Why my drive shows less space than expected?

Have you ever wondered when you plug in your new hard disc drive and it shows less free space then expected? This ain’t the question you thought for the first time. If you google this query, you’ll find over 3.7 million results, and this post that you are reading is probably the first one. so you are in-a-way, landed on the right page Smile

Well that’s not very obnoxious. Its pretty simple, rather a mere numbers game.

In order to determining the disc drive capacity you have to count on the operating system. This is mainly because of the file system they user. When dealing with systems based on Windows and Mac, you will often see both binary measurements and decimal measurements of the capacity of your drive. In both cases, the capacity of a drive is measured by using the total number of bytes available on it. As long as the drive displays the correct number of bytes (approximate), you are getting the full capacity of that drive.

Decimal vs. Binary:
First up, the hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes for the obvious sake of simplicity and consistency. This is industry standard is basically a decimal (base 10) measurement. However, certain systems like BIOSs, FDISK and Windows define specifically a megabyte (MB) as 1,048,576 bytes and a gigabyte (GB) as 1,073,741,824 bytes. Mac and most other systems also use these values. These are binary (base 2) measurements.

To Determine Decimal Capacity:
A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).

To Determine Binary Capacity:
A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2).

This is the main reason why different utilities actually report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make an MB and a GB.

Important: If you are dealing with drive sizes smaller than the Approximate Binary Capacity of your Drive Size, then you may be dealing with either a BIOS limitation, or a Windows drive size limitation. For more information, please see links below.

Following table shows a relative count for the capacities measured by different file systems.

Various Drive Sizes and their Binary and Decimal Capacities

Drive Size	Approximate Total Bytes	Decimal Capacity (bytes/1,000,000,000)	Approximate Binary Capacity(bytes/1,073,724,841)
10 GB	10,000,000,000	10 GB	9.31 GB
20 GB	20,000,000,000	20 GB	18.63 GB
30 GB	30,000,000,000	30 GB	27.94 GB
36 GB	36,000,000,000	36 GB	33.53 GB
40 GB	40,000,000,000	40 GB	37.25 GB
60 GB	60,000,000,000	60 GB	55.88 GB
74 GB	74,000,000,000	74 GB	68.91 GB
80 GB	80,000,000,000	80 GB	74.51 GB
100 GB	100,000,000,000	100 GB	93.13 GB
120 GB	120,000,000,000	120 GB	111.76 GB
150 GB	150,000,000,000	150 GB	139.69 GB
160 GB	160,000,000,000	160 GB	149.01 GB
180 GB	180,000,000,000	180 GB	167.64 GB
200 GB	200,000,000,000	200 GB	186.26 GB
250 GB	250,000,000,000	250 GB	232.83 GB
300 GB	300,000,000,000	300 GB	279.40 GB
320 GB	320,000,000,000	320 GB	298.02 GB
400 GB	400,000,000,000	400 GB	372.52 GB
500 GB	500,000,000,000	500 GB	465.65 GB
640 GB	640,000,000,000	640 GB	595.84 GB
750 GB	750,000,000,000	750 GB	698.47 GB
1 TB (1000 GB)	1,000,000,000,000	1 TB (1000 GB)	931.30 GB
1.5 TB (1500 GB)	1,500,000,000,000	1.5 TB (1500 GB)	1396.95 GB
2 TB (2000 GB)	2,000,000,000,000	2 TB (2000 GB)	1862.6 GB
3 TB (3000 GB)	3,000,000,000,000	3 TB (3000 GB)	2793.97 GB
4 TB (4000 GB)	4,000,000,000,000	4 TB (4000 GB)	3725.29 GB
6 TB (6000 GB)	6,000,000,000,000	6 TB (6000 GB)	5587.94 GB
8 TB (8000 GB)	8,000,000,000,000	8 TB (8000 GB)	7450.58 GB