Why do drives show less space than advertised?

Isn’t it annoying to discover that a recently purchased drive has less space than advertised? Most users complain that their SSD or a hard disk drive shows less space than what is indicated on the label. Possibly, you also got a drive with a capacity of 1TB but Windows explorer shows only 931GB of available space. Now you may wonder where all remaining gigabytes went. Likewise, if you buy a 512GB external SSD and it will only show 476GB of total available space when plugged into the PC. So why does it happen?

Cheat from manufacturer?

When you notice that a drive’s capacity isn’t as large as advertised, you may think that the manufacturer has cheated on you. But you’re not the only one who faces this issue. Whether you buy a thumb drive, hard drive, or SD card, when you plug it into a Windows computer, the total available space displayed on the computer is always slightly less than the advertised capacity of the device. However, unlike a common belief, less storage capacity doesn’t mean that you got a defective drive. Also, it doesn’t mean that your SD card came preloaded with some unwanted files or it’s the manufacturers that are cheating you. The actual reason for capacity discrepancy lies in math and marketing. Other possible reasons for this can be invisible shadow files, formatting overhead, and hidden recovery partitions. So, let’s understand why advertised capacity is different than actual drive storage capacities.

Check out our hard drive capacity calculator to calculate real usable space vs. advertised. We also have a RAID calculator

It only happens in Windows or older MacOS

The simple answer to this question lies in the Decimal system vs. the Binary system. Common programs like Windows, some old versions of macOS, and other operating systems use the binary number system. In the binary format, 1 GB amounts to 1024 MBs, not 1000 MBs. Humans understand the capacity in the decimal numeral system. But computers work the binary numeral system. For the sake of consumers’ understanding, data storage companies label their products in the decimal system. This means that if a device is labeled as one megabyte which is equal to 1,000,000 bytes, in the computer, this one megabyte is 1,048,576 bytes.

As you might know, MBs, GBs, and TBs are the units used to express the capacity of storage devices. ‘Bit’ is the smallest entity used to measure the capacity of storage devices, and 8 bits make up a ‘byte’ while 1000 bytes make up a kilobyte (KB). All computer systems work on binary math and represent base-2 amounts which means there’s an increment of 2 to the 10th power or 1,024 in each level, that is:

• Kilobyte (KB) = 1,024 bytes
• Megabyte (MB) = 1,024 kilobytes or 1,048,576 bytes
• Gigabyte (GB) = 1,024 megabytes or 1,073,741,824 bytes
• Terabyte (TB) = 1,024 gigabytes or 1,099,511,627,776 bytes

So, manufacturers rate the drive capacities based on the standard base-10 numbers, which means that one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. If you buy a hard disk of 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space. But for your computer, a KB is 1024 bytes, an MB is 1024 KB, and a GB is 1024 MB. This means that your hard drive of 500,000,000,000 (500 GB) will be shown as 500,000,000,000 / (1024*1024*1024) = 465.66 GB in the PC.

As mentioned above, with each level, there’s an increase in the prefix, which also increases the total space discrepancy. Here is the difference in each level to compare the actual and the advertised space:

• Megabyte difference = 48,576 bytes
• Gigabyte difference = 73,741,824 bytes
• Terabyte difference = 99,511,627,776 bytes

According to this difference, for each gigabyte of space in your hard drive, there will be 73,741,824 bytes or approx 70.3 MB of less disk space available in Windows.

Another reason why drive can show less space

There can also be other reasons for space discrepancies, such as hidden software partitions. Possibly, you are using a program that needs to create a separate partition on the drive for proper functioning, and that might reduce the available space. Example: data encryption software.

Key takeaway

• Reason for capacity discrepancy lies in the decimal vs. binary system.
• Binary system shows less capacity than stated on the label.
• Windows and older MacOS using a binary system.
• You can use a calculator to calculate real available space.