MB or MiB, Mbps or MB/s: file sizes and speeds explained

Updated: 29 October 2026

A new 1 TB drive shows as 931 GB, a 100 Mbps connection downloads at 12 MB/s, and the same file is 4.7 GB on one computer and 4.4 GB on another. Nothing is missing in any of these cases. Two different things are going on: sizes are counted in steps of 1,000 by some and 1,024 by others, and speeds are measured in bits while files are measured in bytes. This guide sorts out both, with the numbers worked out.

A byte is eight bits

A bit is a single 0 or 1; a byte is a group of eight bits, enough for one letter of plain text. The symbols differ only by case: b is a bit and B is a byte, so Mb is a megabit and MB a megabyte, eight times larger. Storage and file sizes are counted in bytes; connection speeds are counted in bits per second, written Mbps, Mb/s or Mbit/s.

1 B = 8 bitOne byte.
100 Mb = 12.5 MBA hundred megabits are 12.5 megabytes.
1 MB/s = 8 MbpsA file transfer of one megabyte per second needs an 8 megabit connection.

Two ways to count: 1,000 or 1,024

In the International System of Units, kilo means 1,000, mega a million and giga a billion, and the BIPM, which keeps the system, says these prefixes refer strictly to powers of 10: “one kilobit represents 1000 bits and not 1024 bits”. Computer memory, however, comes in powers of two, and for decades 1,024 bytes were called a kilobyte. To end the ambiguity, the IEC approved separate binary prefixes in December 1998: kibi (Ki), mebi (Mi), gibi (Gi), tebi (Ti), now part of IEC 80000-13.

Decimal	Bytes	Binary	Bytes	Binary is larger by
`1 kB`	1,000	1 KiB	1,024	2.4%
`1 MB`	1,000,000	1 MiB	1,048,576	4.9%
`1 GB`	1,000,000,000	1 GiB	1,073,741,824	7.4%
`1 TB`	1,000,000,000,000	1 TiB	1,099,511,627,776	10.0%

The gap grows with every step, so it is small for a photo and large for a drive. Many programs still use the binary values with the decimal names, which is where the confusion comes from: a “GB” on screen may mean either.

Human-readable units

Who counts which way

Drive and memory-card makers, and internet providers, use decimal units: a 1 TB drive holds 1,000,000,000,000 bytes, and Apple's own packaging note reads “1GB = 1 billion bytes”. Apple devices have shown storage in decimal units too since OS X Snow Leopard and iOS 11; Apple's support page says iOS 10 and earlier and OS X Leopard and earlier counted in binary.

Windows File Explorer shows binary values with decimal names: its KB is 1,024 bytes, a choice Microsoft's Raymond Chen explained in 2009 as following existing practice. On Linux, ls -h, df -h and du -h use powers of 1,024 with letters K, M and G, while --si switches to powers of 1,000, as the GNU coreutils manual describes. Memory chips follow JEDEC, the semiconductor standards body, whose dictionary defines mega for storage capacity as 1,048,576, so 8 GB of RAM is 8 GiB.

Where	Counts in	A 1 TB drive shows as
Drive packaging, internet speeds	decimal (1,000)	1 TB
macOS and iOS today	decimal (1,000)	1 TB
Windows File Explorer	binary (1,024), labelled KB, MB, GB	931 GB
Linux ls -h / df -h	binary (1,024), labelled K, M, G	932G (931.32 rounded up)
RAM modules	binary (1,024)	—

Why a new drive looks smaller

A drive sold as 1 TB has 1,000,000,000,000 bytes. Divided by 1,073,741,824, the bytes in a gibibyte, that is 931.32, which Windows shows as 931 GB. The bytes are all there; only the unit has changed. The formatted capacity is a little lower still, because the file system keeps some space for its own records.

Sold as	Bytes	Shown in binary units
64 GB	64,000,000,000	59.60 GiB
256 GB	256,000,000,000	238.42 GiB
512 GB	512,000,000,000	476.84 GiB
1 TB	1,000,000,000,000	931.32 GiB
2 TB	2,000,000,000,000	1,862.65 GiB (1.82 TiB)
4 TB	4,000,000,000,000	3,725.29 GiB (3.64 TiB)

Connection speeds: Mbps against MB/s

Internet and network speeds are quoted in bits per second, and always in decimal units: 100 Mbps is 100,000,000 bits per second. To compare with a download counter in megabytes per second, divide by eight. Download programs and browsers often show binary units, which makes the figure smaller again.

Connection	Megabytes per second (MB/s)	Mebibytes per second (MiB/s)
10 Mbps	1.25	1.19
50 Mbps	6.25	5.96
100 Mbps	12.5	11.92
300 Mbps	37.5	35.76
500 Mbps	62.5	59.60
1 Gbps	125	119.21

These are ceilings. Every packet also carries addresses and control data: an ordinary 1,500-byte packet has at least 20 bytes of IPv4 header and 20 of TCP header, so at most 1,460 bytes, about 97%, are file data, and Ethernet or Wi-Fi add framing of their own. Wi-Fi signal, a busy server or other devices on the line usually cost more than that, which is why a 100 Mbps line rarely downloads faster than about 11 to 12 MB/s.

How long a download takes

Multiply the size in bytes by eight to get bits, then divide by the speed in bits per second. A 4 GB game on a 100 Mbps connection: 4,000,000,000 × 8 ÷ 100,000,000 = 320 seconds, 5 minutes 20 seconds at best. 50 GB at 300 Mbps takes 22 minutes 13 seconds. Upload speeds on home connections are often much lower than download speeds, so sending the same file can take many times longer.

4 GB at 100 Mbps5 min 20 s.
50 GB at 300 Mbps22 min 13 s.
700 MB at 20 Mbps4 min 40 s.

Download time

Writing sizes without ambiguity

When the exact number matters, in a contract, a specification or a support ticket, write the binary prefix when you mean 1,024: 512 MiB, 16 GiB. When you write MB or GB, use them in the decimal sense, as storage makers and network operators do. For speeds, keep bits and bytes apart with the case of the letter, and write Mbit/s or MB/s if the reader might confuse them.

Frequently asked questions

Is a kilobyte 1,000 or 1,024 bytes?

Officially 1,000; 1,024 bytes is a kibibyte (KiB). In everyday use, and in Windows, kilobyte often still means 1,024, so check which one a program uses.

Why does my 100 Mbps internet download at 12 MB/s?

Because the speed is in megabits and the download in megabytes: 100 divided by 8 is 12.5 MB/s, and packet headers and other traffic take a little more.

Why does the same file show different sizes on two computers?

One is counting in decimal units and the other in binary. A file of 4,700,000,000 bytes is 4.7 GB on a Mac and about 4.4 GB in Windows, which divides by 1,073,741,824.

Is 8 GB of RAM the same as 8 GB of storage?

No. Memory is counted in binary, so 8 GB of RAM is 8 GiB, 8,589,934,592 bytes; 8 GB of storage is 8,000,000,000 bytes.

What do Mbps, Mb/s and Mbit/s mean?

All three are megabits per second. MB/s, with a capital B, is megabytes per second, eight times more.