How much willpower do gravely wounded soliders lose?

In my current Classic Iron Man game using both Iron Will and New Guy from the Training school I have:

• A Heavy Squaddie that was gravely hurt once at 36 will
• A Heavy Captain that was gravely hurt 2 or 3 times at 17 will!
• A newly recruited Heavy Squaddie (never hurt) is 45 will.

I'm sure I had the Captain before I bought Iron Will, but at what point I don't know. I can't go back and calculate exact values because of the whole Iron Man thing.

I'd guess its around -10 to -12 Will for a grave injury. It's probably a range.

Needless to say that Captain with 17 Will has seen her last mission. She panics when a window breaks.

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Update:

I’ve been tracking these stats the last couple of nights: I had a my supper star support Colonel get gravely hurt twice. His will went from 91 to 81 and then to 67.

So new confirmed range is: -10 to -14.

I just had a major critically wounded and after the mission his Will sat at 70. Before the mission it was 100 and i know this because i just had him qualify for Psi-abilities from the psi-labs prior to the mission.

I suppose the drop in will isn't an issue in this circumstance since he's already psi-qualified and 70 is sufficient to not have issues in a mission. But 30 is a damn lot.

Even though this question is a year old i thought i'd reply anyway since it ranks high in the google search and the game has been on steam sale recently and probably seeing a lot more activity.

There is a difference between "gravely" and "critically" wounded. Gravely wounded soldiers lose some willpower for the remainder of the mission; but not permanently. Critically wounded lose their willpower permanently.

Citation:

Injured:
If a soldier is injured during combat, for the remainder of the mission they are subject to a Will penalty, visible as "Battle Fatigue" when viewing the soldier info interface. If injured by less than 50% of their total health, the penalty is -5 Will. If injured by more than 50% of their total health, the penalty is -10 Will.

Critically Wounded
When a soldier loses all of their HP during a mission, they either die or become critically wounded. Soldiers of a higher rank are more likely to be critically wounded instead of dying. Critically wounded soldiers must be stabilized or revived with a Medikit or they will bleed out and die in three turns (including the turn they were injured), if the mission is not completed by then. Critically wounded soldiers, even if saved, incur a permanent -10 (-15 on Classic or Impossible) reduction to Will. In XCOM: Enemy Within, this penalty can be avoided with the Secondary Heart Gene Mod.

Critically wounded soldiers are not targeted by enemies, but can be killed before bleeding out in the event of an explosion. Take care while fighting enemies that are equipped with explosives or explode upon death (such as Cyberdiscs), as well as near combustible objects in the environment (such as vehicles).

Gravely Wounded
When a soldier is heavily (but not critically) wounded, they are considered to be gravely wounded. Other than an increased recuperation time in the Infirmary, this status is the same as wounded. It does not require stabilization or entail critically wounded's Will reduction penalty.

Source: http://xcom.wikia.com/wiki/Soldier_(XCOM:_Enemy_Unknown)#Mortality