Bits, Bytes, KB, MB, GB, TB: Data Units Explained

A bit is the smallest unit of digital data – a single binary digit valued 0 or 1 – and 8 bits make 1 byte, the unit that holds one character of text. Larger sizes (KB, MB, GB, TB, PB) just stack bytes, but they are counted two ways: powers of 1,000 (decimal) by drive makers and powers of 1,024 (binary) by your operating system, which is why a “1 TB” drive shows as 931 GB in Windows.

What Is a Bit?

A bit (binary digit) is the smallest unit of data, holding just one of two values: 0 or 1:

• One on/off state: every number, letter, image, and sound a computer handles breaks down into bits.
• Stored physically: one transistor (or one magnetic/flash cell) holds one bit at a time.
• Lowercase b: the bit is written b, and network speeds are counted in bits per second (bps), not bytes.

What Is a Byte?

A byte is a group of 8 bits, the standard unit for one character of text:

• 8 bits: fixed by the IBM System/360 in the 1960s and used by every modern computer since.
• 256 values: 8 bits give 2⁸ = 256 combinations (0-255), enough for one ASCII character.
• Nibble: half a byte (4 bits) = one hexadecimal digit (0-F); rare outside low-level work.
• Uppercase B: the byte is written B, which is how file sizes and storage are measured.

What Are KB, MB, GB, TB, and PB?

KB, MB, GB, TB, and PB are bytes stacked in groups, each step about 1,000 times larger than the last:

Each unit also has a binary twin (KiB, MiB, GiB…) counted in 1,024s, covered next. The full ladder up to the yottabyte is in the table below.

Complete Data Unit Table: Bit to Yottabyte

The table lists every unit from bit to yottabyte with its symbol and both the binary (1,024-based) and decimal (1,000-based) values:

What Is the Difference Between Binary and Decimal Prefixes?

The difference is the base they count in: decimal prefixes use 1,000, binary prefixes use 1,024:

• Decimal / SI (1,000): 1 KB = 1,000 bytes, 1 MB = 1,000 KB, 1 GB = 1,000 MB – used by drive makers and network speeds.
• Binary / IEC (1,024): 1 KiB = 1,024 bytes, 1 MiB = 1,024 KiB, 1 GiB = 1,024 MiB – how computers actually address memory.
• The IEC fix (1998-99): kibi/mebi/gibi/tebi were defined to end the old ambiguity where RAM used 1,024-based “MB” but disks used 1,000-based “GB”.
• The catch: most operating systems still show 1,024-based values but label them “KB/MB/GB”, which keeps the confusion alive.

Why Does a Hard Drive Show Less Space Than Advertised?

A drive shows less because the maker counts capacity in 1,000s while Windows counts in 1,024s – the same bytes, two labels:

• On the box: a “1 TB” drive holds 1,000,000,000,000 bytes (decimal).
• In Windows: that is divided by 1,073,741,824 and labeled “GB”, so it reads about 931 GB (really GiB).
• Not lost space: the ~6.9% gap is a counting difference, not a defect or hidden partition; it grows with capacity.
• macOS and Linux differ: since 2009 macOS reports decimal, so the same drive shows ~1,000 GB there.

Real-World Data Size Examples

These examples map the units to everyday files so the scale is easy to picture:

• 1 ASCII character = 1 byte.
• 1 tweet (280 characters) ≈ 280 bytes.
• 1 page of plain text (2,000 words) ≈ 12 KB.
• 1 MP3 song at 128 kbps (4 minutes) ≈ 4 MB.
• 1 JPEG photo (12 megapixel) ≈ 3-6 MB.
• 1 hour of 1080p video (H.264) ≈ 4-8 GB.
• 1 hour of 4K video (H.264) ≈ 45 GB.
• 1 AAA game install ≈ 50-150 GB.
• 1 “1 TB” HDD displays as 931 GB in Windows (decimal vs binary).

Bits vs Bytes: Why Internet Speed and File Size Differ

Internet speed and file size use different units: speed is counted in bits, storage in bytes, and 8 bits = 1 byte:

• Speed = bits (lowercase b): ISPs sell Mbps and Gbps – megabits and gigabits per second.
• Storage = bytes (uppercase B): files and drives are in MB, GB, TB.
• Convert: divide the bit figure by 8. 100 Mbps ÷ 8 = 12.5 MB/s max; 1 Gbps = 125 MB/s.
• Real world: overhead, Wi-Fi loss, and server limits trim ~5-20%, so 100 Mbps lands near 10-12 MB/s in practice.

Data Transfer Rate Units at a Glance

Transfer rates split by field: bits per second in networking, bytes per second in storage:

• Mbps (megabits/s): ISP internet speeds. 100 Mbps downloads at 12.5 MB/s max, often over USB or Wi-Fi.
• MBps (megabytes/s): storage device speeds. An SSD rated 500 MB/s moves 500 × 8 = 4,000 Mbps.
• Gbps (gigabits/s): Ethernet links. 1 Gbps Ethernet = 125 MB/s maximum throughput.

Last Thoughts on Bits, Bytes, and Data Units

Bits and bytes are the building blocks of all digital data: a bit is one 0 or 1, and 8 bits make a byte. The only real trap is counting – drive makers and network speeds use 1,000-based decimal units, while your operating system reports 1,024-based binary values under the same “KB/MB/GB” labels, which is why a 1 TB drive reads 931 GB. Keep two habits and the rest follows: divide Mbps by 8 to get MB/s, and remember that “advertised” size is decimal while “reported” size is binary.

Key Takeaways:

• A bit is 1 binary digit (0 or 1); a byte is exactly 8 bits.
• Data units scale by 1,024 (binary/IEC) or 1,000 (decimal/SI) depending on the context.
• A “1 TB” hard drive shows as 931 GB in Windows because manufacturers use decimal (10¹²) while Windows reports in binary (2⁴⁰).
• Networking speeds use bits per second (Mbps); storage speeds use bytes per second (MBps).
• A 100 Mbps internet connection downloads at a maximum of 12.5 MB/s.
• IEC introduced KiB, MiB, GiB, TiB to unambiguously denote binary multiples.

Frequently Asked Questions (FAQs)