How the Bandwidth Calculator Works
How long will a download take? The answer comes from two numbers: how much data there is (the size of the file), and how fast it comes to you (the speed of your connection). The Bandwidth / Download Calculator divides the first by the second, and it takes care of the fact that they use different units.
Bits and bytes
The size of a file is measured in bytes (KB, MB, GB), and the speed of an internet connection is measured in bits per second (Kbps, Mbps, Gbps). One byte has 8 bits. This is the reason that a plan of 100 Mbps does not download 100 megabytes in a second: it downloads 12.5 megabytes.
MB per second = Mbps / 8
A small "b" means bits and a capital "B" means bytes, so Mbps is megabits and MB/s is megabytes per second. Each block of the chart is 3.1 MB/s.
10 Mbps █ 1.25 MB/s 50 Mbps ██ 6.25 MB/s 100 Mbps ████ 12.5 MB/s 500 Mbps ████████████████████ 62.5 MB/s 1000 Mbps ████████████████████████████████████████ 125 MB/s
The formula
time (seconds) = file size (bytes) x 8 / speed (bits per second)
Example, the default of the calculator: a file of 1,000 MB (1,000,000,000 bytes) on a connection of 50 Mbps. The file has 1,000 x 8 = 8,000 megabits. At 50 megabits per second, the time is 8,000 / 50 = 160 seconds, that is 2 minutes and 40 seconds.
The units of the calculator
| Unit | Size |
|---|---|
| 1 KB, 1 MB, 1 GB, 1 TB | 1,000; 1,000,000; 1,000,000,000; 1,000,000,000,000 bytes |
| 1 MiB, 1 GiB | 1,048,576 bytes; 1,073,741,824 bytes |
| 1 Kbps, 1 Mbps, 1 Gbps | 1,000; 1,000,000; 1,000,000,000 bits per second |
| 1 MB/s | 8 megabits per second |
The units MB and GB, as they are used here, are decimal (1 MB = 1,000,000 bytes). Some programs, such as Windows, show sizes in binary units (MiB and GiB, where 1 GiB = 1,073,741,824 bytes) but call them GB. This makes a file about 7% bigger than you would think. Choose the unit that your program uses.
Table: how long common files take
| File | 10 Mbps | 50 Mbps | 100 Mbps | 1 Gbps |
|---|---|---|---|---|
| A song (5 MB) | 4 s | 800 ms | 400 ms | 40 ms |
| A photo album (200 MB) | 2 min 40 s | 32 s | 16 s | 1.6 s |
| A movie in HD (4 GB) | 53 min 20 s | 10 min 40 s | 5 min 20 s | 32 s |
| A big game (50 GB) | 11 h 6 min 40 s | 2 h 13 min 20 s | 1 h 6 min 40 s | 6 min 40 s |
At 100 Mbps, the time of each file is below. Each block is about 100 seconds, and a bar has at least one block.
A song █ 400 ms A photo album █ 16 s A movie in HD ███ 5 min 20 s A big game ████████████████████████████████████████ 1 h 6 min 40 s
Chart: 1 GB at different speeds
Each block is 20 seconds. Your default speed of 50 Mbps is green. Ten times the speed takes one tenth of the time.
10 Mbps ████████████████████████████████████████ 13 min 20 s 25 Mbps ████████████████ 5 min 20 s 50 Mbps ████████ 2 min 40 s 100 Mbps ████ 1 min 20 s 500 Mbps █ 16 s 1000 Mbps █ 8 s
Why the real time is longer
The formula gives the best case. The real download is slower because:
- Overhead. Every packet of data has headers and other information, and this uses about 5% to 10% of the speed. Enter it in the calculator to see the effect: with 10% overhead, the time of the example goes from 160 seconds to 178 seconds.
- The speed of the plan is a maximum. Your real speed can be lower at busy hours, and Wi-Fi can be much slower than a cable.
- The server that sends the file may be slow, or may limit the speed.
- Other devices in your home share the same connection.
Also note that many plans have a different speed for the download and for the upload, and the upload is often much slower. Use the upload speed when you send a file.
Tips
- Test your real speed with a speed test, and use that number.
- Use a cable instead of Wi-Fi for a big download.
- Add 5% to 10% for overhead to get a realistic time.
What this calculator does not include
- A speed that changes during the download.
- Pauses, a resume after a stop, and limits on the data of your plan.
- The time to unpack, to install or to check the file.
This tool gives an estimate. The real time depends on your network and on the server.