Data Storage
Bytes, KB, MB, GB, TB, PB
A drive sold as 1 TB shows up in Windows as 931 GB, and nothing has been stolen. The manufacturer counted 1,000,000,000,000 bytes, using decimal SI prefixes. Windows divided that by 1,073,741,824 — 1024³ — and then labelled the result "GB" anyway. Two different units, one shared abbreviation.
This converter takes a deliberate position on that: byte units use binary steps of 1,024, because that is how operating systems report file sizes and memory, while bit units use decimal steps of 1,000, because that is how networking hardware and ISPs count. The mixture is not an inconsistency, it is the two conventions each used where it actually applies.
The other thing to watch is the case of a single letter. Mb is megabits and MB is megabytes, and they differ by a factor of eight — the reason a 100 Mb/s connection tops out at 12.5 megabytes per second — and, because the file manager counting those megabytes is using powers of 1,024, displays as about 11.9 MiB/s.
Why your 1 TB drive shows 931 GB
Storage manufacturers have used decimal prefixes since the industry began, so 1 TB means 10^12 bytes. Windows File Explorer computes in powers of 1,024 but prints the decimal abbreviations, so the same drive reads 931 GB. The division is 10^12 ÷ 2^30 = 931.32, not a whole number; File Explorer truncates the display to 931. macOS switched to decimal reporting in Snow Leopard (10.6, 2009), so the identical drive reads as 1 TB on a Mac and 931 GB on a PC.
The IEC settled the naming in 1998 with standard 60027-2: the binary units are the kibibyte (KiB, 1,024 bytes), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB), leaving kB, MB, GB and TB to mean the decimal powers of ten. Adoption is patchy. Linux tools split the difference — ls -lh reports binary sizes, df -H reports decimal. RAM is always binary, because memory is addressed by binary address lines and an 8 GB module is unavoidably 8 GiB.
The gap compounds up the ladder: each step is 2.4% (1,024 against 1,000), so a decimal figure read as binary is about 7% low at the gigabyte and about 9% low at the terabyte. If you paste a manufacturer’s decimal capacity into this converter, expect that discrepancy — it is the same discrepancy your operating system is showing you.
Two ladders, and the factor of eight between them
The byte ladder used here climbs in powers of 1,024:
1 KB = 1,024 B1 MB = 1,048,576 B(1,024²)1 GB = 1,073,741,824 B(1,024³)1 TB = 1,099,511,627,776 B(1,024^4)1 PB = 1,125,899,906,842,624 B(1,024^5)
One caveat on the last row: the converter stores the petabyte as 1.1259 × 10^15 bytes rather than the exact 1,125,899,906,842,624, which overstates it by about 93 million bytes — eight parts in a hundred million. Every other byte step is exact.
The bit ladder climbs in powers of 1,000 and then divides by eight to reach bytes:
1 Kb = 1,000 bits = 125 B1 Mb = 1,000,000 bits = 125,000 B1 Gb = 1,000,000,000 bits = 125,000,000 B
Divide megabits per second by eight for megabytes per second, and remember that the answer is in decimal megabytes while the file size your download manager shows is probably in binary ones — a 4.8% mismatch that makes real transfers look marginally slower than the arithmetic promised.
A download, and a floppy disk that was neither
How long is that game? A 25 GB download on a 300 Mb/s connection: 25 GiB is 26,843,545,600 bytes, and 300 Mb/s is 37,500,000 bytes per second, so 26,843,545,600 ÷ 37,500,000 = 716 seconds, just under 12 minutes at full line rate. Ethernet, IP and TCP headers consume a few percent of the link, and no residential connection sustains its rated speed continuously, so budget 13 to 15 minutes.
A 4K stream. At 25 Mb/s, two hours consumes 25,000,000 ÷ 8 × 7,200 = 22.5 decimal gigabytes, or 20.95 GiB. Against a 50 GB monthly data cap that is two and a bit films, and a plan advertising 50 GB supports roughly 4.4 hours of 4K streaming a month.
The 1.44 MB floppy. The 3.5-inch high-density disk held 2,880 sectors of 512 bytes, which is 1,474,560 bytes. That is 1,440 KiB — and the marketing then divided by 1,000 to get "1.44 MB". The figure is neither decimal nor binary but a hybrid of both: the true size is 1.406 MiB, or 1.475 MB. It is the cleanest surviving example of why the IEC bothered to define separate prefixes.
