Download Time Calculator

Download Time Calculator turns a file size and a connection speed into a transfer time. The only trick in it is the unit mismatch that trips everyone up: file sizes are in bytes and connection speeds are in bits per second, with eight bits to the byte. A 100 Mbps line moves at most 12.5 megabytes a second, so a 1 GB file takes about 80 seconds at the absolute best.

That best case almost never happens, which is what the overhead box is for. TCP headers, acknowledgements, TLS, retransmissions and DNS all consume part of the line, and anywhere between 8 and 12% is normal for a transfer over the open internet. Wi-Fi, a busy server or a contended cable segment take more. Setting overhead to zero gives the theoretical figure; leaving it at ten gives something closer to what you will actually see.

The comparison table runs the same file past a range of typical connections, from ADSL to gigabit fibre, which is the quickest way to answer the real question behind most of these calculations: is it worth upgrading the line, or is the file simply too big to move this way?

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows transfer time, with overhead, effective throughput, file size in bits and data per hour together, so you are not running it twice
• A results table you can read across
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The download time calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Estimating how long a large backup or video will take
• Deciding whether to download now or overnight
• Comparing transfer times across connection speeds
• Checking whether a line upgrade is worth it