How to use it
Type the size of the file and pick its unit, then type your connection speed and its unit. The big number is the time the transfer should take. The slider is the share of the line speed you actually get: routers, Wi-Fi, the server at the other end and protocol overhead all take a cut, so 100% is a best case. The tool also shows the speed in bytes per second, the best-case time, and how much data moves in an hour at that rate.
Bits, bytes and the capital B
Internet providers sell speed in bits per second (Mbps, with a small b). File sizes and download managers use bytes (MB, with a capital B). One byte is 8 bits, so a 100 Mbps line moves at most 12.5 MB each second. If a download shows 12 MB/s on a 100 Mbps plan, nothing is wrong. That is the plan working as sold.
The prefixes come in two flavors. KB, MB, GB and TB use 1,000 (what storage makers and most speed ratings use). KiB, MiB, GiB and TiB use 1,024 (what many operating systems show). A "1 TB" drive holds about 931 GiB, which is why it looks smaller once plugged in.
Questions
Why is my download slower than the calculator says? The other end may limit your speed, other devices on your network may be using the line, and Wi-Fi drops with distance and walls. Try a lower percentage to match what you see. Does this work for uploads? Yes, use your upload speed, which is often much lower than your download speed. Is anything sent anywhere? No. The numbers stay in your browser.