KB, MB, GB & TB Explained

You buy a “1 TB” drive, plug it in, and your computer says 931 GB. You pay for a “100 Mbps” connection and your downloads run at 12 MB/s. Neither is a scam, and neither is a fault. Both come from the same small set of confusions about how data is measured — and once you see them, a lot of computing stops being mysterious.

Bits and bytes are not the same thing

Start here, because it is the root of half the confusion.

A bit is a single 0 or 1 — the smallest possible piece of data. Eight bits make one byte, which is enough to store one character of text.

The critical detail is the notation, and it is diabolically easy to misread:

• Lowercase “b” means bits. Mb, Mbps.
• Uppercase “B” means bytes. MB, MB/s.

One character of difference, a factor of eight in meaning.

Why your internet is “slower” than advertised

This follows directly. Internet speeds are sold in bits per second; file sizes are measured in bytes. So a 100 Mbps connection downloads at about 12.5 MB/s — divide by 8.

Nothing is broken. You are simply comparing two different units. A 1 GB file over a perfect 100 Mbps line takes about 80 seconds, not 10.

Is the bits convention deliberately flattering? It produces a bigger number, so make of that what you will. It does have a legitimate engineering history — networks genuinely move bits, not bytes — but nobody in marketing has been in a hurry to change it.

The real culprit: two counting systems

Now the drive-size question. There are two different standardsfor what “kilo” means in computing, and both are in active use.

Decimal (SI) — the everyday metric meaning. Each step is 1,000:

• 1 KB = 1,000 bytes
• 1 MB = 1,000 KB
• 1 GB = 1,000 MB
• 1 TB = 1,000 GB

Binary (IEC) — each step is 1,024:

• 1 KiB (kibibyte) = 1,024 bytes
• 1 MiB = 1,024 KiB
• 1 GiB = 1,024 MiB
• 1 TiB = 1,024 GiB

Why 1,024? Because computers count in powers of two, and 1,024 is 2¹⁰ — the power of two closest to 1,000. Memory is addressed in binary, so binary-sized chunks are what the hardware naturally produces. Early engineers found 1,024 conveniently close to a kilo and borrowed the prefix, and the ambiguity has haunted us ever since.

Why 1 TB shows as 931 GB

Now it resolves cleanly. The two parties are using different systems:

• The manufacturer uses decimal.A “1 TB” drive holds exactly 1,000,000,000,000 bytes. That is what you bought, and you got all of it.
• Windows reports in binary. It divides by 1,024 three times, giving about 931— and then, unhelpfully, labels the result “GB” when it really means GiB.

Not a single byte is missing.The drive contains precisely what was advertised. Only the label on your screen is inconsistent — and the mislabelling is Windows's fault, not the manufacturer's.

The gap grows with size, because the 2.3% discrepancy compounds at every step:

• 1 KB vs 1 KiB — about 2.3% apart
• 1 MB vs 1 MiB — about 4.6%
• 1 GB vs 1 GiB — about 6.9%
• 1 TB vs 1 TiB — about 9.1%

Which is why nobody complains about a kilobyte and everybody complains about a terabyte.

Worth noting: macOS and Linux now use decimal, so the same drive shows as “1 TB” on a Mac and “931 GB” on a PC. Same drive, same bytes, different arithmetic.

RAM is the exception.Memory really is manufactured in binary sizes, so an “8 GB” stick genuinely is 8 GiB — 8,589,934,592 bytes. Memory is the one place where the binary meaning is the honest one.

A sense of scale

Numbers this large stop meaning anything without anchors:

• 1 byte — one character.
• 1 KB — a short paragraph.
• 100 KB — a well-compressed web image.
• 1 MB — a large document, or a compressed photo.
• 5 MB — a typical phone photo, straight from the camera.
• 50 MB — a minute of 1080p video.
• 1 GB — roughly half an hour of HD video, or about 200 songs.
• 1 TB — around 250,000 photos, or 200 hours of HD video.

Where this actually bites you

• Buying storage. Expect a stated GB figure to read about 7% smaller on Windows — and a TB figure about 9% smaller — purely from the labelling difference, plus a little more lost to formatting.
• Estimating download times. Divide your Mbps by 8 to get MB/s.
• Upload limits.A form capping uploads at “10 MB” may mean either standard — but the difference is only 5%, so it rarely matters.
• Cloud billing. Providers almost always use decimal, which works slightly in your favour compared to what your operating system reports.

Frequently asked questions

How many MB are in a GB? 1,000 in decimal; 1,024 MiB in a GiB. The answer depends on which system is being used.

Is my hard drive manufacturer cheating me?No. They use the standard metric meaning of “tera.” The confusing label is on your operating system.

Why is my 100 Mbps internet only 12 MB/s? Bits versus bytes. 100 ÷ 8 = 12.5. That is correct, not slow.

Should I use KB or KiB? KiB is unambiguous and technically correct for binary sizes, but KB is what almost everyone writes. Just be aware which one is meant.

Convert data sizes now

Use our Data Storage Converter to switch between bits, bytes, KB, MB, GB, TB and PB in both decimal and binary modes — so you can see the exact gap for yourself. Curious why computers count in powers of two? Read binary, decimal and hex explained.