The formula
time = size in bits ÷ (speed × efficiency)
Where it comes from
The perennial confusion: connections are sold in megabits per second and files are measured in megabytes. A byte is eight bits, so a 300 Mbps connection downloads at 37.5 MB/s at most. And not even that: protocol overhead takes around 10 %.
How to work it out by hand
- Convert the file size to bits by multiplying by 8
- Take the speed in megabits per second
- Knock off around 10 % of overhead
- Divide the size by the effective speed
What is worth knowing
The advertised speed is a theoretical maximum, not a guarantee. Wi-Fi, the source server, the cabling and how many people share the line all bear on it. Over Wi-Fi the loss can be far more than 10 %: a cable is still noticeably faster and steadier than any domestic wireless network.