A TechLore "Ask the Experts" Question...

Question:

I recently purchased a new 512MB memory card for my digital camera. My computer tells me the max capacity is only 485MB. Why does my card have less capacity than what it's supposed to? Did I get ripped off?

-submitted by jack2312

Answer:

What you're running into is a difference of opinion in how data storage capacity is defined. More specifically, whether or not you calculate capacity using a binary (base 2) or decimal (base 10) system.

Since the prefix "kilo" is defined as 1000 in a base-10 system, then 1 kilobyte would be equal to 1000 bytes. "Mega" is defined as a million, so 1 megabyte would be equal to 1 million bytes. This is the logic that most memory card and hard drive manufacturers follow when defining how much data a particular card or drive can hold.

What makes things complicated is that the computer world is binary, where 1 kilobyte is actually equal to 1024 (2 to the 10th power) bytes. While it may not make a huge difference when you're only talking about 1 kilobyte, the 24 byte difference starts to add up as capacity increases.

To look at your situation specifically:

The card maker defines a kilobyte 1000 bytes. So your 512MB memory card will have around 512,000,000 bytes. It's normal for that number to be a little less. Card makers only guarantee that the formatted capacity will be close.

Windows recognizes data storage in binary, where 1 kilobyte equals 1024 bytes. So in their eyes, your 512MB card (which contains around 512 million bytes) is recoginzed as 488MB.

To see exactly how many bytes your card contains, go to my computer, and right click on the drive letter for your card. Choose "Properties", and look in the section labled "Capacity". You should see that the total number of bytes will be somewhere close to 512 million. In the image above, I ran a scenario on a 128MB Compact Flash card. Windows recognizes it as 121 megabytes, but the total number of bytes is 127,772,672; close to 128 million bytes.

In other words, you did not get cheated by the manufacturer of the card. Like all other card makers, they want to be able to tout the highest amount of capacity using the least amount of bytes.

Matt Whitlock - Editor, TechLore.com

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