The first thing you’ll do is figure out how much space you’re currently using. An easy way to do that is to go to the Apple Menu in the upper-left of your screen and choose “About This Mac.”
If you’re running Yosemite, you should see the redesigned window pictured below (the one where I’ve pointed out the “Storage” tab) right away, but if you’re using an earlier version of the OS, click “More Info” on the next box to get there.
Then you’ll select the “Storage” tab to get an idea of how much space you’re already using on your drive.
Yikes. I’m getting pretty close there.
If you guys are using an even older version of OS X (10.6 or earlier), the process is different. Select your Macintosh HD (or whatever you’ve renamed your drive) on your desktop and press Command-i, and then you can see your capacity and available space there.
In any case, what I usually recommend that people do is take the amount of storage they’re currently using, then add on at least 50 percent more to give themselves room to grow on a new Mac. In my case, I’m using about 105GB (from a capacity of 120GB minus 15GB free), so if I were to get a new laptop from Apple, I’d want to upgrade to a 256GB drive for sure.
Of course, this isn’t a hard-and-fast rule—if you’re a person who takes a ton of photos or downloads media often, you may want to be less conservative in estimating how much you’ll need. It’s always MUCH easier to pay for a bit more than absolutely necessary than to have to start shuffling data around when you run out of room, especially if you let it get to the point where your Mac slows to a crawl because it doesn’t have any free space to work with. Don’t do that, OK?