As I conjectured back when it was first rumored that the new iPhone 6s would get 4K video recording capabilities, iPhone 6s owners will have a hard time maintaining space if they shoot even a moderate amount of Ultra High-Definition video on a 16GB iPhone 6s.

First and foremost, the 16GB iPhone features about 12 GBs or so of usable storage space. 4K video, while amazing looking, uses 375 MB of storage for every minute of video shot. It doesn’t take a mathematician to quickly discern that 4K video will eat through 12GBs of storage space in relatively short order.

Some simple math shows us that shooting just 32 minutes of 4K video would completely exhaust 12 GBs of storage. Yes, one 32 minute video—one dance recital, one baseball game, one camping adventure—would completely fill up a 16GB iPhone 6s. And that’s assuming that you have nothing else stored on your phone.

What if you don’t normally shoot long form videos? Just thirty 1~ minute videos would almost completely exhaust your storage.

Fortunately, users aren’t forced to shoot 4K video. Apple has made it so that we have the option of switching between 720p at 30fps, 1080p (30 or 60 fps), or 4K at 30fps.

Lesser resolution videos use a lot less storage per minute than 4K videos. Here’s the full breakdown:

• 720p30 = 60MB/minute (1MB/second)
• 1080p30 = 130MB/minute (2.2MB/second)
• 1080p60 = 200MB/minute (3.3MB/second)
• 4k30 = 375MB/minute (6.25MB/second)

Here’s the deal: 16GB iPhones are great if that’s all you can spring for in the money department, just know what to expect when it comes to shooting video. If you want to take full advantage of the iPhone’s new capabilities, I’d highly suggest you opt for a 64GB iPhone 6s if at all possible.

If you plan on using the phone extensively for shooting 4K videos, I’d have to go ahead and recommend a 128GB iPhone. That phone would probably have somewhere in the ballpark of 120GB of usable space, which means you’d be able to shoot 320 minutes (5.3 hours!) of 4K video before exhausting your space.

The 64GB iPhone would get somewhere in the ballpark of 155 minutes (2.5 hours) of 4K video, definitely not shabby, but 128GB is where it’s at for extra-prolific video shooters. Remember, it’s not like you can go and slap an SD Card inside one of these phones. Get as much space as you can reasonably afford.

With all of this in mind, which iPhone will you opt for during Saturday morning’s pre-order frenzy? I shoot a lot of video, but I think 128GB is too much for my shooting habits. I’ll be opting for a 64GB iPhone 6s Plus. Why a Plus? Here’s why the 6s Plus is better for video.