Who needs this?
Anyone who has bought a drive, filled a memory card or wondered why a download crawls compared with the speed on the internet bill:
- Drive buyersThe box says 2 TB and File Explorer says about 1.8 TB. Nothing is broken, and nobody kept the difference.
- Photographers and videographersHow many hours of 4K fit on a 128 GB card before the shoot ends early?
- Phone ownersTwenty gigabytes free sounds like plenty until the camera is set to 4K.
- Anyone with internetSpeeds are sold in megabits per second, files are sized in megabytes. That factor of 8 explains a lot.
- IT and supportA user's "missing" space is a units question about half the time.
Bits, bytes and the two kinds of gigabyte
A bit is a single yes-or-no, a 1 or a 0. A byte is a group of 8 bits, enough to store one of 256 values, such as a letter of the alphabet. Files are measured in bytes. Network speeds are measured in bits per second.
The abbreviations differ by one letter case: b is a bit, B is a byte. So 100 Mb is 100 megabits, and 100 MB is 100 megabytes, eight times as much.
The bigger source of confusion is the prefix in front. Think of two grocers selling eggs, one by the ten and one by the dozen. Both say "a box," but the boxes aren't the same size. Storage has the same problem with "kilo," "mega" and "giga":
- Decimal (SI) prefixes count in thousands: 1 kB = 1,000 bytes, 1 MB = 10⁶, 1 GB = 10⁹, 1 TB = 10¹² bytes. Drive and card makers use these, and so do network speeds.
- Binary prefixes count in 1,024s, because computer memory is addressed in powers of two: 1 KiB = 1,024 bytes, 1 MiB = 1,048,576, 1 GiB = 2³⁰ = 1,073,741,824 bytes. Their proper names are kibibyte, mebibyte and gibibyte.
Windows divides by 2³⁰ but still prints "GB," so a 1 TB drive (10¹² bytes) shows up as about 931 GB. That's really 931 GiB with the wrong label. macOS switched to decimal in version 10.6 (2009), so a Mac shows the same drive as 1 TB, or close to it. Neither one is wrong about the number of bytes. They divide by different numbers.
After that, formatting takes a little more. The file system needs room for its own tables and directories, so the usable space on a freshly formatted drive or card is a bit below even the binary figure.
Doing the math, step by step
GB to GiB
Multiply by 10⁹ to get bytes, then divide by 2³⁰. In one step, multiply by 0.9313.
- Turn GB into bytes
1,000 GB × 1,000,000,000 = 1,000,000,000,000 bytes - Divide by 1,073,741,824
10¹² ÷ 2³⁰ = 931.32 GiB
The gap grows with each prefix: 2.4% for kilo, 4.9% for mega, 7.4% for giga and 10% for tera.
Video size from bitrate
Video files are sized by bitrate × time. Bitrate is in megabits per second, so divide by 8 to get megabytes.
- Bits for one hour
100 Mbps × 3,600 s = 360,000 megabits - Divide by 8
360,000 ÷ 8 = 45,000 MB - Into gigabytes
45,000 MB ÷ 1,000 = 45 GB
That gives a handy constant: every 1 Mbps of bitrate adds 450 MB per hour, so GB per hour = Mbps × 0.45.
The same divide-by-8 is behind download times. Moving that 45 GB file over a 100 Mbps connection takes exactly as long as it took to record, one hour at full speed. The download time guide covers real-world slowdowns.
File size: 45 GBInternet speed: 100 MbpsOverhead and slowdowns: 0%
Real situations, worked out
10¹² bytes ÷ 1,073,741,824 = 931.32. Windows is counting in GiB and labeling it GB. After formatting, the free space reads a little lower again. A 2 TB drive comes out at 1,862.6 GiB, which Windows shows as about 1.8 TB.
1 TB = 931.3 GiB (shown as "931 GB")128 GB × 8 = 1,024,000 megabits. ÷ 100 Mbps = 10,240 seconds, which is 2 hours, 50 minutes and 40 seconds. That ignores formatting and the camera's own files, so plan on a few minutes less.
about 2.8 hours (2 h 50 min) of recordingRAW files from a 24 MP sensor run roughly 25 to 30 MB each, depending on the camera and the scene. 64,000 MB ÷ 30 = 2,133 and 64,000 MB ÷ 25 = 2,560.
roughly 2,100 to 2,500 RAW photosOne minute is 50 × 60 ÷ 8 = 375 MB. 20,000 MB ÷ 375 = 53.3 minutes. Some phones list the approximate size per minute right in the camera settings, which saves you the bitrate math. A 256 GB phone also never has 256 GB free: the operating system and built-in apps take their share first.
