Why Do Hard Drives Show the Wrong Capacity in Windows?

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If you’ve ever purchased a computer with a hard disk capacity of 500 GB and opened Windows Explorer only to find that its capacity looked more like 440 GB, you may be wondering where all those gigabytes went.

There are several reasons Windows could display the wrong amount of available space, from invisible shadow files, formatting overhead, and hidden recovery partitions to misleading (though technically accurate) storage capacities advertised by hard drive manufacturers.

Image Credit: Norlando Pobre

Why Your Hard Drive Shows Less Space Than Advertised

If you’ve paid attention to hard drives, USB flash drives, and other storage devices, you may have noticed that they always have less space than promised once they’re formatted. The reason for this difference lies in the way hard drive manufacturers advertise their devices, versus the way Windows computers actually use the storage devices. There’s also some overhead required when Windows formats your drive, for the file system and boot data, though in comparison to today’s large hard drives, it’s not a lot.

To a hard disk manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. Essentially, if a hard disk is advertised as 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space. The hard disk manufacturer thus advertises the disk as a 500 GB hard disk.

However, manufacturers of RAM don’t sell it in even groups of 1000 – they use groups of 1024. When you’re buying memory, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB. To work back from the 500,000,000,000 bytes above:

500,000,000,000 / (1024*1024*1024) = 465.66 GB

Keep in mind that the hard drive manufacturers are using the accurate description of the terms–the prefix giga, for instance, means a power of 1000, whereas the correct term for powers of 1024 is gibibyte, though it isn’t often used. Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000.

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That’s a difference of nearly 35 GB over what the average buyer would be led to believe a hard drive contains. If hard disks were advertised in terms of the amount of space they actually contained when you connected them to your Windows computer, a 1 TB hard drive would be labeled a 931 GB hard drive instead.

Alternatively, Windows could update their UI to use the correct definition of gigabyte–other operating systems, like OS X, have already changed their representation to correctly state the right amount of space.

Why Your Computer Shows the Wrong Amount of Free Space

You’ll probably notice something odd about the amount of free space your hard drive contains, if you look closely. If you right-click your C: drive in Windows, you’ll see a certain amount of space referred to as “Used Space” – in the screenshot below, the hard disk contains 279 GB of files.

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However, if you select all the files on your C: drive (including hidden files and Windows system files), right-click them, and select Properties, you’ll notice something odd. The amount of space used by files doesn’t match up with the amount of used space on your hard drive. Here, we have 272 GB worth of files on our C: drive – but Windows is using 279 GB of space. That’s a difference of 7 GB or so – where did all those GBs go?

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It turns out that certain types of files don’t appear in Windows Explorer. Files in Windows’ aptly named “shadow storage,” also known as “shadow copies,” don’t appear here. The shadow storage contains System Restore points and previous versions of files for the Previous Versions feature in Windows Explorer.

To view the exact amount of storage used by shadow files on every hard drive attached to your system, you can run the command below. You’ll need to run it as Administrator – to open a Command Prompt window as Administrator, search for Command Prompt in the Start menu, right-click the Command Prompt shortcut, and select Run as administrator.

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Run the following command in the Command Prompt window:

vssadmin list shadowstorage

As we can see in the command below, about 9 GB of space is used in our hard drive by the Windows Shadow Copy Storage. The difference above looked more like 7 GB, but that can be explained by rounding.

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To adjust the amount of hard drive space used by the shadow copy service (System Restore and Previous Versions of files), follow this guide: Make System Restore Use Less Drive Space in Windows 7

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Other Partitions

Laptops and desktop computers often come with several partitions, including a hidden recovery partition. If you’re wondering why a new computer has less free space than its hard drive specifications would lead you to believe, there’s a good chance some of that is taken up by a separate recovery partition.

To check for partitions, use the Disk Management application included with Windows. Click Start, type partitions, and select the Create and format hard disk partitions shortcut to open it.

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The hard drive should report its correct size in the Disk Management window. As we can see in the screenshot below, nearly 11 GB of the hard drive’s space is reserved for a hidden recovery partition. This is fairly typical of laptops and other computers you don’t build yourself.

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Each of these factors can take a bite out of your available hard drive space, leaving you with less space than expected for your own use.

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.