Two systems, one number
Ask how big a “1 TB” hard drive is and you can get two different answers, both correct. The manufacturer says one trillion bytes. Your operating system says about 931 “GB”. Neither is lying — they are counting in different systems. Storage and networking use the decimal system, where each step up is a factor of 1000. Operating systems and programming languages historically use the binary system, where each step is a factor of 1024. The Data Size Converter shows both at once so the discrepancy stops being a mystery.
Bits and bytes
The smallest unit is the bit — a single 0 or 1. Eight bits make one byte, which is enough to store one character of text. Everything larger is a multiple of the byte. The one place bits still dominate is networking: internet speeds are quoted in megabits per second (Mbps), not megabytes. Because there are eight bits in a byte, a 100 Mbps connection moves data at roughly 12.5 megabytes per second. Mixing the two up is why a download often looks “eight times too slow” — the speed was in bits, the file size in bytes.
The decimal system (powers of 1000)
The decimal units follow the international SI prefixes:
- 1 kilobyte (KB) = 1,000 bytes
- 1 megabyte (MB) = 1,000 KB = 1,000,000 bytes
- 1 gigabyte (GB) = 1,000 MB = 1,000,000,000 bytes
- 1 terabyte (TB) = 1,000 GB = 1,000,000,000,000 bytes
- 1 petabyte (PB) = 1,000 TB
This is the system hard-drive, SSD, and USB-stick makers use, and the one networking equipment uses. When you buy “1 TB” of storage, you are buying exactly one trillion bytes.
The binary system (powers of 1024)
Computers address memory in powers of two, so it was natural early on to group bytes by 1024 (which is 2 to the power 10) and still call the result a “kilobyte”. To remove the ambiguity, the IEC introduced explicit binary prefixes:
- 1 kibibyte (KiB) = 1,024 bytes
- 1 mebibyte (MiB) = 1,024 KiB = 1,048,576 bytes
- 1 gibibyte (GiB) = 1,024 MiB ≈ 1.074 billion bytes
- 1 tebibyte (TiB) = 1,024 GiB
- 1 pebibyte (PiB) = 1,024 TiB
The gap between the two systems compounds at each level. At the kilobyte level it is only 2.4%, but by the terabyte level a TiB is about 10% larger than a TB. That 10% is exactly why a 1 TB drive shows as roughly 931 of the OS’s “GB”. You can watch the gap grow by converting the same value at each scale in the Data Size Converter.
Worked example: 1 GiB
Take one gibibyte. In bytes that is 1,024 × 1,024 × 1,024 = 1,073,741,824 bytes. In decimal units that same byte count is 1.073741824 GB — a little over one decimal gigabyte. In bits it is 8,589,934,592. The converter lays all of these out in one table, so you can read across from the binary column to the decimal column and see precisely how the two systems describe the identical amount of data.
When to use which
Reach for decimal units when you are matching advertised storage capacities, quoting network speeds, or following SI conventions in documentation. Reach for binary units when you are matching what an operating system, a file manager, or a programming language reports — for example when checking whether a file will fit in a memory buffer. The mistake to avoid is comparing a decimal number from one source with a binary number from another; a “500 GB” backup target and a “500 GB” dataset may not actually be the same size if one figure is really GiB.
Converting without the headache
The rules are simple once separated: divide or multiply by 1000 for decimal steps, by 1024 for binary steps, and by 8 to move between bits and bytes. Doing it by hand across several levels is where errors creep in, which is what the Data Size Converter is for — type a value once and read every unit at a glance. If you want to go deeper into how computers represent these numbers underneath, our binary converter guide explains the base-2 number system that makes the 1024 grouping natural in the first place.