Data storage units come in two systems that use the same names. In the decimal (SI) system, each step is 1,000: 1 kilobyte (kB) = 1,000 bytes, 1 megabyte = 1,000,000 bytes, and 1 gigabyte = 1,000,000,000 bytes. In the binary system, each step is 1,024: 1 kibibyte (KiB) = 1,024 bytes, and 1 gibibyte (GiB) = 1,073,741,824 bytes. That gap is why a 1 TB drive shows up as about 931 GB in Windows.
Bits and bytes
Everything starts with the bit, a single 0 or 1. A byte is 8 bits, enough to store one character of plain English text.
Lowercase b means bits and uppercase B means bytes. That one letter is a factor of eight: 100 Mb is 12.5 MB. Storage and file sizes are measured in bytes; network speeds are measured in bits per second.
The two systems side by side
| Decimal (SI) | Bytes | Binary (IEC) | Bytes | Binary is larger by |
|---|---|---|---|---|
| kilobyte (kB) | 10³ = 1,000 | kibibyte (KiB) | 2¹⁰ = 1,024 | 2.4% |
| megabyte (MB) | 10⁶ | mebibyte (MiB) | 2²⁰ = 1,048,576 | 4.86% |
| gigabyte (GB) | 10⁹ | gibibyte (GiB) | 2³⁰ = 1,073,741,824 | 7.37% |
| terabyte (TB) | 10¹² | tebibyte (TiB) | 2⁴⁰ ≈ 1.0995 × 10¹² | 9.95% |
| petabyte (PB) | 10¹⁵ | pebibyte (PiB) | 2⁵⁰ ≈ 1.1259 × 10¹⁵ | 12.59% |
The gap grows at every step, from 2.4% at the kilo level to almost 10% at the tera level.
Where each system is used
| Context | Convention |
|---|---|
| Hard drive, SSD and USB drive capacity | Decimal |
| Network and internet speeds | Decimal (bits per second) |
| macOS (since 2009) and iOS | Decimal for file and disk sizes |
| Linux command-line tools such as df -h and ls -lh | Binary, labeled K, M, G |
| Windows File Explorer | Binary values, labeled KB/MB/GB |
| RAM and processor cache | Binary |
The binary names (kibi, mebi, gibi and so on) were introduced by the International Electrotechnical Commission (IEC) in 1998 and are now part of the IEC 80000-13 standard. NIST recommends them to keep the SI prefixes decimal, as they are in every other unit; see metric prefixes explained.
Why your drive looks smaller
A drive sold as 1 TB holds 1,000,000,000,000 bytes. Windows divides by 1,024³ to report gigabytes:
1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.3 GiB, displayed as "931 GB"
The capacity is not missing; it is the same number of bytes in a larger unit. Formatting and system files then take a little more. A 2 TB drive shows about 1.82 TB in Windows, and a 256 GB memory card plugged into a Windows PC shows about 238 GB, because 256,000,000,000 bytes is 238.4 GiB.
The data storage converter converts between every decimal and binary unit, including bits.
Download times: bits vs bytes
Internet plans are sold in megabits per second (Mbps), while files are measured in megabytes. Divide the speed by 8 to compare them:
A 4 GB file on a 100 Mbps connection
4,000,000,000 × 8 = 32,000,000,000 bits
32,000,000,000 ÷ 100,000,000 = 320 s ≈ 5.3 minutes at full speed
Real downloads take longer because of protocol overhead, Wi-Fi conditions and server limits; 80 to 90 percent of the advertised speed is typical on a good wired connection. A 500 Mbps plan peaks at 62.5 MB/s. The download time calculator handles the unit juggling, and the bandwidth calculator estimates monthly data use.
How big are common files?
