What are bits and bytes?
A bit is the smallest piece of digital data: a single 0 or 1. A byte is a group of 8 bits, enough to store one letter of plain English text. Larger units are built from these: a kilobyte, megabyte and gigabyte are a thousand, a million and a billion bytes. Network speeds are counted in bits per second, and file sizes in bytes. Mixing the two up is the most common reason a download seems eight times slower than the speed you pay for.
Common conversions
| Amount | Equals |
|---|---|
| 1 GB | 1,000 MB |
| 1 GiB | 1,024 MiB |
| 1 TB | 1,000 GB |
| 1 MB | 8,000,000 bits |
| 1 MB | 1,000 KB |
| 1 GB | about 0.931 GiB |
| 1 Gb (gigabit) | 125 MB |
Decimal and binary units
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 KB (kilobyte) | 1,000 | 1 KiB (kibibyte) | 1,024 |
| 1 MB (megabyte) | 1,000,000 | 1 MiB (mebibyte) | 1,048,576 |
| 1 GB (gigabyte) | 1,000,000,000 | 1 GiB (gibibyte) | 1,073,741,824 |
| 1 TB (terabyte) | 10¹² | 1 TiB (tebibyte) | 1,099,511,627,776 |
The gap grows with size: a gibibyte is about 7% larger than a gigabyte, a tebibyte about 10% larger than a terabyte.
How long does a download take?
Time to download each file at full speed, worked out with the same calculator as the tool above:
| File | 10 Mbps | 25 Mbps | 100 Mbps | 300 Mbps | 1 Gbps |
|---|---|---|---|---|---|
| 1 GB file | 13 min 20 s | 5 min 20 s | 1 min 20 s | 27 s | 8 s |
| 4 GB HD film | 53 min 20 s | 21 min 20 s | 5 min 20 s | 1 min 47 s | 32 s |
| 50 GB game | 11 h 6 min 40 s | 4 h 26 min 40 s | 1 h 6 min 40 s | 22 min 13 s | 6 min 40 s |
| 150 GB game | 1 d 9 h 20 min | 13 h 20 min | 3 h 20 min | 1 h 6 min 40 s | 20 min |
Real downloads usually take a little longer. Part of every connection carries network overhead rather than your file, Wi-Fi is often slower than the line itself, and a busy server may not send at your full speed. Many browsers, game launchers and download managers show progress in megabytes per second (MB/s), so a steady 12 MB/s on a 100 Mbps line is close to the maximum of 12.5.
Why a new drive shows less space than advertised
Drive makers count in decimal units, where 1 TB is 1012 bytes. Windows counts in binary units (1 GiB = 1,0243 bytes) but labels them “GB”, so the same drive shows a smaller number. Nothing is missing. This is how each size appears in Windows before formatting and system files take their share:
| Sold as | Shown in Windows |
|---|---|
| 128 GB | 119.2 GB |
| 256 GB | 238.4 GB |
| 512 GB | 476.8 GB |
| 1 TB | 931.3 GB |
| 2 TB | 1.82 TB |
| 4 TB | 3.64 TB |
macOS, iOS and Android use decimal units, so there a 1 TB drive shows close to 1 TB.
Speeds and sizes
- Internet speed is in megabits per second: 100 Mbps = 12.5 megabytes per second.
- A 4 GB film takes about 5 minutes 20 seconds at 100 Mbps.
- Phone storage and data plans are in decimal gigabytes.
Measure your own connection with the internet speed test, then plug the result into the download time calculator.