Quick answer: Drive makers use 1 TB = 10^12 bytes while Windows counts in binary units, so 1 TB = 931.32257 GiB (shown as "931 GB").
How it's calculated
A byte is 8 bits. The SI prefixes are powers of 1,000: 1 kB = 1,000 B, 1 MB = 1,000,000 B, 1 GB = 10^9 B. The binary (IEC) prefixes are powers of 1,024: 1 KiB = 1,024 B, 1 MiB = 1,048,576 B, 1 GiB = 1,073,741,824 B.
Storage makers and macOS use the decimal units; Windows and many programs use the binary ones but label them KB, MB and GB, which is why a new drive looks smaller than advertised.
Network speeds are in bits per second (Mbps), not bytes. To estimate a download time, use the download time calculator.
Worked examples
- A 1 TB drive in Windows: 1 TB = 931.32257 GiB
- A 512 GB phone: 512 GB = 476.83716 GiB
- A 100 MB file in bits: 100 MB = 800,000,000 bit
Common data size conversions
| From | To |
|---|---|
| 1 B | 8 bit |
| 1 kB | 1,000 B |
| 1 KiB | 1,024 B |
| 1 MB | 1,000 kB |
| 1 GB | 1,000 MB |
| 1 TB | 1,000 GB |
| 1 GB | 0.93132257 GiB |
| 1 TB | 0.9094947 TiB |
| 512 GB | 476.83716 GiB |
| 1 GiB | 1,073.7418 MB |
Frequently asked questions
Is 1 GB 1,000 MB or 1,024 MB?
In SI units 1 GB = 1,000 MB. 1,024 belongs to the binary units: 1 GiB = 1,024 MiB.
Why does my 1 TB drive show 931 GB?
The drive holds 10^12 bytes; Windows divides by 1,024³ and shows 931.32 'GB', which are really GiB.
How many bits in a byte?
Eight.
What's the difference between Mb and MB?
Small b is bits, capital B is bytes, and a byte is 8 bits. So a 100 Mbps line moves at most 12.5 megabytes a second.
What comes after a terabyte?
The petabyte, then the exabyte, zettabyte and yottabyte, each a thousand times the last. The binary versions are PiB, EiB, ZiB and YiB, each 1,024 times the last.
Why is my phone's free space smaller than its capacity?
The operating system and built-in apps take their share first, and the phone may count in binary units. A 128 GB phone always shows a fair bit less free before you've added a thing.