How is the damage for shield slam and shield bash calculated?

Question

Shield slam and bash are unique spells which utilize the defense value of the shield worn to calculate the damage.

Is the other factor in calculating damage based on weapon skill or shielding skill? Are these spells stronger in protector or blood rage stance?

Answer 1

It appears shielding and defense value have largest impact on damage dealt. 

Testing with higher melee skill values could bring different results.

A test was conducted using a level 20 elite knight with base skills fist 38, sword 26, shielding 10. Shields utilized included; plate shield, bone shield, mastermind shield, mastermind shield with +4 shielding imbuement. Damage was tested against a summoned monk under a variety of conditions including; no stance with shielding reduced to 0 with berserker potion, no stance with shield equipped only, no stance with powerful blockade imbued helmet and/or shield to boost shielding, protector and blood rage stances and powerful blockade imbued helmet/shield, no stance and spike sword with total +3 def (+2 base, +1 proficiency perk). Hopefully someone who loves statistics more than I do can dig deeper into these numbers even with the relatively small sample size of hits. 

Answer 2

For these two spells, the formula for your average damage is as follows:

F + FLOOR(S * D * P / 1000 + P / 4)

Where:

F	Flat damage
S	Shielding skill
D	Shield defense
P	Base power of spell

Note that shield defense can be modified by your weapon, and there is also a weapon proficiency perk to further increase it.

Example: level 100, shielding skill 100, great shield (def 50), magic sword (+3 def modifier), wheel of destiny with one gem (+1 to flat damage). Total flat damage will be 21, total def will be 53. For shield slam (base power 52):

21 + FLOOR(100 * 53 * 52 / 1000 + 52 / 4) = 309

If you take the +1 defence modifier perk on the magic sword's proficiency tree, it will increase D from 53 to 54, and the average damage will increase from 309 to 314 in this instance.

Note that this is your average damage (there is some variation), and then the damage will also get affected by the creature's physical resistance, armor, and mitigation.