How long will that download take?

Why two different numbers?

Internet speed is sold in megabits per second (Mbps), but files are measured in bytes. Eight bits to a byte, so a 100 Mbps line moves about 12.5 megabytes a second at absolute best. That division is where most people’s mental maths goes wrong — a 50 GB game on a “fast” 100 Mbps line is still an hour, not a few minutes.

There’s a second, sneakier gap. Internet speeds use decimal megabits (1,000,000 bits), while your operating system reports file sizes in binary units — what Windows calls “50 GB” is really 50 × 2³⁰ bytes, about 7.4% more data than 50 decimal GB. We use the binary reading, because it matches the file you’re actually waiting for.

On top of that, roughly 5% of your line is spent on the packet headers that make the internet work (TCP/IP and Ethernet framing) rather than on your file. The “realistic” figure above accounts for both. It’s still an optimistic number — a best case for a healthy line with nothing else competing.

What makes real downloads slower still

• The server on the other end. You can only receive as fast as it sends. A busy or distant server is the most common reason a fast line feels slow.
• Wi-Fi. Often the real ceiling. Test it with our Wi-Fi or line? tool before blaming your ISP.
• Other people and devices sharing the line at the same time.
• Peak-hour congestion, especially on international traffic in the evenings.
• Your device — an old laptop or a slow drive can cap a gigabit line.

Download times by line speed

Realistic times, including the overhead above. Useful for sanity-checking whether a faster package is worth it.

The pattern worth noticing: doubling your speed halves the time, but the saving shrinks fast. Going from 25 to 50 Mbps saves hours on a big download; going from 500 Mbps to 1 Gbps saves minutes. Our speed calculator works out what your household actually needs, and am I overpaying? checks whether you’re paying over the odds for it.