You buy a 1 TB drive, plug it in, and the computer reports 931 GB. Nothing was stolen and nothing is broken; you have just met the oldest accounting dispute in computing, where one side counts in thousands and the other in 1,024s. This guide settles what MB, GB, and TB actually mean, where the missing gigabytes go, and how bits sneak into the story whenever an internet provider is talking. Our free data storage converter speaks both dialects.
In this guide
The unit ladder
A byte is 8 bits, and everything above it climbs by a fixed step. The catch is that two ladders exist, and they look almost identical:
| Decimal (SI) | Bytes | Binary (IEC) | Bytes | Gap |
|---|---|---|---|---|
| 1 KB | 1,000 | 1 KiB | 1,024 | 2.4% |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 | 4.9% |
| 1 GB | 1,000,000,000 | 1 GiB | 1,073,741,824 | 7.4% |
| 1 TB | 1012 | 1 TiB | 240 | 10.0% |
Notice the gap growing with every rung: 2.4% at kilobytes, a full 10% by terabytes. The two ladders agree at the byte and drift apart from there, which is why the confusion gets more expensive as drives get bigger. Why computers love 1,024 in the first place is a powers-of-two story, told properly in our number systems guide.
The 1024 confusion, settled
The official rule since 1998: KB, MB, GB, TB are powers of 1,000; KiB, MiB, GiB, TiB are powers of 1,024. Storage manufacturers follow the rule, memory makers and operating systems only partly. The single most confusing player is Windows, which measures in binary units but labels them with decimal names: its “GB” is really GiB. macOS switched to honest decimal counting years ago, and Linux tools generally write GiB when they mean it. So the same file can show different “GB” sizes on two healthy machines while being identical to the byte.
Where the missing gigabytes go
The famous disappearing space is unit translation, not loss:
- A 500 GB drive holds 500,000,000,000 bytes = 465.7 GiB, which Windows displays as “465 GB”.
- A 1 TB drive holds 1012 bytes = 931.3 GiB, displayed as “931 GB”.
Every byte the box promised is present; the box counted in thousands and the operating system counted in 1,024s and mislabeled the result. A small additional slice goes to the filesystem’s own bookkeeping after formatting, but the headline gap, nearly 69 “GB” on a terabyte drive, is purely the two ladders disagreeing.
Bits vs bytes: the provider’s favorite unit
Internet speeds are sold in bits per second, file sizes live in bytes, and the lowercase b carries the entire difference: Mbps is megabits, MB/s is megabytes, and one byte is 8 bits. So a “100 Mbps” connection moves at most 12.5 MB/s, and a 5 GB game download at that speed takes 5,000 × 8 / 100 = 400 seconds, about 6.7 minutes, not the 50 seconds the optimistic reading promises. Gigabit fiber, 1 Gbps, peaks at 125 MB/s. The divide-by-8 step is the whole trick, and it is the most common speed-related support call in existence.
What things actually weigh
Rough modern reference points, useful for sanity-checking storage plans:
| Item | Typical size |
|---|---|
| Plain text email | a few KB |
| Smartphone photo | 2 to 5 MB |
| MP3 song | 3 to 10 MB |
| Hour of HD streaming | 1 to 3 GB |
| Modern game install | 50 to 150 GB |
| Hour of 4K phone video | 20 to 45 GB |
These are ranges, not constants, since compression settings dominate. The useful skill is order-of-magnitude fluency: photos live in megabytes, video lives in gigabytes, and a text document essentially weighs nothing. Data units are also the one family where the metric prefixes betray you, which earns them their own trap entry in our unit conversion pillar.
Frequently asked questions
So was the drive manufacturer lying?
No, and this surprises people: the manufacturer’s decimal GB is the standards-compliant usage. The mismatch comes from the operating system using binary units under a decimal name. Both numbers describe the same physical bytes.
Why do computers count in 1,024 at all?
Because memory addressing is built on powers of two, and 210 = 1,024 was close enough to 1,000 that early engineers borrowed the metric prefixes. The borrowed names stuck for decades before the KiB family was introduced to clean up the ambiguity.
Is a megabyte of RAM the same as a megabyte of disk?
Physically yes, a byte is a byte. By labeling convention, RAM is virtually always quoted in binary units (a “16 GB” RAM kit is 16 GiB), while disks are quoted in decimal. Same word, two industries, two ladders.
How do I estimate a download time correctly?
Size in gigabytes × 8,000 gives megabits; divide by your connection’s Mbps for seconds. Real downloads run a bit slower than the headline speed, so round up. The converter handles the unit side of the arithmetic.