These are typical, approximate sizes; actual sizes depend heavily on compression settings.
| Item | Typical size |
|---|---|
| One page of plain text | about 2 to 4 kB |
| Smartphone photo (JPEG) | about 2 to 5 MB |
| One minute of MP3 audio at 128 kbps | about 1 MB |
| One hour of 1080p streaming video at 8 Mbps | about 3.6 GB |
| A modern video game | often 50 to 150 GB |
Video size follows directly from bitrate: 8 megabits per second × 3,600 seconds = 28,800 megabits, or 3.6 gigabytes. The video file size calculator works out sizes from bitrate and length.
How much fits on a drive?
Divide the capacity by the typical file size, keeping both in the same system. A 128 GB phone holds roughly 128,000 MB ÷ 4 MB ≈ 32,000 photos at 4 MB each, minus whatever the operating system and apps occupy. A 2 TB backup drive holds about 2,000 GB ÷ 3.6 GB ≈ 555 hours of 1080p video at 8 Mbps. These are estimates: compressed formats vary widely, and a drive should never be filled completely, because file systems slow down when nearly full.
Smaller and larger units
Below the byte, a nibble is 4 bits, exactly one hexadecimal digit. Processors handle data in words, typically 32 or 64 bits on modern machines. At the large end, cloud providers and data centers talk in petabytes and exabytes; a single exabyte is a billion gigabytes. Network equipment sometimes quotes gigabits per second (Gbps); a 10 Gbps link moves at most 1.25 GB/s.
Converting step by step
To convert within one system, multiply or divide by 1,000 (decimal) or 1,024 (binary) once per step. To switch systems, go through bytes:
500 GB to GiB: 500 × 10⁹ bytes ÷ 2³⁰ ≈ 465.66 GiB
16 GiB to GB: 16 × 1,073,741,824 bytes ÷ 10⁹ ≈ 17.18 GB
In Excel, the CONVERT function understands both systems: =CONVERT(1,"Tbyte","Gibyte") returns about 931.32, and =CONVERT(500,"Mbit","Mbyte") returns 62.5.
The powers of two behind the binary units are covered in exponent rules, and binary numbers themselves in how to convert binary to decimal.
RAID and usable capacity
When drives are combined in a RAID array, usable space is less than the sum of the drives because some capacity goes to redundancy. Four 4 TB drives in RAID 5 give about 12 TB usable, which Windows would display as roughly 10.9 TB. The RAID calculator compares levels.
Common mistakes
- Confusing Mb and MB, which differ by a factor of 8.
- Assuming a drive is defective because it shows less than its label.
- Mixing systems mid-calculation, such as dividing decimal bytes by 1,024.
- Expecting full advertised speed on every download.
Frequently asked questions
Is 1 GB 1,000 MB or 1,024 MB?
Both are in use. Under the SI (decimal) definition used by drive makers, networks and macOS, 1 GB = 1,000 MB = 1,000,000,000 bytes. Under the binary convention used by Windows and for RAM, the same label means 1,024 MB. The unambiguous binary name is the gibibyte (GiB), which is 1,073,741,824 bytes.
Why does my 1 TB hard drive only show 931 GB?
The drive holds 1,000,000,000,000 bytes, the decimal terabyte. Windows divides by 1,024³ instead of 1,000³ but still labels the result GB, so it reports 931 GB. Nothing is missing; it is the same number of bytes counted in different units.
What is the difference between Mbps and MB/s?
Mbps is megabits per second and MB/s is megabytes per second. A byte is 8 bits, so divide Mbps by 8 to get MB/s. A 500 Mbps internet plan transfers at most 62.5 MB/s.
How many bytes are in a kilobyte?
1,000 bytes under the SI definition, or 1,024 bytes under the older binary convention. The IEC introduced the kibibyte (KiB) for 1,024 bytes so that kilobyte (kB) could mean exactly 1,000.
Is RAM measured in binary or decimal units?
Binary. Memory chips are built in powers of two, so a module sold as 16 GB actually holds 16 GiB, which is 17,179,869,184 bytes.