How to use the bandwidth calculator
- Choose what you want to work out: download or upload time, data transferred in a set time, or website bandwidth per month.
- For a transfer time, enter the file size and pick its unit. Decimal units (kB, MB, GB) and binary units (KiB, MiB, GiB) are both listed.
- Enter the connection speed. Internet plans are sold in megabits per second (Mbps); download managers often show megabytes per second (MB/s).
- For data transferred, enter how long the connection runs.
- Optionally add an overhead percentage for protocol and network losses.
- For a website, enter the page views per month and the average page size, plus any extra allowance.
How transfer time is calculated
The key step is turning bytes into bits. File sizes are measured in bytes, but connection speeds are measured in bits, and one byte is eight bits.
Decimal prefixes step by 1,000: 1 GB is 1,000,000,000 bytes and 1 Mbps is 1,000,000 bits per second. Binary prefixes step by 1,024: 1 GiB is 1,073,741,824 bytes. Windows labels binary sizes "GB", which is why a 1 TB drive shows up as about 931 GB.
Worked example
The defaults are a 5 GB file on a 100 Mbps connection with no overhead:
- File size in bytes: 5 × 1,000,000,000 = 5,000,000,000 bytes
- In bits: 5,000,000,000 × 8 = 40,000,000,000 bits
- Speed: 100 Mbps = 100,000,000 bits per second, or 12.5 MB/s
- Time: 40,000,000,000 ÷ 100,000,000 = 400 seconds = 6 min 40 s
Switch to data transferred and the same 100 Mbps running for 1 hour moves 100,000,000 × 3,600 ÷ 8 = 45 GB, which is 41.91 GiB. For a website with 100,000 page views a month and 2.5 MB pages, monthly bandwidth is 100,000 × 2.5 MB = 250 GB. Spread over a 30-day month, that averages about 0.77 Mbps.
Download time for 1 GB
| Connection | Time for 1 GB |
|---|---|
| 10 Mbps | 13 min 20 s |
| 25 Mbps | 5 min 20 s |
| 50 Mbps | 2 min 40 s |
| 100 Mbps | 1 min 20 s |
| 300 Mbps | 26.7 s |
| 500 Mbps | 16 s |
| 1 Gbps | 8 s |
These are ideal times. Double the size and the time doubles; double the speed and it halves.
Why real downloads take longer
Your plan's speed is the most the line can carry, not what every file gets. TCP/IP headers, Wi-Fi retransmissions and encryption take a share, usually 5–15%. The server sending the file may also be slower than your line, and other devices on the same network share the bandwidth. Uploads are often much slower than downloads on cable and DSL plans, so check your upload speed separately before timing a backup or video upload. A speed test shows what you actually get.
Frequently asked questions
What is the difference between Mbps and MB/s?
Mbps is megabits per second and MB/s is megabytes per second. Divide Mbps by 8 to get MB/s, so a 100 Mbps plan downloads at up to 12.5 MB/s. The lowercase b means bits and the capital B means bytes.
How much bandwidth does my website need?
Multiply your monthly page views by your average page size, then add room for spikes, search engine bots and growth. A site with 100,000 views of 2.5 MB pages uses about 250 GB a month. Images and video make up most page weight, so compressing them cuts bandwidth the fastest.
Does a file shown as 4.7 GB in Windows take longer than I expect?
Windows reports sizes in binary units but labels them GB. A file Windows calls 4.7 GB is really 4.7 GiB, about 5.05 decimal GB. Choose GiB in the calculator for sizes copied from Windows so the time comes out right.
Why is my upload so much slower than my download?
Many cable, DSL and some fiber plans are asymmetric: the upload speed is a fraction of the download speed, such as 300 Mbps down and 20 Mbps up. Enter your upload speed to time an upload. At 20 Mbps, a 5 GB backup takes about 33 minutes.
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