Data storage units:
bits, bytes and the gigabyte mix-up

Bits and bytes: the factor of eight

A bit is a single 0 or 1, the smallest piece of information a computer handles. A byte is a group of 8 bits, which is enough to represent one character in basic text or one value from 0 to 255. Everything bigger is built from bytes.

The prefix letters are the first place people trip. A lowercase b means bits and a capital B means bytes, so 100 Mb is 12.5 MB. Internet providers, network speeds and some streaming bitrates are quoted in bits per second, while file sizes, memory and storage are given in bytes.

So the first conversion in most problems is a division by 8. A line rated at 200 megabits per second moves at most 25 megabytes per second, and a file of 4.7 GB is 37.6 gigabits. Everything else in the page comes from this plus the prefixes.

The byte count of 8 has been the standard for decades, and the word 'octet' appears in networking standards to say exactly eight bits without ambiguity. Early machines used other byte sizes, but you will not meet them in everyday computing.

Decimal prefixes versus binary prefixes

Two families of prefix exist. The decimal (SI) ones step by 1,000: kilobyte, megabyte, gigabyte, terabyte. The binary ones step by 1,024: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). The binary names were introduced to remove ambiguity.

The gap widens with each step, which is why the confusion grows at larger sizes. At kilobytes it is a minor matter, and at terabytes it is a tenth of the drive.

Network speeds always use the decimal prefixes, with 1 Gbps meaning 1,000,000,000 bits per second. That is another reason to keep the two families apart: transfer rates are decimal, while memory and some file-system figures are binary.

The conversion formulas

Whenever a tool or operating system reports 'GB', check which of the two it means. Windows has long shown binary values with the label GB. macOS and most phone systems use decimal values, and storage manufacturers always use decimal.

Worked example: 12 gigabits and a 1 TB drive

Apply it to storage. A drive sold as 1 TB holds 10¹² bytes. In binary units that is 10¹² ÷ 2³⁰ = 931.3 GiB. An operating system that labels binary values with GB therefore shows about 931 GB, which looks like a loss of 69 GB but is only a different unit.

And to download time: a 4.7 GB file at 200 Mbps is 4.7 × 8 = 37.6 gigabits, and 37,600 megabits ÷ 200 = 188 seconds, a little over 3 minutes. Real downloads are slower than the line rating because of protocol overhead, server speed and Wi-Fi conditions.

From a bitrate to gigabytes per hour

Streaming services and video settings quote bitrate in megabits per second, but your data allowance is in gigabytes. The conversion multiplies by the seconds in an hour, then divides by 8 for bytes and by 1,000 for gigabytes.

Phone storage shows the same split between decimal and binary reporting. A 256 GB phone has 256 × 10⁹ bytes, which is 238.4 GiB; a 512 GB model is 476.8 GiB. The operating system, preinstalled apps and the system partition take further space, so the free amount at first use is lower still.

Where you meet each unit

• Bits: network speeds (Mbps, Gbps), video bitrates, encryption key lengths.
• Kilobytes and megabytes: text files, photos, emails, app sizes.
• Gigabytes: phone storage, RAM, movie files, data plans.
• Terabytes: laptops, external drives, cloud storage, camera archives.

A phone data plan counts decimal gigabytes in most cases, but the exact definition is in the operator's terms, and some apps report usage in binary units. When two numbers differ by about 7%, a GB versus GiB mismatch is the first thing to rule out.

Cloud storage plans and backups are priced in gigabytes or terabytes, usually decimal. A 2 TB plan holds about 1.82 TiB, so when comparing a plan with a drive displayed in binary units, convert both to one convention before deciding whether the capacity is enough.

Mistakes that cause confusion

• Reading Mbps as MB per second and expecting an 8-times faster download.
• Comparing a binary figure with a decimal figure without converting.
• Forgetting that RAM is almost always binary-sized, so 16 GB of RAM is 16 GiB in practice.
• Assuming a file's size on disk equals its content size. Small files occupy whole allocation blocks on some file systems.
• Treating bits per second as a size. A 5 Mbps stream uses 5 ÷ 8 = 0.625 MB each second, or about 2.25 GB per hour.

Marketing also mixes units on one page. A router box may list a speed in Mbps next to a storage figure in GB, and a game's download size may be given in GB while your plan's cap is in GiB. Whenever you plan around a limit, convert the figures into the same unit first and keep the working visible.