# Lab Notes: What's a 'Gibibyte,' Anyway?

When is two gigabytes not two gigabytes? When you’re transferring files back and forth between a Mac and PC, that’s when. If you’ve ever packed a 100GB external hard drive to the brim with data from your MacBook only to plug it into a friend’s PC and discover what appears to be a gimped 92GB of capacity, you’re not alone.

But don’t worry, your data hasn’t gone anywhere; the PCWorld Lab crew ran into the same problem recently when sharing a data set between Mac and PC, and we’re here to explain this storage mystery. Be warned, though--there’s math involved.

Here in the PCWorld Lab we’re always working on improving our tests to keep pace with the demands of consumer technology. So when the Macworld Lab team moved in with us, it was like the ultimate nerd sleepover--except instead of pillow forts, we built new storage testing procedures that unified two disparate testing methodologies.

If you take a look at our How We Test page, you’ll see that up until recently we were throwing 3.6GB of hard data at every storage unit that crossed our testbeds. But the Macworld guys were testing their storage drives with just 1GB of data and could therefore test a greater variety of drives on behalf of their readers.

We wanted to use identical test files, regardless of whether we were testing on a Mac or a PC, so we compromised on a new 2GB data set packed with photos and Johnny Cash albums. But when we started running the new storage tests, what looked like 2GB in Snow Leopard OS X 10.6 turned out to be only 1.85GB in Windows 7.

Our data seemed to have been stolen, and the culprit? Why it was binary, dear reader.

After a little bit of research we realized that while hard drive manufacturers universally describe storage capacity in powers of 10, Windows 7 still measures data in powers of 2. Since the closest you can get to 1,000 in exponential binary is 1,024 (2^10), every Windows 7 gigabyte (1,000MB) is actually 1,024MB--which should really be referred to as a gibibyte (GiB).

A GiB equals 1.074GB. Mac OS measured data the same way as Windows does until Snow Leopard, last year’s Mac OS X upgrade which (among other improvements) began measuring data in exponents of 10 instead of 2, just like the drive manufactures.

So the truth is that whether your OS reports file size in powers of 10 or powers of 2, your actual data remains the same. For example, if you transfer 10GB of images from Snow Leopard to an SD Card, it may be reported differently on the Windows hard drive you copied them to.

Our tests show OS X is simply more accurate than Windows when it comes to displaying storage capacity; much as America holds to an outdated imperial measurement system, Windows still clings to a binary counting convention in the face of a metric majority.