Bits, bytes, and why each field uses its own
Serial links move one bit at a time, so link rates are natively bits per second: Ethernet, DOCSIS, DSL, LTE and 5G all specify in bits, and an ISP quoting 940 Mb/s on a gigabit line is describing what the wire carries after framing overhead. Storage counts bytes because memory and files are byte-addressed, and a byte is the smallest unit a program can normally load.
The eight-bit byte is a convention rather than a law of nature. Machines with 6, 7, 9 and 36-bit words all shipped commercially before the IBM System/360 made eight bits the standard in 1964. Standards documents avoid the ambiguity by saying octet instead, which is why network RFCs count octets and not bytes.
Text is where byte counts surprise people. UTF-8 uses one byte for ASCII, two for most accented Latin, Greek and Cyrillic, three for the main CJK ranges and four for emoji and rarer scripts — so a 280-character post can be anywhere from 280 to over 1,100 bytes. The Unicode Converter tool shows the underlying code points.
Sizes to sanity-check a figure against
| Item | Typical size |
|---|---|
| A page of plain text | ~2 KB |
| A 12 MP JPEG from a phone | 4–6 MB |
| The same shot as camera RAW | 25–50 MB |
| A 3-minute MP3 at 128 kb/s | 2.9 MB |
| The same track at 320 kb/s | 7.2 MB |
| A minute of 1080p video at 8 Mb/s | 60 MB |
| A single-layer DVD | 4.7 GB decimal (4.38 GiB) |
| A single-layer Blu-ray | 25 GB decimal (23.3 GiB) |
| A two-hour 4K HDR stream | ~20–25 GB |
Audio sizes follow directly from the bitrate: 320 kb/s for 180 seconds is 57,600,000 bits, or 7.2 MB. Video is the same arithmetic at a larger bitrate, which is why the only number that matters when estimating a video file is bits per second, not resolution.
Sizing a plan, a bucket, or a backup
Cloud storage. Consumer tiers from Google, Apple and Dropbox are quoted in decimal GB and TB, and object storage bills per GB-month decimal, as does egress. Comparing a provider’s decimal TB against the binary TB your file manager reports overstates what you can actually fit by about 9%.
Data caps. A mobile plan with a 50 GB soft cap sounds generous until it is converted into hours: at a 25 Mb/s 4K bitrate that is 4.4 hours, and at 5 Mb/s for 1080p it is about 22 hours. The cap is a bit-rate budget wearing byte units.
Backups. Sizing a backup target from a source figure needs both prefixes handled consistently. A 4 TB decimal drive holds 3.64 TiB, so a 3.8 TiB source will not fit onto it despite the numbers appearing to leave room. This is the single most common way a backup plan fails at the last step, and it is entirely a units problem.
Frequently asked questions
Why does my 1 TB drive show as 931 GB?
The manufacturer counted 10^12 bytes using decimal prefixes; Windows divides by 1024^3 but still prints the label GB. 10^12 divided by 2^30 is 931.32, which it truncates to 931. macOS reports decimal capacities, so the same drive reads as 1 TB there.
Is a kilobyte 1,000 or 1,024 bytes?
Both definitions are in use. The IEC standard reserves kB for 1,000 bytes and calls 1,024 bytes a kibibyte (KiB). This converter uses 1,024 for byte units because that matches how operating systems report file sizes.
What is the difference between Mb and MB?
A factor of eight. Lowercase b is bits, uppercase B is bytes. A 100 Mb/s connection moves 12.5 decimal megabytes per second before protocol overhead, which a file manager counting in powers of 1,024 shows as about 11.9 MiB/s.
How long does a 25 GB download take on a 300 Mb/s line?
About 12 minutes at full line rate: 26.84 billion bytes divided by 37.5 million bytes per second is 716 seconds. Allow 13 to 15 minutes in practice, since protocol overhead and variable throughput both take a cut.
Why does this converter mix binary bytes with decimal bits?
Because the two industries genuinely count differently. Operating systems report file sizes in powers of 1,024, while network equipment and ISPs specify link rates in powers of 1,000 bits per second.
Was the 1.44 MB floppy really 1.44 megabytes?
Not in either system. It held 1,474,560 bytes, which is 1,440 KiB divided by 1,000 to reach the marketing figure. In consistent units it is 1.406 MiB or 1.475 MB.