Are critical hits calculated on hit or on action?

If they are calculated on action I will link my Whirling Blade to Power charge on critical. Currently I have it linked on my Reave, and that limits my potential damage.

On action. If you roll crit on skill use, every hit with that skill will crit (or at least attempt to in case of attacks). If the skill repeats, crit will repeat too.

PCoC can give you at most one charge per skill use, so multistrike will indeed slow down its generation.

To be fair, I wasn't using multistrike yet. I should buy a 5L chest (I haven't found anything worth of being linked).

Thanks for the answer. The wiki said something similar but I wanted to confirm.

just an addendum, since it can be related:

'%chance to ignite/shock/freeze' is calculated on hit.

It can be kinda important when considering getting shocks or freezes for HoI and HoT (or that new Innervate gem and presumably the upcoming others like it), or getting ignites applied to prep Flame Surge. It means something like Lightning Tendrils can have 4 (or 8 with Spell Echo) chances per cast (and per target) to roll shock, as opposed to the one chance per cast to roll crit.