How this works

File size measurements have two incompatible systems that use the same unit names, which creates persistent confusion. The decimal system (used by drive manufacturers and most storage marketing) defines 1 KB as 1,000 bytes. The binary system (used by operating systems when displaying file and volume sizes) defines 1 KB as 1,024 bytes — because 1,024 is 2¹⁰, a round number in binary arithmetic.

Both systems are internally consistent; the confusion arises only because they share labels. The IEC tried to resolve this in 1998 by introducing new names (kibibyte, mebibyte, gibibyte — abbreviated KiB, MiB, GiB) for the binary units, but these never caught on in common usage, so the ambiguity remains.

The decimal vs binary gap at each unit

Kilobyte (KB)

Decimal: 1,000 bytes · Binary: 1,024 bytes · Gap: 2.4%

Megabyte (MB)

Decimal: 1,000,000 bytes · Binary: 1,048,576 bytes · Gap: 4.9%

Gigabyte (GB)

Decimal: 1,000,000,000 bytes · Binary: 1,073,741,824 bytes · Gap: 7.4%

Terabyte (TB)

Decimal: 1,000,000,000,000 bytes · Binary: 1,099,511,627,776 bytes · Gap: 9.9%

This is why a "1TB" drive shows as approximately 931GB in Windows or macOS — the OS reports in binary gigabytes, the manufacturer advertised in decimal gigabytes: 1,000,000,000,000 ÷ 1,073,741,824 ≈ 0.931.

Which system each context uses

Hard drives, SSDs, USB drives (manufacturer specs)

Decimal. Always. This is why drive capacities always appear smaller in the OS than advertised.

Windows file sizes and volume display

Binary (but labelled "GB" without the "i"). A file Windows shows as "1.00 GB" is actually 1,073,741,824 bytes.

macOS (10.6 and later)

Decimal. Apple switched from binary to decimal display in Snow Leopard, which makes macOS disk sizes match manufacturer specs.

RAM specifications

Binary. Memory chips are manufactured in exact powers of two. "8GB RAM" means exactly 8,589,934,592 bytes.

Internet speeds and data caps

Decimal megabits and gigabits. Internet speeds (e.g. 100 Mbps) use bits, not bytes, and use decimal prefixes. To find the byte download speed, divide by 8.

Frequently asked

Why does my new drive show less space than advertised?

The drive is full size — the apparent discrepancy is a labelling mismatch. Manufacturers measure in decimal gigabytes; Windows reports in binary gigabytes using the same "GB" label. A 1TB drive contains exactly 1,000,000,000,000 bytes, which is 931.32 binary gigabytes. No storage is missing.

Which should I use for file uploads or email attachments?

Most web services and email attachment limits are stated in decimal megabytes or gigabytes. Use the decimal setting in this converter when checking whether a file fits within a stated limit. The exception is RAM-related contexts, where binary is standard.

What are kibibytes and mebibytes?

IEC 80000-13 standardised kibibyte (KiB = 1,024 bytes), mebibyte (MiB = 1,048,576 bytes), and gibibyte (GiB = 1,073,741,824 bytes) to remove the ambiguity. These terms are used in Linux distributions, some technical documentation, and engineering contexts, but haven't replaced the ambiguous KB/MB/GB labels in everyday use.