about 53 minutes of 4KDoing it in your head
For GB to GiB, take off 7%. For TB to TiB, take off about 9%. Both rules land within a fraction of a percent.
| Labeled size | Exact GiB | GB × 0.93 | Off by |
|---|---|---|---|
| 32 GB | 29.80 | 29.76 | 0.04 |
| 64 GB | 59.60 | 59.52 | 0.08 |
| 128 GB | 119.21 | 119.04 | 0.17 |
| 256 GB | 238.42 | 238.08 | 0.34 |
| 512 GB | 476.84 | 476.16 | 0.68 |
| 1,000 GB | 931.32 | 930.00 | 1.32 |
Video pegs. Bitrate × 0.45 = GB per hour. 25 Mbps is about 11 GB an hour, 50 Mbps is 22.5 GB and 100 Mbps is 45 GB. For speeds, divide by 8: a 1 Gbps line moves 125 MB a second at best.
Common mistakes
Reading Mbps as MB per second. A "500 Mbps" plan downloads at most 62.5 MB per second, not 500. Download windows usually show MB/s, so the speed looks eight times slower than the ad. It isn't.
Thinking the drive maker shorted you. The bytes are all there. A 1 TB drive holds 1,000,000,000,000 bytes, and the operating system counts them in larger 2³⁰-byte chunks. The only real loss is the small slice formatting takes.
Mixing GiB from the computer with GB from the camera. If Windows says a card has 119 "GB" free, that's 119 GiB, or 128 GB in the units the bitrate math uses. Mix the two and your recording estimate comes out about 7% short.
Treating kB, KB and KiB as the same. Lowercase k is the SI kilo, 1,000. "KB" with a capital K often means 1,024 in memory specs, and KiB always does. When the difference matters, write KiB.
A bit of history
- 1948Claude Shannon's paper founding information theory uses the word "bit," and credits it to his colleague John Tukey, a contraction of "binary digit."
- 1956Werner Buchholz at IBM coins "byte" during the design of the Stretch computer, spelled with a y so it couldn't be misread as "bit."
- 1964IBM's System/360 makes the 8-bit byte the norm. Earlier machines had used other sizes.
- 1998The IEC introduces binary prefixes (kibi, mebi, gibi) so 1,024-based units get their own names.
- 2009Apple's Mac OS X 10.6 Snow Leopard switches to decimal units, matching what's printed on the drive.
Calling 1,024 bytes a "kilobyte" was harmless when the gap was 2.4%. At terabyte scale it's 10%.
Reference table
| Labeled size | Bytes | Binary (what Windows shows) | 100 Mbps video |
|---|---|---|---|
| 32 GB | 32 × 10⁹ | 29.8 GiB | 43 min |
| 64 GB | 64 × 10⁹ | 59.6 GiB | 1 h 25 min |
| 128 GB | 128 × 10⁹ | 119.2 GiB | 2 h 51 min |
| 256 GB | 256 × 10⁹ | 238.4 GiB | 5 h 41 min |
| 512 GB | 512 × 10⁹ | 476.8 GiB | 11 h 23 min |
| 1 TB | 10¹² | 931.3 GiB | 22 h 13 min |
| 2 TB | 2 × 10¹² | 1,862.6 GiB (1.82 TiB) | 44 h 27 min |
| 4 TB | 4 × 10¹² | 3,725.3 GiB (3.64 TiB) | 88 h 53 min |
Video times are before formatting overhead, rounded to the nearest minute.
Questions people ask
Why does my 1 TB drive only show 931 GB?
The drive holds 10¹² bytes. Windows divides by 1,073,741,824 (2³⁰) instead of 1,000,000,000, so it reports 931 and still calls them GB. On a Mac you'll see close to 1 TB.
Is a gigabyte 1,000 or 1,024 megabytes?
By the SI and IEC standards, 1 GB is 1,000 MB. 1,024 MiB make a gibibyte (GiB). In everyday use people still say "gigabyte" for both, which is the root of the confusion.
How many bits are in a byte?
Eight. That's why you divide a speed in megabits per second by 8 to get megabytes per second: 100 Mbps is 12.5 MB/s.
How much 4K video fits on a 128 GB card?
It depends on the bitrate. At 100 Mbps it's about 2 hours 50 minutes; at 200 Mbps, about 1 hour 25 minutes; at 50 Mbps, about 5 hours 41 minutes. Multiply Mbps by 0.45 to get GB per hour, then divide the card size by that.
Why does a freshly formatted card show less space than the label?
Two reasons stack up: your computer may show binary units (128 GB is 119.2 GiB), and the file system reserves a small amount for its own bookkeeping.