How long that file will actually take — with the Mbps-versus-MB/s trap handled for you, plus a realistic overhead allowance so the estimate matches reality.
File size
Connection speed
ISPs advertise in Mbps — megabits. Browsers and download managers show MB/s — megabytes, eight times bigger.
%
Protocol overhead, congestion and disk speed. 0% for a theoretical best case, 10–20% for a realistic estimate.
Download time
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Effective speed—
Theoretical best—
File size in bytes—
Total seconds—
Downloaded in 1 minute—
Downloaded in 1 hour—
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Megabits are not megabytes
This is where almost every download estimate goes wrong. Internet speeds are sold in megabits per second (Mbps). File sizes are measured in megabytes (MB). There are eight bits in a byte, so a 100 Mbps connection tops out at about 12.5 MB/s — a factor of eight, not a rounding error.
Notice the case, too: Mb is megabits, MB is megabytes. A 1 Gbps fibre line delivers roughly 125 MB/s at best, so a 50 GB game is around seven minutes, not fifty seconds.
Why real downloads are slower than the maths
Protocol overhead — TCP/IP headers, acknowledgements and TLS handshakes consume 3–8% of raw throughput before any file data moves.
The server's limit — you only get your full line speed if the source can serve it. Busy servers throttle per connection.
Wi-Fi — real Wi-Fi throughput is typically half to two-thirds of the link rate advertised by the router, and worse through walls.
Disk write speed — an old spinning hard drive can bottleneck a gigabit connection all by itself.
Contention — everything else on the network is sharing the same pipe.
The overhead percentage covers all of this in one number. Zero gives the theoretical floor; 10% is a fair everyday assumption on a wired connection; 20–30% is realistic on congested Wi-Fi.
Windows says a file is 8 MB when it actually holds 8,388,608 bytes — it is using binary megabytes (2²⁰). Storage manufacturers, macOS and most network tools use decimal megabytes (10⁶ = 1,000,000 bytes). The gap is about 4.9% at the megabyte level and grows to roughly 10% at terabytes, which is precisely why a "1 TB" drive shows up as 931 GB. Switch the convention above to match wherever you read the file size.
Rough guide at 100 Mbps
At a real-world 100 Mbps with 10% overhead you get about 11.25 MB/s: a 5 MB photo is instant, a 700 MB video is roughly a minute, a 5 GB film is about seven and a half minutes, and a 100 GB game is around two and a half hours. Halve the speed and every number doubles.
On a 100 Mbps connection, about 90 seconds at the theoretical maximum and closer to 100 seconds in practice. On 25 Mbps it is around six minutes; on 1 Gbps fibre roughly nine seconds, assuming the server can keep up.
Why is my download slower than this calculator says?
Set the overhead higher. The usual causes are Wi-Fi rather than Ethernet, a server that limits per-connection speed, other devices sharing the line, or a slow disk. Try 20–30% overhead on Wi-Fi for a more realistic figure.
What is the difference between Mbps and MB/s?
Mbps is megabits per second, MB/s is megabytes per second, and there are 8 bits in a byte. Divide Mbps by 8 to get MB/s — so 100 Mbps is 12.5 MB/s. ISPs quote the bigger-sounding bit figure; download managers show bytes.
Should I use binary or decimal file sizes?
Match whatever showed you the file size. Windows file properties and most download managers use binary; storage capacities, macOS and network speeds use decimal. The difference is about 5% per unit step, so it only matters for large files.
Does upload speed work the same way?
Yes — the arithmetic is identical, just substitute your upload speed. Most home connections are asymmetric, with upload far slower than download, so check the right figure before estimating a big file transfer out.