No Internet? No Problem: YouTube Getting Ready to Let You Watch Videos Offline, on Your Phone.

YouTube streams more than six billion hours of video a month. Soon it will let people watch some of those videos even if they’re not connected to the Web.

Next month, Google’s video site will let viewers save clips on their phones and other mobile devices for up to 48 hours, so they can watch them when they’re not online.

The videos will still be free, and Google will run ads on the clips, which will be available via its mobile apps.

That’s a big change for the eight-year-old company, which has previously required a live Web connection to watch its videos (though there are plenty of workarounds).

YouTube announced the move in a blog post published late last night, and started telling its partners about its plans via email, as well; YouTube said it will allow uploaders to opt out of the offline option.

The practical benefit for YouTube viewers is that they’ll be able to watch videos in places where they’ve previously been unable to do so, like planes or cars.* This should presumably boost overall viewing for the site, which already generates more than a quarter of its views — that’s more than a billion views a day — via mobile.

If so, that may put more pressure on YouTube’s ad rates by opening up more inventory.

Here’s the text of YouTube’s post announcing the move:

We’re always exploring ways to bring more viewers to your content. As part of this effort, later this year we’ll launch a new feature on YouTube’s mobile apps that will help you reach fans — even when they’re not connected to the Internet. This upcoming feature will allow people to add videos to their device to watch for a short period when an Internet connection is unavailable. So your fans’ ability to enjoy your videos no longer has to be interrupted by something as commonplace as a morning commute. This is part of our ongoing updates to give people more opportunities to enjoy videos and channels on YouTube mobile. Check out the YouTube blog when this launches in November for more details on how this will work for viewers.

* I’ll check, but I don’t believe the move is relevant to Google’s Chromecast gadget, which is designed to work in conjunction with mobile devices, but which relies on an Internet connection to play content on your TV. UPDATE: That’s correct.