Are loose head tfillin kosher?

I had to borrow someones t'fillin today. The head straps were way too big. They still sat properly on my forehead just the straps were way too loose. Was I yotzei b'dieved.

Shalom!

Thank you for your question.

In order to answer your question on whether your shel rosh tefillin were properly worn, we must define what the proper fit and placement of the shel rosh tefillin is meant to be.

The shel-rosh is to be placed on the head between the eyes. No part of the tefillin box may fall below the hairline. If it does, one does not fulfil the mitzva. The highest point on the head for the tefillin to sit is “the end of the soft area of a baby’s skull.” Any higher than this also renders the mitzva invalid.

So too, the knot of the shel rosh is to be placed at the back of one’s head at the bottom of the skull where the skull meets the neck. This should be in an area where hair grows.

As such, the strap around the head must be snug enough to ensure the above-mentioned positions of the shel rosh box at the top of the head and the knot at the back of the head. This will only be possible if the straps are not too loose nor too tight.

However, if when you wore the Tefillin it was placed at its right place then you were Yotse even if the straps were loose.

One should only borrow tefillin with permission.

OC 27 and commentaries