Bits, Bytes, KB vs KiB: Digital Storage Units Explained

Buy a “1 TB” external drive, plug it in, and Windows reports something like 931 GB. Sign up for a “100 Mbps” connection and files arrive at around 12 MB per second. Neither is a bug. Both come from two conventions that share the same letters but mean different things. This guide takes them apart so the numbers stop being mysterious.

Bits and Bytes

A bit is a single binary digit — a zero or a one. A byte is a group of eight bits, enough to hold one character of basic text or a number from 0 to 255. Stored data is almost always measured in bytes; data in transit is almost always measured in bits per second.

The standard symbols are lowercase b for bit and uppercase B for byte. Mixing them up changes a number by a factor of eight: 50 MB/s and 50 Mb/s are very different speeds.

Decimal (SI) vs Binary (IEC) Prefixes

The everyday metric prefixes — kilo, mega, giga, tera — are powers of 1,000. They are part of the SI and mean the same thing for bytes as they do for metres. Drive manufacturers, network speeds, macOS and most Linux desktops use them.

Computers, however, address memory in powers of two, and for decades “kilobyte” was informally used for 1,024 bytes. To remove the ambiguity, the International Electrotechnical Commission (IEC) introduced the binary prefixes in 1998 — kibi, mebi, gibi, tebi, pebi — written KiB, MiB, GiB, TiB and PiB. The names are deliberately awkward so they cannot be confused with kilo, mega, giga and tera. RAM sizes, Windows file sizes and many programming tools count in these binary units.

Why a 1 TB Drive Shows as 931 GB

The drive really does hold one trillion bytes — 1,000,000,000,000 — exactly as the decimal label says. Windows measures capacity in binary units (GiB/TiB) but still labels them “GB” and “TB”. So the same trillion bytes becomes:

1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.32

Nothing has been lost; two different units are sharing one abbreviation.

Use the GB to GiB converter to check any other size, or MB to GB and GB to MB for decimal conversions.

Network Speed: Mbps vs MB/s

Internet plans are sold in megabits per second (Mbps); download managers usually show megabytes per second (MB/s). To convert:

• Divide megabits per second by 8 to get megabytes per second.
• Allow roughly 5–10% for protocol overhead (TCP/IP headers, encryption, retransmissions).

If a download is much slower than the right-hand column, the bottleneck is elsewhere — Wi-Fi signal, the source server, or disk write speed. The Mbps to MB/s converter does the division for any value.

Common Mistakes

• Mixing bits and bytes. Always check whether the unit is b or B. The factor of eight is large.
• Comparing capacity across operating systems. A drive may seem to “shrink” or “grow” depending on which system reports it; the bytes are the same.
• Forgetting overhead. File-system metadata, recovery partitions and reserved space all take a share. A 256 GB SSD never gives you 256 GB of free space.
• Assuming “K” always means 1,024. In RAM specifications it usually does; in drive and network specifications it does not. When precision matters, write KiB or kB explicitly.

Worked Example: Will It Fit?

You have 480 GiB of data on a laptop and want to back it up to a “500 GB” external drive.

1. 480 GiB × 1,073,741,824 ≈ 5.154 × 10¹¹ bytes — your data.
2. 500 GB = 5 × 10¹¹ bytes — the drive.
3. Your data is about 3% larger than the drive. The copy will fail before it finishes.

Pick a drive whose decimal capacity is comfortably above your binary usage — a 1 TB unit here.

Related Guides

• SI base units & prefixesThe full decimal prefix table, quecto to quetta.
• GB to GiBDecimal to binary gigabytes.
• Mbps to MB/sTurn plan speeds into download speeds.

Frequently Asked Questions

What is the difference between a bit, a byte and a kilobyte?

A bit is one binary digit (0 or 1). A byte is 8 bits. A kilobyte (kB) is 1,000 bytes, or 8,000 bits, in the SI definition; in older and Windows usage “KB” often means 1,024 bytes, which is properly called a kibibyte (KiB).

How many bytes are in a kibibyte?

Exactly 1,024 bytes (2¹⁰). A kibibyte (KiB) is the binary counterpart of the kilobyte, which is exactly 1,000 bytes. 1 MiB = 1,024 KiB = 1,048,576 bytes.

Is a kilobyte a unit of speed or storage?

A kilobyte (KB) is a unit of storage — an amount of data. Speed is data per unit of time, written as kilobytes per second (KB/s) or, more often for networks, kilobits per second (kbps).

What is the difference between KB, kB and Kb?

Capital B means byte and lowercase b means bit, so KB or kB is a kilobyte (8,000 bits) while Kb or kb is a kilobit (1,000 bits). Strict SI style writes kilo as lowercase k (kB); “KB” with a capital K is common in software and often implies 1,024 bytes.

When should I use KB versus KiB?

Use KiB, MiB and GiB when you mean powers of 1,024 — RAM, memory addresses, file sizes reported by Windows or programming tools. Use kB, MB and GB when you mean powers of 1,000 — drive capacities, network speeds, macOS and Linux file sizes. Writing the IEC unit removes any ambiguity.

Why does a 1 TB hard drive show only 931 GB?

The drive holds 10¹² bytes, which is 1 TB in decimal units. Windows divides by 1,024³ (a gibibyte) but still prints “GB”, so 10¹² ÷ 1,073,741,824 ≈ 931.3. No capacity is missing; the two numbers use different units.

How do I convert Mbps to MB/s?

Divide by 8. A 100 Mbps connection moves at most 12.5 MB/s; after protocol overhead expect around 11–12 MB/s. A 1 Gbps line tops out at 125 MB/s, typically 110–118 MB/s in practice.

How do I work out total memory capacity in KiB?

Multiply the number of memory locations by the bytes stored in each, then divide by 1,024. For example, 65,536 addresses × 2 bytes (16-bit words) = 131,072 bytes, and 131,072 ÷ 1,024 = 128 KiB. If each location holds bits rather than bytes, divide by 8 first.