Download Time Calculator
With file size 5 GB, connection speed 100 Mbps, share of the speed you actually get 85 percent, download time comes to 7.84 minutes — download time. It is reached in 8 steps, the last of which is 470.58824 / 60, and each one is printed on the page with its numbers filled in. The formula is the one published by NIST Special Publication 811, not an approximation fitted to it.
How long a file takes at a given connection speed, with the bits-and-bytes factor of eight that makes everyone’s estimate wrong.
Formula and sources checked · How we check
File size 5 GB, Connection speed 100 Mbps, Share of the speed you actually get 85
7.84 minutes
Download time for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Megabytes a second, at the advertised speed
100 / 812.5 MB/s- Realistic throughput
100 / 8 * 85 / 10010.625 MB/s- File size in megabytes
5 * 10005,000 MB- Seconds
5000 / 10.625470.588 s- Minutes
470.58824 / 607.843- Hours
470.58824 / 36000.131- Seconds at the full advertised speed
5000 / 12.5400 s- Files like this in a 1.2 TB monthly cap
1200 / 5240
Worked example
A 5 GB file on a 100 Mbps line takes 400 seconds at the advertised rate and about 7.8 minutes in practice. The trap is the unit: 100 megabits a second is 12.5 megabytes a second, so the download is eight times slower than the number on the bill suggests.
How to work it out yourself
- 1.Note which unit you have. Connection speeds are in megabits (Mb, small b) and file sizes in megabytes (MB, capital B) — one is eight times the other, and the notation is the only thing distinguishing them.
- 2.Leave some headroom in the efficiency figure. Protocol overhead alone takes a few per cent, and Wi-Fi, contention and a slow server take more.
- 3.For an upload, use your upload speed. On most cable and DSL connections it is a fraction of the download rate, which is why sending a file takes far longer than fetching one.
A 5 GB file at different speeds
| Connection speed (Mbps) | Download time | Minutes | Realistic throughput |
|---|---|---|---|
| 10 | 78.43 minutes | 78.43 | 1.06 MB/s |
| 25 | 31.37 minutes | 31.37 | 2.66 MB/s |
| 50 | 15.69 minutes | 15.69 | 5.31 MB/s |
| 100 | 7.84 minutes | 7.84 | 10.63 MB/s |
| 200 | 3.92 minutes | 3.92 | 21.25 MB/s |
| 300 | 2.61 minutes | 2.61 | 31.88 MB/s |
| 500 | 1.57 minutes | 1.57 | 53.13 MB/s |
| 1000 | 0.78 minutes | 0.78 | 106.25 MB/s |
At 85% of the advertised rate.
The formula
- Megabytes a second, at the advertised speed
100 / 8 - Realistic throughput
100 / 8 * 85 / 100 - File size in megabytes
5 * 1000 - Seconds
5000 / 10.625 - Minutes
470.58824 / 60 - Hours
470.58824 / 3600 - Seconds at the full advertised speed
5000 / 12.5 - Files like this in a 1.2 TB monthly cap
1200 / 5
Source: NIST Special Publication 811 — Guide for the Use of the International System of Units, IEC 80000-13 — quantities and units: information science and technology (binary prefixes)
Questions people actually ask
- Why is my 100 Mbps connection only downloading at 12 MB/s?
- Because it is doing exactly what it says. Mbps is megabits per second and MB/s is megabytes per second, and there are eight bits in a byte — so 100 Mbps is 12.5 MB/s at best. Nothing is wrong; the two units are simply eight times apart and differ only by the case of one letter.
- Why do I never get the full advertised speed?
- Protocol overhead takes a few per cent before anything else, and then Wi-Fi, the server at the other end, congestion on the route and your own hardware each take a share. Eighty to ninety per cent of the advertised rate over a wired connection is normal; over Wi-Fi at range it can be half.
- Is a gigabyte 1,000 or 1,024 megabytes?
- Both are in use, which is why file sizes never quite agree. A gigabyte as defined by the SI prefix is 1,000 megabytes; the binary equivalent is a gibibyte, 1,024 mebibytes. Storage is sold in the decimal units and operating systems have historically reported the binary ones, which is why a 1 TB drive shows as 931 GB.
- Does upload speed matter?
- For video calls, backups and sending large files, more than download does. Cable and DSL connections are asymmetric — often ten or twenty times faster down than up — which is why a household can stream four films at once and still struggle to send a video. Fibre is usually symmetric and does not have the problem.
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