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Data Size Converter: Bits, Bytes, MB vs MiB, Drive Capacity and Download Time

Convert bits, bytes, the decimal units (kB, MB, GB, TB, PB) and the binary units (KiB, MiB, GiB, TiB, PiB) in one table, all exactly.

Maintained by Hirantha Sandaruwan Lakshan. Updated 13 October 2026.

About this tool

Two helpers sit beside it: a download time estimator that handles the megabit and megabyte mix-up, and a drive capacity calculator that shows why a 500 GB drive appears as 465.66 in Windows. Nothing you type leaves your device.

How it works

Two systems share the same names. The SI prefixes are powers of 1,000: a kilobyte (kB) is 1,000 bytes, a megabyte (MB) is 1,000,000 bytes and a gigabyte (GB) is 1,000,000,000 bytes. The binary prefixes are powers of 1,024: a kibibyte (KiB) is 1,024 bytes, a mebibyte (MiB) is 1,048,576 bytes and a gibibyte (GiB) is 1,073,741,824 bytes. The IEC approved the names kibi, mebi, gibi, tebi and pebi in December 1998, and they are now defined in IEC 80000-13, so that MB and MiB can be told apart. The gap grows at every step, from 2.40% at kilo to 12.59% at peta (see the table).

A byte is 8 bits, so a megabyte is 8 megabits. The standard symbol for the bit is bit and for the byte is B, which NIST writes as 1 B = 8 bit. Because Mb and MB differ only in the case of one letter, they are easy to confuse: Mb or Mbit is a megabit, and MB is a megabyte. Internet speeds are normally quoted in megabits per second, while file sizes and browser downloads are usually shown in megabytes, so a 100 Mbit/s line moves at most 12.5 MB each second (100 divided by 8) and a 1 Gbit/s line at most 125 MB per second.

The download estimator divides the file's size in bits by the line speed in bits per second, after taking off the overhead you enter. Even on a perfect wired link, data travels in packets that carry headers. RFC 6349 (section 4.1.1) works this out for TCP over Ethernet: each 1,538 bytes on the wire carry 1,460 bytes of file data, so a 100 Mbit/s link gives at most 94.9 Mbit/s of file data. That is about 5.1% overhead, so the default allowance is 5%. Wi-Fi signal, a busy server, a VPN and other devices on the line can cut the speed much more, so read the result as the ideal case.

Drive makers count 1 GB as 1,000,000,000 bytes. Windows divides the same bytes by 1,073,741,824 and writes the result with the label GB (it is really GiB). Nothing is missing: a 500 GB drive holds about 500,000,000,000 bytes, which is 465.66 GiB, 6.87% less than the number on the box. At terabyte scale the gap is 9.05%, so a 1 TB drive shows about 931 GB. Seagate's support page shows the same 500 GB to 465 GB example; its table prints 465 where this tool rounds 465.66, so the last digit can differ.

Worked examples

A 500 GB drive in Windows

500 GB is 500,000,000,000 bytes. Divide by 1,073,741,824 and you get 465.66 GiB, which Windows shows as 465 GB. In tebibytes it is 0.455 TiB. The drive has lost nothing: the same bytes are counted in bigger units.

Downloading a 1.5 GB file at 50 Mbit/s

1.5 GB is 12,000 megabits (1.5 x 8 x 1,000). At 50 Mbit/s that takes 240 seconds, or 4 minutes, on a perfect line. With 5% overhead the line carries 47.5 Mbit/s of file data, so it takes 240 divided by 0.95 = 252.6 seconds, which is 4 min 13 s.

Megabits to megabytes

A 100 Mbit/s plan can move at most 12.5 MB per second, so a 700 MB file needs at least 56 seconds (700 divided by 12.5). A browser that shows 11.9 MB/s on that line is working normally, because the most a 100 Mbit/s line can carry as file data is 94.93 Mbit/s, which is 11.87 MB/s.

Decimal and binary prefixes compared

Decimal unitBytesBinary unitBytesBinary is bigger by
kilobyte (kB)1,000kibibyte (KiB)1,0242.40%
megabyte (MB)1,000,000mebibyte (MiB)1,048,5764.86%
gigabyte (GB)1,000,000,000gibibyte (GiB)1,073,741,8247.37%
terabyte (TB)1,000,000,000,000tebibyte (TiB)1,099,511,627,7769.95%
petabyte (PB)1,000,000,000,000,000pebibyte (PiB)1,125,899,906,842,62412.59%

Frequently asked questions

What is the difference between MB and Mb?

MB is a megabyte and Mb (or Mbit) is a megabit. A byte is 8 bits, so 1 MB is 8 Mb. Internet speeds are quoted in megabits per second (Mbit/s or Mbps), while file sizes are in megabytes, so divide the line speed by 8 to get the most megabytes per second you can expect.

How many MB are in a GB?

1,000 MB in a GB using the decimal (SI) units that drive makers and networks use, and 1,024 MiB in a GiB using the binary units. Windows reports binary sizes but labels them MB and GB, which is where the confusion starts.

Why does my 500 GB drive show 465 GB?

The maker counts 500,000,000,000 bytes as 500 GB. Windows divides by 1,073,741,824 bytes and shows 465.66, still labelled GB. No space is missing. File system structures take a little more space on top of this, but the unit difference is by far the biggest part.

How many MB per second is 100 Mbps?

At most 12.5 MB/s (100 divided by 8). After about 5% of TCP and Ethernet overhead you can expect close to 11.9 MB/s on a good wired connection. A slower or shared line gives less.

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

By the international standard, a kilobyte (kB) is 1,000 bytes and a kibibyte (KiB) is 1,024 bytes. Older software used KB for 1,024, and some still does, which is why the two get mixed up. If the unit has an i in it (KiB, MiB, GiB), it is 1,024-based for certain.

How fast is 1 Gbit/s in MB per second?

At most 125 MB/s (1,000 divided by 8), and about 118.7 MB/s once the 5.1% TCP and Ethernet overhead is taken off. A 1 GB file then takes about 8.4 seconds instead of 8.

Does this tool upload my numbers?

No. Everything is calculated in your browser and nothing is sent or stored.

Updated Oct 13, 2026