Hard Drive Capacities – Manufacturer Versus Operating System Reports

I have received a couple of questions about the Seagate FreeAgent Pro 500 GB drive that I recently purchased. One question that I have received multiple times concerns the manufacturer stated capacity versus the capacity reported by the operating system.

Capacity is normally defined using the standard units of metric measurement;

  • Mega – Million
  • Giga – Billion
  • Tera – Trillion

with the unit of measurement being a Byte – consisting of 8 bits.

That is all well and good, but here is where the manufacturers and operating systems such as Windows differ. There are two methods for reporting capacity

  • Binary – Kilobyte = 1024 bytes (Used by Operating Systems such as Windows)
  • Decimal – Kilobyte = 1000 bytes (Used by Drive Manufacturers)

This small discrepancy is barely noticed at smaller drive sizes, but as drives become bigger the difference becomes more noticeable.

Review this chart from the definition of Byte on Wikipedia, and Seagate site on Storage Capacity Measurement Standards.

Name Decimal Capacity Decimal (Equivalent) Binary Capacity Binary Value (in Decimal) Size Difference
KiloByte 10^3 1,000 2^10 1,024 2.40%
MegaByte 10^6 1,000,000 2^20 1,048,576 4.86%
GigaByte 10^9 1,000,000,000 2^30 1,073,741,824 7.37%
TeraByte 10^12 1,000,000,000,000 2^40 1,099,511,627,776 9.95%

As drives capacity increases this discrepancy becomes more obvious, so a 500 GB drive is as follows:

500 GB * 7.37% = 36.85 GB difference or about 465 GB capacity reported by the operating system.

The drive has the same capacity for physical storage, the capacity is just being reported using different standards and can be misleading.

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2 Responses to “Hard Drive Capacities – Manufacturer Versus Operating System Reports”

  1. jameswillisisthebest Says:

    This is my first post
    just saying HI

  2. Thomas Says:

    jameswillisisthebest – Thanks for stopping by and commenting. Welcome to the conversation!


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