File Transfer Calculator
Work out transfer time, file size, or the speed you need. Clear units, fewer surprises.
Calculated locally in your browser
A little context
The numbers, explained.
An estimate for planning your next upload, download, or local transfer.
How is the estimate calculated?
Transfer time = file size in bits ÷ effective speed in bits per second. Effective speed is your connection speed × (1 − overhead ÷ 100). Choose file size or speed to solve the same equation in reverse.
What’s the difference between MB and Mbps?
A capital B means bytes; a lowercase b means bits. There are 8 bits in a byte, so 100 Mbps is 12.5 MB/s before overhead. Decimal units use powers of 1,000 (1 GB = 1,000 MB). Binary units use powers of 1,024 (1 GiB = 1,024 MiB).
Which speed and overhead should I use?
For uploads, use your upload speed; for downloads, use your download speed. The default 10% overhead is an adjustable assumption, not a measured value. If you already know your effective file-transfer speed, set overhead to 0% to avoid counting it twice.
Why might the real transfer take longer?
This is an estimate, not a speed test. Wi-Fi conditions, competing traffic, server limits, storage performance, and many small files can all affect transfers. The calculator assumes a constant speed and doesn’t include connection setup time. Results are rounded; fractional seconds round up.
Are my inputs saved or sent anywhere?
No. Calculations run on this device. Inputs stay in page memory and aren’t added to URLs, uploaded, or stored by Celestia Tools. Copying a result is always your choice. The site still downloads its own assets.