What this calculator works out
This calculator estimates how long a file will take to download, given its size and the speed you actually get rather than the speed on the advertisement.
The arithmetic trips most people up in one specific place: file sizes are measured in bytes and connection speeds in bits, and there are eight bits in a byte. A 100 Mbps line therefore delivers at most 12.5 megabytes a second.
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Before you calculate
- File sizes are quoted in bytes. Connection speeds are quoted in bits. There are eight bits in a byte, which is the single most common cause of confusion here.
- Use the speed you actually get, not the one on the advertisement. A speed test taken at the time of day you plan to download is far more useful.
- Wi-Fi is usually the bottleneck, not the broadband line. If the number matters, test over a cable.
Divide by eight. A 100 Mbps connection moves at most 12.5 megabytes per second, so a 25 GB file takes about half an hour at best rather than four minutes. Providers quote megabits because the number is eight times larger.
How this calculator works
Megabits in the file = gigabytes × 1,000 × 8
Effective speed = advertised speed × realistic share
Seconds = megabits ÷ effective speedThe calculator uses decimal units, where one gigabyte is 1,000 megabytes. This matches how broadband speeds, hard drive capacities and most download pages are quoted. Some software reports sizes in binary units instead, where a gigabyte is 1,024 megabytes, which is why a file can appear about 7% smaller in one place than another.
The realistic share is the honest part. Almost nothing sustains its headline speed: protocol overhead alone accounts for a few per cent, and the server at the other end, your Wi-Fi, and everyone else in the house all take a share.
Worked example: a 25 GB game
A 25 GB download on a 70 Mbps connection running at 80% of its headline speed:
- File size in megabits: 25 × 1,000 × 8 = 200,000 Mb
- Effective speed: 70 × 0.8 = 56 Mbps
- Time: 200,000 ÷ 56 = 3,571 seconds, or just under an hour
At the full 70 Mbps it would be 48 minutes. The gap between the two is a fair measure of how much the advertised figure flatters real life.
Things that affect the result
- Wi-Fi. Distance, walls and interference commonly halve a connection before it reaches the device.
- The server. A busy download server or a throttled free tier will cap you well below your line speed.
- Time of day. Evening congestion is real on shared connections.
- Other devices. Streaming, backups and updates all compete for the same line.
- Upload speed. On most home connections it is a fraction of the download speed, so sending a file takes far longer than fetching one.
Common mistakes
- Confusing megabits with megabytes. Mb is megabits, MB is megabytes, and the difference is a factor of eight.
- Using the advertised speed. Ofcom requires providers to advertise a speed available to at least half of customers at peak time, so half of customers get less.
- Testing over Wi-Fi and blaming the line. Test over a cable before complaining.
- Forgetting the file has to be written to disk. On a slow external drive or an almost-full disk, storage can become the limit.
Frequently asked questions
Why is my download slower than this calculator says?
Usually Wi-Fi, the server at the other end, or other devices sharing the line. Lower the realistic share figure until it matches what you actually see, then use that figure for future estimates — it is a reasonable measure of your connection in practice.
What is the difference between Mbps and MBps?
Mbps is megabits per second and MBps is megabytes per second. Eight megabits make one megabyte, so 80 Mbps is 10 MBps. Broadband is sold in megabits; files are measured in megabytes.
Should I use 1,000 or 1,024 megabytes to a gigabyte?
This calculator uses 1,000, which matches broadband advertising, storage capacities and most download pages. Windows and some other software use 1,024, which makes the same file look about 7% smaller. For estimating a download time the difference is well inside the margin of error.
What share of my advertised speed should I expect?
Over a cable, 80 to 95% is typical for a healthy connection. Over Wi-Fi it varies enormously with distance and interference and can fall below half. Run a speed test at the time of day you care about and use that.
Does this work for uploads?
Yes, if you enter your upload speed instead. On most home broadband the upload speed is a small fraction of the download speed, so backing up a large folder to cloud storage can take many times longer than downloading the same amount.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
These organisations publish the guidance and figures behind this page.
- Ofcom — UK broadband speed reporting and the rules on how speeds may be advertised
- GOV.UK — broadband availability, universal service obligation and consumer guidance
Sources are checked at publication and can change — how I choose and check references.