2.5 inches. I fitted the Samsung 850 EVO, fits perfectly.
A great writeup, worked perfectly.
The rubber feet on my laptop had quite a lot of glue on, so taking those off took some doing.
It may be worth mentioning that at step 15 (backplate removal), you need to be careful with the HDD flexible cable, and know how to remove and replace afterwards. Might seem obvious, but this threw me a bit as a newbie, as it is looped around the backplate. :)