Hey there 21pisto,
Thanks for reaching out to Apple Support Communities. We understand your iPhone storage never seems to finish calculating. We’ll be happy to help with this.
If restarting didn’t help, try force closing the app, then restart your iPhone and see if this helps at all:
How to force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone
Then wait for at least a minute and see if the size completes calculating.
If this doesn’t help, create a backup and reinstall iOS using the steps under “If your device turns on but gets stuck during start up” from these links:
How to back up your iPhone, iPad, and iPod touch
If your iPhone, iPad, or iPod touch won't turn on or is frozen
We hope helps.
Take care.