Spell Success

Version 0.30: This article may not be up to date for the latest stable release of Crawl.

Spell success is the rate at which casting a spell can be expected to succeed. Spell success rate is determined by a complex combination of:

1. Spell level
2. Spell-related skills, Intelligence
3. Penalties from body armour and shields (mitigated by Armour skill / Shields skill)
4. Modifiers like Wizardry and Wild Magic

Base Chance

The source code talks in terms of chance of failure, not success. Therefore, we want the spellFailure chance to be as low as possible - in fact, to reach a 0% success rate, your chance to fail has to be negative. More on that below.

The base failure before any modifiers is as follows:

 spellFailure = 60
              - [6 * spell skills]
              - [2 * Intelligence]
              + Spell difficulty
              + Armour/shield penalty

• 60 is an arbitrary base chance of failure.
• Increasing intelligence and spell skills (spells of that skill and Spellcasting) will decrease failure.
• Higher level spells are more difficult, wearing armour/shields make it more difficult.

spellFailure, at this stage, is capped at 1400.

Skill

"Spell skills" is the combination of Spellcasting and an average of a spells schools (e.g. a Conjurations / Fire spell will take the average of the Conjurations skill and Fire Magic skill). This is calculated with the following two equations:

 Z = [Spellcasting / 2] + [Average(SpellSkills) * 2]

 spell skills = 50 * log_2 (1 + Z / 50)

With 27 in Spellcasting and 27 in relevant skills, the total value of (6 * spell skills) is capped at 370.

Spell level

Higher level spells are more difficult. Each level of spell adds a number to your spell failure rate.

 spellDifficulty =   3 (level 1)
                    15 (level 2)   (+12)
                    35 (level 3)   (+20)
                    70 (level 4)   (+35)
                   100 (level 5)   (+30)
                   150 (level 6)   (+50)
                   200 (level 7)   (+50)
                   260 (level 8)   (+60)
                   330 (level 9)   (+70)

Armour and shield penalties

(Under Construction)

 Armour/Shield Penalty = 19 * (armour penalty) + 19 * (shield penalty)

 armour penalty = 0.4 * (Encumbrance Rating)^2
                  * (45 - Armour skill Lv) / 45
                  / (Str + 3)

 shield penalty = max(0, (EV Penalty) - (Shield Skill Lv) / (Player species factor) )

Player species factor is 5 for normal species, 3 for large species, and 7 for small species, respectively. Spriggans have 9, though they only can use bucklers.

Step down

At this point, the spell failure is put through a stepdown curve. If it's over 45, it's unaffected, otherwise it needs to be progressively lower to improve the overall result. This is a stepwise curve, and complex to describe. In the table is the listed native chance to reach certain points (i.e., spell success bands). This is not the final spell rate; there are more modifiers.

Description	spellFailure	Raw Failure Rate
Fair	45	45%
Good	10	30%
Very Good	-24	20%
Great	-80	10%
Excellent	-140	4%
Perfect	-180	0%

From −60 to −180, this equation is linear; each difference of 20 gives 2% to the final spell success chance. Penalties are more meaningful the higher your spell failure chance is.

Modifiers

The following modifiers are added after the STEPPED DOWN failure rates. However, they are still raw spell failure chance, and are adjusted by a sigmoid function later.

Positive (+spell failure):

• Wild Magic: +4% per level
• Orb of energy: +10%
• Crystal ball of Wucad Mu: +20%
• Vertigo: +7%

Negative (-spell failure):

• Subdued Magic: -2% per level
• Wizardry: -25% for 1 source, -33% for 2, -50% for 3

Vehumet gives 2 levels of wizardry for destructive spells.

After all modifiers have applied, you will get a cumulative raw spell failure chance. This is converted into the final success via the function below.

Final Step

The number we have obtained isn't the final chance of failure. The game doesn't compare that number with a random number between 0 and 99, but with the sum of three numbers divided by three.

(1d101 + 1d101 + 1d100 - 3)/3 < fail chance

This is equivalent to applying this transformation to the chance of failure:

This sigmoid function makes it more difficult to decrease your failure rate when it is high or low, but it will go down very quickly when it is in the middle. Up to raw failure rate of 33%, one can use this formula:

 N                 = (raw_failure_rate) * 3
 Real Failure Rate = N * (N+1) * (N+2) / 6 / 101 / 101 (in unit of %)

This can be tabulated for usefulness.

Raw Fail chance	Actual fail chance
32%	14.9%
30%	12.3%
28%	10.0%
26%	8.1%
24%	6.4%
22%	4.9%
20%	3.7%
18%	2.7%
16%	1.9%
14%	1.3%
12%	0.8%

Strategy

It is debatable whether the following formulae are of real use during a game. However, inflection points such when the extra armour EV penalty disappears are fairly easy to discern. Calculating this stuff on the fly is only for savants! However, one can refer example section of this page for Lv 9 magics, which requires tremendous amount of experience to be stabilized.

Example: Fire Storm

For an actual failure rate of 10%, a raw failure rate of 28% is required. Assume Int=32, a good value for a dedicated caster (if a bit low). Also assume no armour/shield penalty.

• Without any aid: With a raw spell failure of 28%, you'll need a spellFailure value of 2. This is equivalent to 54.0 points of spell skills, requiring (school skill*2 + spellcasting*0.5) = 55.7. With a Spellcasting skill of 27, you'll need an average spell schools of 21.1.

• Wizardry x1: A wizardry ring knocks 1/4 of failure rate before sigmoid function, so the required raw fail rate is 38%. A spellFailure of 32% is stepped down to 38%. This value is quite easy to be achieved now, and average of 19.4 Lv including spellcasting is sufficient for 10% failure rate.

• Vehumet worshipper / Wizardry x2: These enhancements knock off 1/3 of failure rate before sigmoid. Now the raw fail rate required before stepdown is 38%, and (school skill*2 + spellcasting*0.5) = 47.3. Required average skill Lvs, including spellcasting, is 18.9. Worshipping Vehumet really offers tremendous benefits for high-level spells.

• Vehumet + Wizardry / Wizardry x3: Maxed out enhancer effect. Raw fail rate of 56% can be used without stepdown, and (school skill*2 + spellcasting*0.5) = 43.3. Required average skill Lvs, including spellcasting, is 17.3. If one regulates spellcasting up to 15, then 17.9 Lv of each school level is sufficient!

See also

• Spell power
• Miscast effect