Units

How many MB are in a GB?

Both answers are correct, for different units — and knowing which is which explains the "missing" space on every hard drive you have ever bought.

There are 1,000 MB in 1 GB. The SI decimal definition — used by hard drive and SSD makers, network speeds, and macOS, Android and iOS — makes each prefix a power of 1,000. The 1,024 figure belongs to a different unit, the gibibyte (GiB), where each step is a power of 2. Windows displays GiB but labels it "GB", which is the entire source of the confusion: a 1 TB drive holds a true 1,000,000,000,000 bytes, which Windows shows as 931 GB.

Worked out 2026-09-04 (IST) · SI decimal prefixes vs IEC binary prefixes

The two systems side by side

Decimal (SI) vs binary (IEC) data units
Unit	Decimal (SI) — drives, networks	Binary unit	Binary (IEC) — Windows, RAM	Gap
Kilobyte	1 KB = 1,000 bytes	KiB	1 KiB = 1,024 bytes	2.4%
Megabyte	1 MB = 1,000 KB = 10⁶ bytes	MiB	1 MiB = 1,024 KiB = 1,048,576 bytes	4.9%
Gigabyte	1 GB = 1,000 MB = 10⁹ bytes	GiB	1 GiB = 1,024 MiB = 1,073,741,824 bytes	7.4%
Terabyte	1 TB = 1,000 GB = 10¹² bytes	TiB	1 TiB = 1,024 GiB	10.0%
Petabyte	1 PB = 1,000 TB = 10¹⁵ bytes	PiB	1 PiB = 1,024 TiB	12.6%

What advertised drive sizes really show

Advertised capacity vs what Windows reports
Advertised	Actual bytes	Windows shows
500 GB	500,000,000,000	465 GB
1 TB	1,000,000,000,000	931 GB
2 TB	2,000,000,000,000	1.81 TB
4 TB	4,000,000,000,000	3.63 TB
8 TB	8,000,000,000,000	7.27 TB

The one-line rule. If a company sold it to you — a drive, a data pack, a broadband speed — it is decimal, 1,000 to a step. If an operating system is reporting it back to you, it is almost certainly binary, 1,024 to a step, mislabelled as GB.

Check your own numbers

Convert between bytes, KB, MB, GB, TB and their binary equivalents in one step.

Data Storage ConverterBytes, KB, MB, GB, TB and the binary KiB/MiB/GiB units, converted both ways.

Frequently asked

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

Nothing is missing. The manufacturer sold you 1,000,000,000,000 bytes, which is genuinely 1 TB in SI units. Windows divides by 1,024 three times instead of 1,000, giving 931.3 — and then labels the result "GB" instead of "GiB". macOS and Linux with modern tools show 1,000 GB for the same drive.

Is a 100 Mbps connection 100 MB per second?

No, and the gap is a factor of eight. Network speeds are quoted in megabits per second (Mbps), and there are 8 bits in a byte, so 100 Mbps is 12.5 MB/s at best. A 1 GB file therefore takes at least 80 seconds on a perfect 100 Mbps line, before protocol overhead.

Which should I use when calculating storage or bandwidth?

Use decimal (1,000) whenever you are dealing with a purchase, a quota, a data plan or a transfer speed, because that is how vendors define them. Use binary (1,024) when reading memory sizes, file sizes in Windows Explorer, or anything reported by an operating system. Label which one you mean and the ambiguity disappears.

What comes after TB?

Petabyte (PB) = 1,000 TB, exabyte (EB) = 1,000 PB, zettabyte (ZB) = 1,000 EB and yottabyte (YB) = 1,000 ZB. Each has a binary twin — pebibyte, exbibyte and so on — and the gap between the decimal and binary versions widens at every step, reaching about 21% at the exabyte level.

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