	49	53	57	61	65	69	73	77
6	3	10	14	18	22	26	30	34	38	42	46	50	54	58	62	66	70	74	78	82
12	9	16	20	24	28	32	36	40	44	48	52	56	60	64	68	72	76	80	84	88
18	15	22	26	30	34	38	42	46	50	54	58	62	66	70	74	78	82	86	90	94
24	21	28	32	36	40	44	48	52	56	60	64	68	72	76	80	84	88	92	96
30	27	34	38	42	46	50	54	58	62	66	70	74	78	82	86	90	94	98

This only applies to seven Unique items. In practice, minimum slvl is determined by the Unique item level or its base item level, whichever is highest. The table below specifies the minimum ilvl required for particular slvls.

Unique Item	Level	Skill	Rlvl	Slvl														% Chance	Event
Unique Item	Level	Skill	Rlvl	7	8	9	10	11	12	13	14	15	16	17	18	19	20	% Chance	Event
[[D2 Unique Elite sword#Lightsabre\|Template:D2 Unique]]	73	Chain Lightning	18								70	74	78	82	86	90	94	5	On Attack
[[D2 Unique Elite axe#Hellslayer\|Template:D2 Unique]]	73	Fire Ball	12										72	76	80	84	88	10	On Attack
[[D2 Unique Exceptional axe#Stormrider\|Template:D2 Unique]]	49	Charged Bolt	1							49	53	57	61	65	69	73	77	10	On Striking
[[D2 Unique amulet#The Rising Sun\|Template:D2 Unique]]	73	Meteor	24							72	76	80	84	88	92	96		2	When Struck
[[D2 Unique Exceptional belt#Snowclash\|Template:D2 Unique]]	49	Blizzard	24	48	52	56	60	64	68	72	76	80	84	88	92	96		5	When Struck
[[D2 Unique Exceptional axe#Boneslayer Blade\|Template:D2 Unique]]	50	Holy Bolt	6						50	54	58	62	66	70	74	78	82	50	When Struck
[[D2 Unique Elite polearm#Stormspire\|Template:D2 Unique]]	85	Charged Bolt	1														77	2	When Struck

Casting order

Casting order is normally irrelevant: if two or more skills are triggered by the same event, all are cast, even if there are two or more delays: those for each skill are not applied. [4] [5] However, an 11 frame delay is applied from the action flag (the frame in the attack or casting animation in which the action is actually executed). During this delay no active skills can be used, although Scrolls of Identify and Town Portal placed in the belt can be used. [6] [7]

This 11 frame delay does not interrupt an attack sequence if % Chance to Cast is applied during that sequence. The delay can be applied from the action flag of any individual attack in the sequence, so if it is applied by an attack early in the sequence then the delay may end before that sequence ends. If multiple attacks in a sequence apply % Chance to Cast, the delay is applied from the last attack in the sequence to apply it. [8]

Casting order only becomes relevant when two or more skills using next delay missiles are cast (since the next delay of the first cast may prevent those following from applying to the target), or when two or more Curses are cast (since the last cast will normally overwrite those preceding it). The factors affecting casting order for all remaining cases take precedence in the following order:

Trigger event

Event	Curses [9]	Other [10]
Active	On Striking On Attack When You Kill an Enemy When You Level-up	On Attack On Striking When You Kill an Enemy When You Level-up
Passive	When Struck When You Die	When Struck When You Die

Equipping order

When the trigger event is the same, equipping order determines casting order: first equipped, first cast and so on. [11] [12] Only when the user switches weapons, levels-up or dies will the game re-equip weapons or items in the following order: [13]

Head
Neck
Torso
Right Arm
Left Arm
Right Ring
Left Ring
Belt
Feet
Gloves

Skill/stat order

When the trigger event and item are the same, casting order is determined by the displayed order: Curses are cast in descending order (first displayed to last), [14] while other skills are cast in ascending order (last displayed to first). [15] The table lists Curses and skills in casting order, not displayed order:

Curse	Skill
Taunt Howl ¹ Lower Resist Decrepify Attract Life Tap Confuse Terror Iron Maiden Weaken Dim Vision Amplify Damage Hit Blinds Target Hit Causes Monster to Flee	Frost Nova Nova Chain Lightning Teeth Poison Nova Twister ² Volcano ² Tornado ²

¹ Howl may be cast second, but due to its mechanics it is actually applied last. [16]
² Twister, Volcano and Tornado mechanics will often result in variations between casting and application order.

Skill Level

When the trigger event, item and skill are the same, casting order is determined by ascending order of skill level: the lowest is cast first and the highest is cast last. [17]

References

Adeyke, Random level of CTC skill (Technical Discussion)
Onderduiker, % Chance to Cast and Curses Tests (Technical Discussion)