Unit & Measurement Tables
Data Storage Conversion Table
Data storage has two conversion systems that share the same symbols, and that collision is responsible for one of the most durable consumer complaints in computing: the drive that appears to be missing 7% of its capacity. A gigabyte is 1 000 000 000 bytes. A gibibyte is 1 073 741 824 bytes. Both get printed as "GB" by software that has not updated its labels since 1998.
The decimal system is what drive manufacturers use, because the SI prefixes kilo, mega, giga and tera have always meant powers of a thousand. The binary system is what memory addresses and file systems naturally produce, because computing is built on powers of two. IEC 80000-13 introduced the unambiguous names KiB, MiB, GiB and TiB in 1998, and they are slowly replacing the ambiguous ones.
The practical consequence is small in percentage terms and large in absolute terms: the bigger the drive, the bigger the apparent loss. A 500 GB drive reports 465 GiB; an 8 TB drive reports 7.28 TiB. Nothing is broken, and the same arithmetic explains why a 128 GB phone shows about 119 GB of usable space before the operating system takes its share.
Decimal storage units (SI), as used on drive packaging
| Bytes | Kilobytes | Megabytes | Gigabytes | Terabytes |
|---|---|---|---|---|
| 1 | 0.001 | 0 | 0 | 0 |
| 1,000 | 1 | 0.001 | 0 | 0 |
| 1,000,000 | 1,000 | 1 | 0.001 | 0 |
| 1,000,000,000 | 1,000,000 | 1,000 | 1 | 0.001 |
| 1,000,000,000,000 | 1,000,000,000 | 1,000,000 | 1,000 | 1 |
Decimal prefixes step by 1000. This is the convention manufacturers use when they print a capacity on the box.
Binary storage units (IEC), as reported by operating systems
| Bytes | Kibibytes | Mebibytes | Gibibytes | Tebibytes |
|---|---|---|---|---|
| 1,024 | 1 | 0 | 0 | 0 |
| 1,048,576 | 1,024 | 1 | 0 | 0 |
| 1,073,741,824 | 1,048,576 | 1,024 | 1 | 0 |
| 1,099,511,627,776 | 1,073,741,824 | 1,048,576 | 1,024 | 1 |
| 1,125,899,906,842,624 | 1,099,511,627,776 | 1,073,741,824 | 1,048,576 | 1,024 |
Binary prefixes step by 1024. Windows labels these as KB/MB/GB, which is the source of the missing-capacity confusion.
What a drive actually shows: printed capacity vs reported capacity
| Printed capacity | Bytes | Reported as (GiB) | Difference |
|---|---|---|---|
| 500 GB | 500,000,000,000 | 465.7 | −6.9% |
| 1,000 GB | 1,000,000,000,000 | 931.3 | −6.9% |
| 2,000 GB | 2,000,000,000,000 | 1,862.6 | −6.9% |
| 4,000 GB | 4,000,000,000,000 | 3,725.3 | −6.9% |
| 8,000 GB | 8,000,000,000,000 | 7,450.6 | −6.9% |
A 1 TB drive holds 1000 GB by the decimal definition and reports as 931 GiB. Nothing is missing; the two columns use different units with the same symbol.
Data size in context
| Item | Typical size | Bytes |
|---|---|---|
| This page as HTML | about 60 KB | 60,000 |
| A minute of streaming audio | about 1 MB | 1,000,000 |
| A phone photo | about 4 MB | 4,000,000 |
| A minute of 1080p video | about 130 MB | 130,000,000 |
| A two-hour HD film | about 4 GB | 4,000,000,000 |
| A Blu-ray disc | 25 GB | 25,000,000,000 |
| Typical laptop SSD | 512 GB | 512,000,000,000 |
Figures are order-of-magnitude estimates for context; codecs and settings change them by a factor of several.
How to Use These Tables
- When comparing a drive's printed capacity to what your OS reports, divide bytes by 1024³ to get GiB, then compare. Expect roughly 7% less than the printed number.
- When calculating transfer time, convert to bits: 1 byte is 8 bits, and network speeds are quoted in bits per second. A 100 Mbit/s connection moves about 12.5 MB per second at best.
- For cloud storage quotas, assume decimal units — providers bill in GB, not GiB, which is why a 1 TB quota holds 1000 GB of files.
- Do not mix the two systems in one calculation. Converting GB to GiB and back with rounded intermediate values is how a capacity spreadsheet drifts by several percent.
Common Questions
Why does my 1 TB drive show 931 GB?
Because the drive holds 1 000 000 000 000 bytes — exactly 1 TB in decimal — and Windows divides by 1024³ and still calls the result GB. 1 000 000 000 000 ÷ 1 073 741 824 = 931.3. The correct label for that figure is GiB.
Is a kilobyte 1000 or 1024 bytes?
Both, depending on context. SI defines 1 kB as 1000 bytes; IEC defines 1 KiB as 1024 bytes. RAM, file sizes and disk partitions traditionally use 1024, while network throughput and storage marketing use 1000.
How many megabytes are in a gigabyte?
1000 MB in decimal, or 1024 MiB in binary. 500 GiB is 536.9 GB, so a drive advertised at 500 GB is the smaller of the two by about 7%.
How long does it take to transfer 1 GB?
At 100 Mbit/s, about 80 seconds in theory, because 1 GB is 8 Gbit and 8 ÷ 0.1 Gbit/s is 80 s. Real transfers add 10–20% for protocol overhead and slower storage at either end.
Does a 4K video really use 4 GB per hour?
It depends entirely on the bitrate. Streaming 4K runs at roughly 15–25 Mbit/s, which is 6.75–11.25 GB per hour. Locally recorded 4K at 100 Mbit/s is about 45 GB per hour.
More Unit & Measurement Tables
Sources
- ISO/IEC 80000-13:2008 — Quantities and units, Part 13: Information science and technology — International Organization for Standardization
- IEEE 1541-2021 — Standard for Prefixes for Binary Multiples — IEEE Standards Association
- The International System of Units (SI Brochure) — prefixes — Bureau International des Poids et Mesures
Filed with Unit Conversion calculations.