How many MB in a GB: 1000 or 1024

The short answer

• In the decimal system (the SI system): 1 GB = 1000 MB = 1,000,000,000 bytes. This is what drive, memory card and phone manufacturers use.
• In the binary system: 1 GiB (gibibyte) = 1024 MiB = 1,073,741,824 bytes. This is how computers count memory internally, and how some systems display sizes even when they write "GB".

The gap grows with size: 4.9% at megabytes, 7.4% at gigabytes and nearly 10% at terabytes.

Why there are two systems

The prefixes "kilo", "mega" and "giga" mean thousands, millions and billions in the International System of Units. But computers work in base 2, and 1024 (2¹⁰) is very close to 1000, so for decades 1024 bytes were called a "kilobyte". To end the ambiguity, the International Electrotechnical Commission (IEC) approved dedicated prefixes for binary multiples in 1998: kibi (Ki), mebi (Mi) and gibi (Gi), equal to 2¹⁰, 2²⁰ and 2³⁰. So a kilobyte (KB) is 1000 bytes and a kibibyte (KiB) is 1024.

Why your 500 GB drive shows less

Apple explains it in its support pages: manufacturers measure capacity with the decimal system, so "1 GB = 1 billion bytes and 1 TB = 1 trillion bytes", and in addition formatting and the operating system use part of the space. If a program or system displays that capacity in the binary system, the number comes out smaller even though no byte is missing. For example:

• A 500 GB drive (500,000,000,000 bytes) is 465.66 GiB.
• A 1 TB drive is about 931.3 GiB.
• A 1 GB file (decimal) is 953.7 MiB.

Apple notes that some apps and systems, such as iOS 10 and earlier and OS X Leopard and earlier, reported storage capacity using the binary system; its current devices use the decimal one. The result is that, depending on the machine, the same file can show slightly different sizes.

And when a website sets a "10 MB" limit

A form that says "maximum 10 MB" does not always make clear whether that is 10,000,000 or 10,485,760 bytes. That gap of almost half a megabyte can decide whether your file is accepted. So it is wise to stay a little under the limit: if the maximum is 10 MB, aim for 9 or less. With the PDF compressor or the KB, MB and GB converter you can adjust and check the size, and with the tool to add up the size of several files you can see how much they take together (useful when the limit applies to the whole set).

Megabits and megabytes: the other confusion

Internet speed is measured in bits per second (Mbps, megabits per second), while file sizes are measured in bytes. A byte is 8 bits. That is why a "100 meg" connection does not download 100 MB every second but at most about 12.5 MB per second (100 ÷ 8).

• At 100 Mbps, a 100 MB file downloads in theory in 8 seconds (100 × 8 = 800 megabits ÷ 100 = 8 s).
• At 300 Mbps, the theoretical maximum is 37.5 MB/s.
• In practice real time is longer because of the network, the server and protocol overhead.

A trick: in speeds, a lowercase "b" means bits and an uppercase "B" means bytes. Mb ≠ MB.

Quick conversions with examples

To move from one unit to the next in the decimal system, multiply or divide by 1000; in the binary system, by 1024:

• GB to MB (decimal): 2.5 GB × 1000 = 2500 MB.
• KB to MB (decimal): 750 KB ÷ 1000 = 0.75 MB.
• MB to KB (decimal): 3.2 MB × 1000 = 3200 KB.
• GiB to MiB (binary): 1.5 GiB × 1024 = 1536 MiB.
• MB to MiB: divide by 1.048576. For example, 10 MB is about 9.54 MiB.
• MiB to MB: multiply by 1.048576. For example, 10 MiB is about 10.49 MB.

That last pair explains why a file that measures 9.8 MB in one program can show as 9.35 MiB in another without anything having changed, and why a "10 MB" limit read as binary leaves a little more room than as decimal.

Summary: how not to get it wrong

1. If the figure comes from a manufacturer or a website limit, assume decimal (1000) and leave some margin.
2. If you read "MiB" or "GiB", it is binary (1024).
3. To convert between units, always use a tool that tells you which base it applies, and check the final size.
4. For speeds, divide megabits by 8 to get megabytes.

Limits matter most when you send files: PDF too large to email shows how to bring a file under a size limit.

Sources and further reading

• Apple Support: how iOS and macOS report storage capacity (decimal and binary)
• NIST: prefixes for binary multiples (kibi, mebi, gibi), approved by the IEC in 1998

Figures checked on 3 October 2026.