How big is my hard drive, really?


I just bought a new computer that was advertised with a 40 GB hard drive, but when I look at it in Windows, it shows up as only a 37.2 GB. Where is the rest of the space or did I get cheated?

- Shawn


This question was answered on September 22, 2004. Much of the information contained herein may have changed since posting.

There are a combination of factors that will reduce the reported size of your hard drive in Windows from what the manufacturer reports

Since the maxim of more is better tends to rule our society, it is not hard to figure out how this confusing method of reporting drive size has come about.

Any marketing person will tell you that if you could advertise a computer that has a 37.2 GB hard drive as one that has a 40 GB hard drive, its a good thing.

The primary difference in how hard drive manufacturers and software programs view disk space comes down to a basic math component.

The technical method for calculating digital space is by using Binary measurement One Kilobyte (KB) in binary is actually 1024 bytes (2 to the 10th power) This is the method of reporting disk space used by software programs and operating systems (Windows).

Hard drive manufacturers use Decimal measurement for reporting disk space, so one Kilobyte in decimal is only 1,000 bytes (10 to the 10th power)

At this micro level the difference in reported space is 24 bytes, which is negligible, but when you start to increase the amount of space being reported, the difference becomes exponential.

One Megabyte (MB) in binary is 1,048,576 bytes (2 to the 100th power) but in decimal it is only 1,000,000 bytes (10 to 100th power) The reported size difference is now 48,756 bytes

One Gigabyte (GB) in binary is 1,073,741,824 bytes (2 to the 1000th power) but in decimal its only 1,000,000,000 bytes which is a difference of 73,741,824 bytes.

When you get into 40 and 80 GB drives the difference can be quite noticeable (almost 7%) If you divide what a manufacturer calls a 40GB drive (40,000,000,000 bytes) by an actual Gigabyte (1,073,741,824) you will get 37.2 GB of binary measured space

If you look closely at most hard drive labels and websites, they have a disclaimer stating that to them 1,000,000,000 bytes = 1 GB and that operating systems use binary numbering systems, which will result in lower reported capacity.

You did not get cheated; you just got introduced to the marketing math used by every hard drive manufacturer If any of them were to start reporting their drives in binary measurement, they would appear to have smaller hard drives than the competition, so dont count on this changing any time soon.

If you think there is difference in a 40 GB hard drive, wait until we start getting into Terabyte drives! There is almost a 10% dfference in reporting size in a 1 Terabyte drive, which translates to a 100GB difference.

That means when the drive manufactures eventually produce a 900 GB hard drive, it will surely be hailed as the first 1 Terabyte drive and they will get away with it!

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Posted by Ken of Data Doctors on September 22, 2004