Tech & everyday
Download time calculator
A 100 Mbps connection does not download 100 megabytes a second. It downloads about twelve.
Bits versus bytes
Internet speeds are quoted in megabits per second (Mbps). File sizes are measured in megabytes (MB). There are 8 bits in a byte, so:
MB/s = Mbps ÷ 8
A 100 Mbps connection therefore moves at most 12.5 MB per second. A 4.7 GB file — the capacity of a single-layer DVD — takes about 6 minutes at that speed in theory, and closer to 8 in practice.
The notation is the only clue: a lowercase b means bits, an uppercase B means bytes. Mbps and MBps differ by a factor of eight, and marketing material almost always uses the one that produces the bigger number.
Why you never get the advertised speed
The theoretical figure is a ceiling nobody reaches. Real throughput is reduced by:
- Protocol overhead — TCP/IP headers, acknowledgements and retransmissions consume 5-10% of raw bandwidth before any file data moves.
- Server limits — the far end may cap per-connection speed, or simply be busy. Very often the bottleneck is not your line at all.
- Wi-Fi — a shared medium with interference, distance loss and contention. Real Wi-Fi throughput is commonly half the negotiated link rate or less.
- Latency and window size — on high-latency links, TCP cannot keep enough data in flight to fill the pipe, which is why a transatlantic download can crawl on a fast connection.
- Local storage — writing to a slow drive or a USB stick can become the limiting factor on a gigabit line.
The efficiency setting accounts for this. 80% is a fair default for a wired connection to a well-provisioned server; 60% is realistic over congested Wi-Fi.
The other factor of 1.024
There is a second, smaller discrepancy. Drive manufacturers define a gigabyte as 1,000,000,000 bytes, while Windows reports in gibibytes of 1,073,741,824 bytes and labels them "GB". That is why a 1 TB drive shows as roughly 931 GB.
This calculator uses binary multiples (1 GB = 1024 MB) for file sizes, which matches how operating systems report them. The difference is about 7% at the gigabyte scale — smaller than the throughput uncertainty, but worth knowing when numbers do not quite reconcile.
Uploads are usually much slower
Most residential connections are asymmetric. A cable or ADSL line advertised at 100 Mbps down may offer only 10 or 20 Mbps up, so backing up 50 GB can take a full day even though downloading it would take an hour. Fibre-to-the-premises is often symmetric, which is one of its genuine practical advantages.
To estimate an upload, enter your upload speed rather than your download speed. The arithmetic is identical.
Common questions
Why is my 100 Mbps connection only downloading at 12 MB/s?
Because there are 8 bits in a byte. 100 megabits per second is 12.5 megabytes per second at best, and real-world overhead brings it down further. Nothing is wrong.
What is the difference between Mbps and MBps?
The capital B. Mbps is megabits per second, MBps is megabytes per second, and they differ by a factor of eight. Internet plans are advertised in the smaller unit because the number looks larger.
Why do I never reach my advertised speed?
Protocol overhead takes 5-10%, the server may cap or throttle you, Wi-Fi loses a great deal to interference and contention, and high latency limits how much data TCP keeps in flight. 80% of the line rate is a good real-world assumption.
Why is uploading so much slower than downloading?
Most residential connections are asymmetric — a 100 Mbps download plan might only offer 10-20 Mbps upload. Enter your upload speed instead to estimate the time; the arithmetic is the same.