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] (bytes/1,073,741,824) |
| 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
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