What is the consistency of the Rocky Helmet?

Question

Is the damage the same every time or does it vary?

Answer 1

If a Pokémon uses a move that makes contact with a Pokémon holding a Rocky Helmet, the Pokémon that used the move takes damage equal to 1/6 of its maximum HP. Each individual strike from a multi-strike move that makes contact will activate Rocky Helmet.

tldr It's consistent.
source

Comments

And this is why Ferrothorn loves it so much...