Decimal and binary: two different gigabytes
Computers count memory in powers of two, so for decades people used kilobyte to mean 1,024 bytes. Hard drive makers and the SI system use kilo to mean 1,000. Both groups were using the same words for different numbers, and the gap grows with every step: 2.4% at kilo, 4.9% at mega, 7.4% at giga and 10% at tera.
In the late 1990s the International Electrotechnical Commission introduced separate names for the powers of 1,024: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). They are standardized in IEC 80000-13. This converter uses the standard names: kB, MB, GB and TB mean powers of 1,000, and KiB, MiB, GiB and TiB mean powers of 1,024.
- Decimal: drive and SSD makers, memory cards, macOS (since 2009), and network speeds.
- Binary: memory (RAM), and Windows, which counts in 1,024s but labels the result KB, MB and GB.
- Mixed: Linux tools vary. Some print binary sizes, some decimal, and many let you choose.
The two sizes are 1,000,000,000 bytes for a GB and 1,073,741,824 bytes for a GiB, so a GiB is 7.4% bigger. The table below the converter, Decimal and binary units side by side, lists each pair from kB and KiB up to PB and PiB.
Why a 500 GB drive shows as 465 GB
Nothing is missing. The maker counts 500 × 1,000,000,000 = 500,000,000,000 bytes and calls it 500 GB. Windows divides those bytes by 1,073,741,824 to get 465.66 and then prints GB, so you see about 465 GB. A Mac divides by 1,000,000,000 and shows 500 GB, which is why the same drive can look different on two computers.
The gap grows as drives get bigger. A 1 TB drive holds 931.32 GiB, which Windows shows as about 931 GB, and a 2 TB drive shows as roughly 1.8 TB. Formatting, the file system and any recovery partition use a bit more space, and how much depends on the device. The drive card on this page covers only the unit difference.
Bits and bytes: Mbit/s versus MB/s
Internet providers quote speed in bits per second and software shows downloads in bytes per second. Eight bits make a byte, so divide a megabit figure by 8 to get megabytes. A 100 Mbit/s line peaks at 12.5 MB/s, and a 1 Gbit/s line at 125 MB/s, or 119.2 MiB/s if your tool counts in binary. Real downloads run slower because of overhead, other traffic and the server. The case of the letter matters: Mb is a megabit and MB is a megabyte.
To estimate a video file size from its bitrate, try the bitrate calculator.
Which family should you use?
Use decimal when you are matching what is printed on the box, a plan or a price list. Use binary when you are matching what an operating system, a memory module or a file manager reports in 1,024s. When it is for a limit, a quota or a plan with real consequences, check which convention the provider uses, because the two answers differ by several percent. The MB to GB and GB to MB pages show both answers together.