The Apple Watch has 8GB of storage space, but it is broken into reserved segments.
According to 9to5Mac, who sought confirmation from Apple, the wearable has 8GB of storage in total, with 2GB of that available for your music. This translates into around 200 songs, so you should be able to get through your workout or commute.
Meanwhile, 75MB of this storage space is reserved for the Photos app. This may seem rather limited, but Apple wants you to be selective about what photos make it to your wrist. It’s estimated that it should be enough for around 100 photos, so you don’t have to be that picky.
Locally stored music and photos can be played on your Apple Watch without your iPhone. So, if you do forget your phone somewhere, at least you’ll have a small selection of songs to make yourself feel better.
That leaves a little under 6GB of storage space unaccounted for. You can expect that a good chunk of this is taken up by the OS and pre-installed apps. The final portion will probably store miscellaneous data like settings and application caches to ensure a smoother overall experience.
The storage space is the same across all Apple Watch variants, whether you spend $349 or more than $10,000 on the wearable.