Why Your 1 TB Drive Shows 931 GB (and Other Data Unit Mysteries)

You buy a 1 TB hard drive, plug it in and your computer says it has 931 GB. Your internet plan promises 100 megabits, but downloads never go faster than about 12 megabytes per second. Nothing is broken and nobody is cheating you, at least not exactly. The confusion comes from two different ways of counting bytes, and from the difference between bits and bytes.

Bits and bytes

• A bit (lowercase b) is a single 0 or 1.
• A byte (uppercase B) is 8 bits, enough to store one basic character such as "A".

Storage (files, drives, memory) is measured in bytes. Network speed is measured in bits per second. That single letter, b or B, is a factor of 8.

Decimal vs. binary units

There are two systems in everyday use:

Decimal (SI)	Value	Binary (IEC)	Value
kilobyte (kB)	1,000 bytes	kibibyte (KiB)	1,024 bytes
megabyte (MB)	1,000² bytes	mebibyte (MiB)	1,024² bytes
gigabyte (GB)	1,000³ bytes	gibibyte (GiB)	1,024³ bytes
terabyte (TB)	1,000⁴ bytes	tebibyte (TiB)	1,024⁴ bytes

Drive manufacturers use the decimal system. Windows uses the binary system but labels the result "GB". That mismatch is the whole mystery.

The 931 GB math

A 1 TB drive has 1,000,000,000,000 bytes. Windows divides by 1,024³ = 1,073,741,824:

1,000,000,000,000 ÷ 1,073,741,824 = 931.32 "GB" (really GiB)

The same thing happens at every size, and the gap grows with capacity:

Advertised	Shown by Windows	Difference
500 GB	465.66 GB	−6.9%
1 TB	931.32 GB	−6.9%
2 TB	1.82 TB	−9.1%

macOS (since 2009), iOS and many Linux tools display decimal units, so the same 1 TB drive shows as about 1 TB there. On top of that, the operating system, recovery partitions and file system take a little extra space.

Internet speed: Mbps vs MB/s

Internet providers sell speed in megabits per second (Mbps). Browsers and download managers show megabytes per second (MB/s). Divide by 8:

• 100 Mbps = 12.5 MB/s
• 500 Mbps = 62.5 MB/s
• 1 Gbps = 125 MB/s

So a 5 GB game on a 100 Mbps connection takes at least 5,000 MB ÷ 12.5 MB/s = 400 seconds, about 6.7 minutes, and longer in practice because of network overhead and Wi-Fi.

Quick reference

Item	Typical size
A plain-text email	a few KB
A smartphone photo	2–5 MB
A 3-minute MP3 song	about 3–7 MB
One hour of HD streaming	about 1.5–3 GB
A modern video game	50–150 GB

Which system should you use?

For storage you buy, use decimal (what is printed on the box). For RAM and for numbers shown in Windows, use binary. When precision matters, write the IEC names (KiB, MiB, GiB) so nobody has to guess. The CoringaWeb byte converter lets you switch between base 1000 and base 1024 and converts bits, bytes, KB, MB, GB, TB and PB instantly.

Frequently asked questions

Why does my 1TB hard drive only show 931GB?

Manufacturers define 1 TB as 1,000,000,000,000 bytes, while Windows divides by 1,024³ and labels the result GB. The capacity is the same; only the unit differs.

How many MB are in a GB?

1,000 MB in the decimal system, or 1,024 MiB in the binary system.

Is 100 Mbps fast enough?

100 Mbps equals 12.5 MB/s, which is enough for several HD or 4K streams at once in a typical household.

What is the difference between MB and Mb?

MB is megabytes and Mb is megabits. One megabyte equals eight megabits.

Informational content only. For legal, financial or health decisions, consult a qualified professional.