Hard Drive Capacity displays difference from advertised sizes and storage sizing


Explains why a hard drive’s capacity often appears to be less than advertised when showing in (My) Computer or Mac Disk Utility.

Question: What is the actual capacity of a 1TB HDD, is it 1000GB or 931GB?

Many user confuse about the difference in the way many operating systems display hard drive sizes, compared to the way hard drive manufacturers describe them. The reason is Hard drives are specified and sold using “GB” and “TB” in their decimal meaning: one billion and one trillion bytes. Many operating systems and other software, however, display hard drive and file sizes using “MB”, “GB” or other SI-looking prefixes in their binary sense,


Ex: Local Disk Capacity in byte: ~135,000,000,000 (135 GB in decimal prefix) vs 125 GB (GiB in binary prefix). Lower about ~7%

In the table below is examples of hard drive capacity from manufactures vs OS display


Drive Size Approximate (Total Bytes) Manufacture label in GB
[Decimal Capacity] (bytes/1,000,000,000)
OS display in GB or GiB
[Approx Binary Capacity]
100GB 100,000,000,000 100                                                    93.13
250GB 250,000,000,000 250                                                232.83
500GB 500,000,000,000 500                                                465.66
600GB 600,000,000,000 600                                                558.79
750GB 750,000,000,000 750                                                698.49
1TB 1,000,000,000,000 1000                                                931.32
2TB 2,000,000,000,000 2000                                              1,862.65
3TB 3,000,000,000,000 3000                                              2,793.97
4TB 4,000,000,000,000 4000                                              3,725.29
5TB 5,000,000,000,000 5000                                              4,656.61
6TB 6,000,000,000,000 6000                                              5,587.94

In the table below is the difference between decimal and binary prefix


Decimal (Manufacture label) Binary (OS display)
Value Metric Value JEDEC IEC
1000 kB kilobyte 1024 KB kilobyte KiB kibibyte
10002 MB megabyte 10242 MB megabyte MiB mebibyte
10003 GB gigabyte 10243 GB gigabyte GiB gibibyte
10004 TB terabyte 10244 TiB tebibyte
10005 PB petabyte 10245 PiB pebibyte
10006 EB exabyte 10246 EiB exbibyte
10007 ZB zettabyte 10247 ZiB zebibyte
10008 YB yottabyte 10248 YiB yobibyte

Is HDD capacity display important?

Yes! When you sizing the storage for an application like SQL, Email, CCTV… the real display capacity of HDD is important for the decision number of Hard disk, capacity of each hard disk and the RAID level.

Example storage sizing:

  • Your application is required 32TB storage
  • If you choose 8 x 4 TB HDD = 32TB, you will fail in storage sizing
  • Real capacity of 8x4TB HDD = 8 x 3725 GB = 29,802 GB.
  • If you use RAID 5 level: 8x4TB HDD = 7 x 3725 GB = 26,077 GB. (1 HDD for RAID)
  • Other RAID levels: see more at RAID online calculator

Be the first to comment

Leave a Reply

Your email address will not be published.


This site uses Akismet to reduce spam. Learn how your comment data is processed.