Have you ever wondered why computers always indicate that your hard drive or memory card has a smaller storage capacity than what’s advertised on the box (and the card itself)? No, it’s not because you got a defective card, it’s not because your card came preloaded with a bunch of unwanted files (your hard drives, maybe), and it’s not because the manufacturers are cheating you by skimping out on the storage space (well, not directly, at least). The reason has to do with math and marketing.
Humans think about numbers in base 10, the decimal numeral system, because we have 10 fingers and 10 toes. That’s why the parts of numbers are called “digits” — just like the parts of our hands and feet.
Computers, on the other hand, think in base 2, the binary numeral system.
Herein lies the root of the issue. The “brilliant” marketing gurus at data storage companies decided early on that all their products should be marketed in the decimal system, since that’s what consumers understand.
Therefore, one megabyte on their products is equal to 1,000,000 bytes, and one gigabyte is equal to 1,000,000,000 bytes. To a computer, however, a megabyte is 1,048,576 bytes and a gigabyte is 1,073,741,824 bytes.
Thus, for each gigabyte advertised in base 10, you’re actually receiving about 70 megabytes less than a gigabyte in base 2.
Take a close look at the box your hard drive or memory card came in, and you’ll notice asterisks. Look for the fine print, and you’ll see a disclaimer stating that the figures are stated in base 10:
Memory cards aren’t as bad as hard drives when it comes to advertised versus “actual” (it seems that the makers try to match up the two figures): when you buy a card for your camera, you’ll probably getting something close to what it says on the box.
Buy a large external hard drive, however, and you’ll likely see a much bigger difference between what the box tells you and what your computer reports.